GMC Western States

 Tech Center Number 37 - June 30, 2002

Emery Stora

From our Technical Vice President


This rally was very enjoyable for both Michelle and I. We got to see many of our friends and made some new ones. The Cherrys and Pettits put on a really great rally. All the volunteers who contributed to the technical seminars did a super job.

On day one we started out with Jim Bounds of GMC Coop of Orlando whose topic was "GMC Renovation Projects." Jim presented a lot of information on interior and exterior work that can be done to update our GMCs. He also discussed recent developments on the LSR (Land Speed Record) project, which is to be an attempt to make a run at the Bonneville Utah salt flats with a GMC motorhome. Jim suggested that anyone who wants more information on the LSR attempt should contact him.

Next was Chuck Garton's presentation on "Neat Modifications for the GMC." Chuck has some great ideas for modifications and additions to his GMC. Among the many that he presented was the use of an automobile electric side window motor to power the cables for the GMC parking brake which eliminates the inside pull lever by the floor. We had only given Chuck 1-1/2 hours for his presentation, but I think that he could have taken a day or two. I have never seen so many switches and meters in a GMC motor home before.

On day two, Frank Condos presented a very complete seminar on overhauling and properly setting up the Quadrajet carburetor including information on rejetting it to match the power needs and the running altitude at which it would be used. Frank had many, many questions from the audience.

Al Chernoff then had a session demonstrating "Wireless Toys and Other Fun Things" where he discussed numerous items one can use when traveling with our GMCs. This included GPS units (ground positioning satellite), wireless telephones, Palm Pilots, computers, digital cameras, music storage and replay devices, wireless broadcast transmitters, DVD players for the computer, satellite TV and satellite Internet access and a whole host of other things. Al is a very talented individual and is a great source of information on these things. If anyone has any questions on equipment or how to use it you should contact him.

The next day we had Duane Simmons with his update of "What Every GMCer Should Know" which is a presentation of a raft of information for both new owners and seasoned veterans. There were so many questions from the floor that we had to give Duane some extra time the next day. He published a new booklet, which we will have Billy Massey (our Webmaster) get updated on our GMC Western States Internet site soon ( If you don't have computer access to the web and want a copy of this booklet, let me know.

On our fourth day of presentations we had a very thorough review of Safety Considerations by Ed Burner. Ed also made a write-up of his presentation available. He covered many safety areas including brakes, tires, operations, and fire prevention and protection. Ed's write-up is worthwhile reading for any GMC owner.

Jim Kanomata of Applied Air Filters, Fremont, CA, the makers of the 3.55:1 and 3:70:1 ratio final drives, gave a presentation on Knuckles/Front Bearings/Tools. Jim then gave a live demonstration of removing a knuckle from a GMC, disassembling it and replacing the bearing. He found an out of round knuckle and a cracked bearing and replaced the damaged parts and even managed to get it back together and working! Jim did a great job and we also loved his jokes while he was doing the demonstration.

Autopsy of a Transmission by Bebe Pettit

Each afternoon of the rally Bebe Pettit had an ongoing tech session "Autopsy of a