CHASSIS

GMC Security Porch Light

yes , well I have been looking for a long time for a Motion sensor light that ran on 12 volts and was small. Some of our GMCers wanted a sensor that could be used for alarms and outside lights to protect their toys parked outside the Motor home.
http://www.gmcmhphotos.com/photos/showgallery.php?cat=4381&ppuser=

I really like this one, but it is not cheap. I bought it here ($66)
http://www.realgoods.com/renew/shop/product.cfm?dp=3004&ts=4500109&kw=motion

but the manuf. is in Canada, and I hope someone will find a cheaper place to buy them
http://www.spsenergy.com/

Here is the assembly manual
http://www.spsenergy.com/downloads/pdf/M_MotionSensor.pdf

I want two more for interior projects on the GMC but do not want to pay that price. The unit is very flexible with 10 amp isolated contacts to drive other devices like alarms, etc. The sensor head is very small and can be mounted without the junction box. The three adjustments are very good which works for other applications. gene

Engineers spend their whole career building circuits with a single point Ground. This is not the best solution for our GMCs. The GMC is virtually a "Ground Grid" which you can think of as a grounding fishnet with a ground at every joint of the net. The GMC is a mass of corroded ground connections of loosely connected to the chassis. You cannot change all of these grounds. You can distribute good ground connections back to the Mother-of-all-grounds, the engine.

There should be a large battery cable from the engine battery directly to the starter and then to the chassis. The Engine with the Alternator (the source) and Starter(the load), is the MOTHER of all grounds. All grounds including the Batteries should return to this point. Some owners buy Engine Batteries with side and top terminals so there is a convenient place for this cable. This cable will normally fix the infamous "hot Starter " and "only starts with the Boost Switch" problems. There have been reports of the starting current melting the Oil Dipstick and electrical arcing through the bearings in the transmission.

A wire needs to go from the Engine directly to the Dash ground connection. The Dashboard is classic for having a poor ground that creates many mysterious problems. There are many other devices like the headlights and turn lights that should have a separate ground wire. This is a good chance to clean up the old ground connections with star washers and anti-oxidizing goo to seal out the elements. Put the new ground wire directly on top of the old ground attached to the chassis.

One problem is how to distribute all of these grounds. I suggest using a ground terminal strip from home Power Boxes. The screw terminals will take almost any size wire and can be tightened into an almost gas tight connection and the strip connects with several bolts directly to the chassis. You can cut the terminal strip into any length to fit the space so buy the long ones, they are cheaper.

There are other grounding issues inside the coach. Typically a ground wire is carried to each load from the Coach Battery, but not always. Just remember where the Mother-of-all-grounds is located, and when you are working on problems with the clearance lights, toad wiring, Tail lights,etc. look at the grounds first.

This is a very common turn signal switch used by GM on vehicles of that era. It is not GMC motorhome specific.
These are made by Wells. The difference in the two numbers is that one also included a set of contacts for cornering lights. The cornering lights are actually a separate connector that plugs into the same switch that you do not have to use. Wells is actually the manufacturer of the switch for Delco (GM) . My Wells switch came with the Delco inscription on the plastic molding.

The problem you will run into is there was two different plugs used on the connector depending on the year the coach was made. One is slightly longer and will not plug in. These part numbers from Autozone come with an adapter to fit both cable connector styles. MAKE SURE THERE IS AN ADAPTER IN THE BOX WHEN YOU BUY IT. You may need it.

If yours is the opposite plug type and you do not want to use the adapter, you can remove the pins from the old plastic plug and insert the new cable pins into it. The pins are the same only the plastic housing is different.

SHORE POWER
The GMC Shore Power Cable was built by the factory with #6 AWG wire, which makes it very stiff due to the type of insulation used & the number of strands within the copper wire. #6 AWG wire is rated at 50 amps for a closed environment...even higher for outside application w/o housing around it. The Shore Power Source Circuit Breaker (CB) is what counts in terms of safety....50 amp circuit breakers at the park's breaker box is just fine for the #6 wire. Any smaller cable wire will require a smaller shore power circuit breaker rating, such as 30 amps for a #10 AWG cable. The OEM (most coaches) shore power source is normally a 4 wire service with two hot wires at 240 vac (lead to lead), a center tap neutral wire (return wire for either leg of the 240 v ac...120 vac use). Since our coach does not use 240 v ac for any application, we theoretically have 50 amps for each Hot leg (100 amps of 120 v ac power). So if the source is rated for 240 v ac at 50 amps, we do have 100 amps at 120 v ac available. The coach main CB box provides a main switch/CB of 40 amps, which limits
the coach to two 40 amp source (80 amps total). If a over load occurs within the coach, the 40 amp main CB will trip before the shore power CB trips.

An inductor is a coil of wire that will restrict/impede the flow of AC current & will heat up the same as an equivalent resistor value (as in an electric heater). Operating the coach at maximum load (80 amps) can be a bit sporty with the shore power cable coiled up nicely in its electrical box (creates an inductor). It has been reported at an FMCA rally, technical seminar, that fires have occurred due to the coiling of the shore power cable into a small electrical storage area. When I plug into shore power, I always pull all the wire out of the storage area & lay it on the ground in a straight line to prevent the coil effect. This will also minimize the voltage drop (maximize the ac voltage available) of the shore power cable while running under loads.
Type "SO" wire/cable is a much better choice than the OEM for the shore power cable, since it has a neoprene rubber jacket & has very fine wire strands. Type "SO" is rated at a much broader range of temperature operation w/o becoming stiff & brittle.Duane Simmons

        1) W/O any AC power at the coach, remove the cover from the AC Circuit Breaker Box & attach an ohm meter to the Neutral Buss (white wire)
& the Chassis Ground Buss (Green or Bare wire).
        2) These two electrical points should be isolated...should have high resistance.
        3) If not, then the two points are connected together somewhere in the coach & must be isolated from each other.
        4) W/Ohm meter attached, start removing one small white wire (not the incoming power cable wire) until the two points become isolated from each other (Low resistance). The last small wire removed is the circuit that contains the connection between the Neutral Buss & The Chassis Ground. This connection must be removed to enable a GFI Shore Power source to function w/o tripping.
        5) Now reattach the small white wires that were removed except for the one that was removed to isolate the two points. With all small white wires attached, except the last one, there should be high resistance between these two points.
        6) Trace the last wire (disconnected) circuit to determine where the Chassis & Neutral wire are connected.

HEADLIGHT SWITCH(4/4/06)
According to my files the correct headlight switch is #HL6554 or Delco D1558
or Standard BlueStreak# DS155.

The Borg-Warner S427 and Napa# HL6613 are correct for the 74-76 Toronados. DENNY

http://www.gmcmhphotos.com/photos/showphoto.php?photo=16284&ppuser=9

You can ad WELLS SW144 to the list of headlight switches too. I just
got one at Auto Zone for $10.98. They were able to cross reference the
Borg-Warner number. It looks just like the one Arch got at Napa. It is
even missing the same blade. richard

NON-WORKING DASH LIGHTS might be related to the installation of a new radio.

For anyone putting in a new radio, bear in mind that the GMC original radios used a separate wire for the panel light in the radio so that it could be dimmed when one wanted to dim the dash lights. Not all newer radios use this feature.

There is a grey wire that connected to the original radio for the panel light. If one were to test this wire with the dash lights off, it would appear to be a ground wire as it would ground through several instrument panel light filaments to ground. If this wire is used to connect to the radio ground wire it turns out that the radio will work just fine as the resistance of several bulb filaments in parallel will not put much resistance in the circuit.

The problem is that when the lights are turned on, the current through the dash light fuse is immediately shorted to ground through the radio and will burn out and the dash lights will not work. If anyone is planning to install a new radio you may want to keep this in mind.

The little tags over the Fuel Tank selector, Battery Boost Switch, Light Switch, etc, were originaly lighted with a fiber optic assembly. This never worked on my GMC. I did not want to remove the dash board to replace this assembly with lights, so I did the follong procedure using the LED solution

This upgrade can be done from the front of the dash board and the cost is about $ 5. gene

Think about the water analogy for a moment. If you have a water tank and its always empty the next morning, THAT WATER HAD TO HAVE GONE SOMEWHERE! If you look at each tube that goes somewhere from the water tank, and see if water is flowing thru it, you WILL find where that
water is going!

Batteries are pretty much like that. Either the electricity is leaking out thru a wire somewhere, or else the battery is old and sick and is leaking internally, in which case its a goner. Get a new battery.

The way you measure electricity flow is the ammeter. It is one of the functions available in those little handheld "Multimeters" you can get at Radio shack for $25.

Here's what you do: Disconnect the wire in question and connect the ammeter across the gap you have just created. (Connect the ammeter's red wire to the side that is closest to the battery.) Caution: Make sure that everything is turned off when you do the test. If there is, say, 15 Amps flowing somewhere, you could blow out the meter.If it reads zero, there's no leak here. If it reads something really small, like 10 milliamps (0.01 amp), that's not the problem either.

So to start, disconnect the big red wire from the battery and connect the ammeter between the battery and the big red wire. Any current flowing? If so, you are on the trail of your leak. Reconnect the big red wire and go to its other end and check each of the wires that go off somewhere from it. Another place to do this is at a fuse panel. One by one, remove a fuse and connect the ammeter in its place. If current is flowing, you are getting closer to your leak. Keep stalking the leak.

If no current was flowing, its probably the battery. Leave it disconnected over night. If its' dead in the morning, get a new battery. If its not dead, then the leaker only comes on occasionally. Keep
looking for it.

When I bought my GMC, all 3 batteries would run down over night. The RV shop guys couldn't figure it out. I checked everything with the ammeter and found that:
     1. One of the 2 house batteries was very sick.
     2. It was draining the other one down. (They were connected in
parallel.)
     3. The dashboard rocker switch, which had lost its labels, was in
the position that connected the house batteries to the engine battery,
thereby running it down too.
Got a new battery and the problem was solved.

BTW, did you ever wonder why the positive direction of flow of electricity is considered to be from the positive terminal to the negative terminal, when, in fact, the electrons really flow from the negative terminal to the positive terminal? It was Benjamin Franklin who decided that and it has stuck ever since.
The same Benjamin Franklin who wanted to name the turkey our national bird.

I finally got to work on the GMC tonight, in the dark. I put in a fresh fully charged 12V engine battery with only the POS cable connected. Then I rigged a test light using a small dash light bulb and test leads between the NEG Cable and the NEG post on the battery. The light showed through the windshield. I pulled every fuse in the glove box fuse block until the light went out on the last fuse. It was the Dash Radio/8Track Player circuit. This radio does not light up and seemed to be dead. I replaced the fuse and was able to turn the test light on or off with the radio on/off switch. Tomorrow, in the daylight, I'll reconnect the engine battery and monitor it for a while. Amazing! Jack

Windshield Wipers rebuild
Bill Telgen, Dixielanders member, rebuilds windshield wiper motors. If I
recall correctly from Bean Station, he gets $150 exchange. Older
Thunderbirds & some Lincolns used the same basic motors. He can convert
those for GMC. He may even be in the market for cores.

Speaking of which, the GMCMI roster and the petrified Parts Interchange
Index (with my numerous notes) is worth the cost of my membership -- .Ken Henderson

Battery Tray - Front mount--non-sliding
A new battery tray is available from Angelo Ragusa, Santa Ana, CA
949-261-5898. The Tray fits the OEM battery bracket & allows
installation of one Engine Battery & two Golf Cart batteries (6 volt ,
T105 size) for the house battery. Made of aluminum & cost $68. This
arrangement is very popular with the 23 ft Birchaven (all batteries up
front).Duane Simmons

Steverson Sliding Battery Tray
The sliders have plastic wheels
Made of steel so do corrode
Lifetime is about 5 years

The tray fits in the compartment beautifully and I did take out the aluminum cover as it does help to have that there anyway. The tray holds the two golf cart batteries and there is still room on top. The tray is metal and has sides and bolts on the end to fasten down the batteries. The tray is on roller bearings and they are strong enough to let the entire tray come out and still be able to see the back water fill opening. I tried the ragusa but, as you said, it doesn't come out all the way. The Stevensen comes from trailer trays they sell. It comes out all the way, and then when it is in it can be locked in place. The lock is a slide pin like you see many other applications.

The slide tray width is 9 3/4 inches wide.
Looks like they cut off a little of the flange arround the GMC opening for the slider. This allows the slider to get closer to the rear of the coach.

The height of the tray is
3 1/4 inches
The batteries sit down into the tray almost to the bottom or about 1/2 inch up from the floor. There is a lot of top room for the batteries shown. The whole slide tray sits on a 1/2 inch of plywood in the bottom of the opening. There are 3/4 inch boards arround the sides of the batteries at the bottom for spacers.

From what you say that the coach had be sitting 3or 4 years and these are the same batteries that have also set the same time length it would be better to replace them.

For the chassis battery (engine starting) get the biggest, highest CCA (cold charging amps) you can find. Walmart sells a 1000 CCA battery for about $60. This size battery will provide a reserve starting power than the lessor rated sizes.

As for coach batteries it depends on the layout of your electrical system. If you have the single big battery in the rear with a small one for the onan most GMC'ers have change to 2 6volt golf cart batteries in series and eliminated the starting battery for the onan, instead using the coach batteries for starting. You should get 6 volt golf cart batteries of at least 220 to 225 aH size. Interstate and Trojan are the most popular brands, but I am sure that you may find batteries of the same rating and Costco or other discount suppliers. The choice is yours.J.R. Wright
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Not a Myth! The book "Managing 12 Volts, How to upgrade, operate, and troubleshoot 12 volt electrical system has a very good explanation on pages 50 thru 52 why connecting batteries in parallel has many problems.

I will paraphrase the section, "An easy way to increase capacity is to parallel a new 12 volt battery with an existing battery or two new 12 volt batteries to increase capacity. The main problem with paralleling batteries is if the batteries are not exactly equal in construction or
degree of sulfation on the plates the battery internal resistance's are different. The battery with a lower internal resistance supplies a greater amount of current to a load than the battery with a higher resistance. Also if one battery has a higher voltage that the other a small circulation current develops between the two parallel batteries. The higher voltage battery is recharging the battery with a lower state of charge. The 2 batteries react just like two water tanks, one being
full and the other being half empty. water seeks it own level so the full tank flows into the half empty tank until each has an equal amount. The same with the batteries, you now have two partially charged batteries. If you put a new battery with a dying battery you will kill
both of the batteries. It is recommended that when a high capacity battery bank is needed 2 6 volt high capacity batteries in series work better and have a much longer life than a parallel battery layout. J.R. Wright
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Golf car batteries are an excellent choice as they are designed to be drawn down rather far then recharged. Starting batteries are designed for lower internal resistance so they can provide high power density for short periods of time. On my coach I put an auxiliary battery holder on the driver's side and have the starting battery there and have plenty of room for a pair of golf cart batteries on the other side. Jerry
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For house batteries use two six volt golf cart batteries in series. If you use two twelve volts in parallel they will try to charge each other and result in fewer available coulombs than if you go the six volt route.

I run 2 golf car batteries (Trojan 104s) in the rear battery compartment of our Palm Beach. The larger capacity batteries are too tall to fit into the compartment without some modifications. I've gone to the TrueCharge 40+ battery charger available from West Marine under their brand, or from Statpower (the manufacturer) in British Columbia.

Using this setup I can go 2 days during the summer before I need to recharge. During the 2 days, our load is primarily determined by the fridge. We still have the original fridge, so I don't have the option of propane. We used 2 lights (originals) for 2-3 hours per day, ran the radio a bit (1-2 hours per day) and still had enough juice left in the batteries to start the genset. Once I had to start the coach first in order to start the genset - after we'de run 3 days without any recharge.

2. when boondocking use the "middle 50%" of the battery bank capacity. This means discharging until about 30% capacity is left and then recharging to 80% using the genset. You can recharge the 50% in about 2 hours depending on the charger. Shore powered recharge to 100%.

3. look at your DC systems that include dimmers or speed controls. If they use load resistors for controlling speed consider replacing with an electronic PWM controller, it's more energy efficient but more expensive.

4. switch to a charger like the TrueCharge that can recharge to an 80% level in a few hours ( I try to use only about the middle 50% of the battery bank capacity since it recharges quickest)
You can also get a good feel for your own power consumption by installing an ammeter on the house battery and watching your load. Alternatively, run a few specific tests like I did to determine how long your battery banks last under what you think will be your typical usage. Lastly, make certain that your house and coach batteries are isolated - that way when you make a mistake, you can still get everything started. Or, if you're really paranoid like me, carry a portapak booster battery just to make sure. Henry

I have a sad tale to tell. The coach caught fire when the cable from the house bat that runs down the driverside of the rig to the isolator rubbed through the insulation------shorted out and caught fire. The fire burned up the cockpit drivers seat side wall and end cap in cockpit. There is a service bulletin on this problem. You are supposed to put the cable in the plastic conduit. The cable where it crosses over the frame in the front was dry and hard. When you pulled on it cracked and poped. Then I went inside. Just behind the drivers seat where the cable begins to travel down the sidewall there it was. The conduit and wire had worn through. There was green crud where the copper had been exposed. No signs of arcing or anything just bare wire. Then I found a place where mice had eaten through everything. I replaced it with 2/0 and conduit. After it leaves the driverside of the engine compartment it goes down that side of the cockpit wall T BTW it takes 25 feet of wire to replace the entire cable. NAPA has it in boxes with 25 feet in it. MEASURE IT before you leave the store. It is not a fun job running the cable and conduit down the wall I started up front threaded it through the wall and I was about 18 inches short.

The number of the bulletin is 77-TM-4. It is listed under 12-chassis- Electrical-2. It was issued June 1977..Arch

Reading this forum made me look for the pinch point behind the drivers wall panel and fuel filler. Found the big wire from the isolator in front and the batteries in the back was pinched. The protective cover had the split towards the sharp edge and the wire had slipped out. Image at

It appears that the house battery cable on some coaches from 1974 to 1977 serial 1456 had an abrasion problem. This is also noted in GMC Motorhomes International Newsletter Issue 24 where the "fix" proceedure is described. My coach falls in thes era --- going to check it out PDQ. Gary

There is a wire in the rear, drivers side of the coach that gets pinched between the body and the frame. Here is a picture of the wire by Hardie
http://www.gmcmhphotos.com/photos/showphoto.php?photo=5031
Here is a description of the problem

I went out to do a pre trip on my coach about an hour ago and have no tail / parking / clearance / dome lights. The 20 amp fuse to that circuit was blown. I've gone through 3 additional fuses but as soon as I pull the headlight switch out to its first stop (parking/clearance) the fuse pops. The headlights are fine but I also do not have any dash instrument lights and don't seem to have any power to the short little 4A fuse marked instrument lites yet the fuse appears to be ok. Dick

Battery Isolator Testing
The Engine Alternator charges the house battery thur the 2 Battery Isolators while driving. A simple test to see if your battery is being charged is to attach a DC Volt meter across the battery in question & read the voltage. Now plug in to shore power & the Battery Voltage should increase more than 3/4 volts. Remove shore power & start up Engine, the battery voltage again should increase more than 3/4 volts. Stop Engine & plug in for Onan Power (operate Onan), the battery voltage again should be increase more than 3/4 volts.

Note: The Engine is isolated from the House battery & should not be charged from the Shore Power Or Onan Power. Also if you want to see if your Onan battery is functional, remove Onan Power to Coach & see if you have 3/4 volts increase with Onan operating. Duane

If needed, buy a low power pure-sinewave inverter for the noise sensitive stereo. Laptop (LT) computers charge an internal battery and do not require a clean power. I have used my Desk top (DT) computer on a simulated sinewave inverter with no problems. However if you are concerned about the power to your computer it is a relatively small load that could run on your stereo inverter. .


DEVICE	WATTS	SIM SINE	SINE

BLENDER	300	X
CAN OPENER	100	X
COFFEY MAKER	1000	X
DRILL 3/8 "	500	X
FRYING PAN	1000	X
FOOD MIXER	300	X
HAIR DRYER	1000	X
IRON	1000	X
MICROWAVE	1000	X
READING LAMP	100	X
TELEVISION	700	X
TOASTER	1500	X
VACUUM CLEAN	1000	X
VCR	50	X
STEREO	500		X
COMPUTER (LT)	100	X
COMPUTER (DT)	300		X
SOLDERING IRON	1000	X
IMPACT WRENCH	1000	X

From this list you can see you do not need to buy a $1500 sinewave inverter to run your $100 microwave. Most of the AC loads in the Motor Home do not require pure sinewave power. The simulated sinewave inverters are more than adequate for most of the loads and if they create RFI (radio frequency interference) noise turn the invertors off when not in use. The 1000 watt inverters are typically on sale for $300 to $500.

When I first bought my GMC I thought to my self, "that DIGI-PANEL is the first thing I am going to remove". Now it is the last item I would remove. Mine is mounted under the top lip of the dashboard over the instrument cluster on the right side. This set of digital indicators monitors:

I love my OEM analog gauges but it is very comforting to compare the analog reading with digital resolution. There are different colors for WARNING, and DANGER levels. The audio alarm will sound when the DANGER levels are reached. This warning broke me out of my revery to warn me that my alternator had failed and my battery was down to the critical level. It also became clear that I had very low oil pressure because when I was stopped with the engine running the alarm sound.

The photo shows the four pointers made from white tape that I added to my DIGI-PANEL. I stole this idea from Claude Brousson. These pointers allow me to take a quick glance at the indicators and immediately see that I am running at the nominal levels. The Audio alarm will tell me I have a problem when I am not looking.
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For those of you odering the digi-panel, suggest you buy an additional sensor for the transmission. I did for bout $8 and I added a small switch which I use to change from one cooling line to the other. That way I can measure the cool, and hot lines from the trasmission by flicking a switch. I used one of the miniature switches and used the blank hole next to the on off switch to make the connection. Works great.al

REPAIR
Carefully remove the shrink fit tubing and check / resolder the connections to the sensor. I have had to repair two of these connections. gene

This picture shows the coolest mounting I have seen for the DIGI-PANEL. Jim Bounds at the WWW.GMC COOP.COM has mounted the digital indicator over the new radio. Go visit him, I am sure he would be glad to do it for you also.
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3. Write
Art Woodall
Degi-Devices
23815 Avenue 26, Chowchilla, CA 93610
or P.O. Box 178, Madera, CA 93639.

MODEL J FOR GMC MOTORHOMES -they are made specifically for GMC's - plus the NEW install instructions will be included --EACH one of you will need to make your own arrangements for payments -- Art takes Credit Cards or you can mail the check to him Pete

The Model J has been modified to use the same Oil Pressure sender that the dash Gauge is using. This makes it easy to connect the DigiPanel. The Dash gauge supplies the voltage to for the DigiPanel to read the oil pressure. The dash gauge must be connected for the DigiPanel to read the oil pressure.

The original DigiPanel supplied a constant current source for the oil pressure sender and must use a separate sender. The sender to use is :
NAPA OP6636.
These are also available from General Automotive and DigiPanel. This is the sender to use for the Dash gauge in the GMC. gene
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Talking to Art I found that he also has another option. He has the panel set
up for an oil temperature sensor as well. He uses a switch to read either
transmission temp or oil temp. It comes with an extra probe that can be used
for either oil temp or for a second location on the transmission. It is
$27.50 more.Emery Stora

This is Mr.C under the coach installing the pressures sender. We elected to put the Pressure sender at the fuel pumps which are located just after the tank selector valve. The sender is the same one (part number shown above) used for the Oil pressure on the GMC coach and was purchased from Digi-Panel for $13.00 .

The Sender supplies a variable resistance of about one ohm per pound. The Howell system operates at a fuel pressure between 16 and 24 pounds. This will give about a one half scale reading on the Digi-Panel. This measurement does not require extreme accuracy since the application is show if there is a problem from the normal operating conditions. It is possible to read the resistance of the sender with a digital meter using the one ohm per pound calibration.

The Digi-Panel has a hole already punched in the front panel that can be used to insert a switch. This switch is wired to select the alternate senders for the Oil/Gas Pressure, and the In/Out transmission oil temperatures. I will supply a wiring diagram soon.

The new switch is added to the right of the Alarm switch on the Digi-Panel . I have labeled the switch as "OIL" in the up position. When the switch is in the down position, the Fuel Pressure is measured. My Howell TBI is currently running at 20 ohms and 20 pounds on the Digi-Panel.gene

CHASSIS ELECTRICAL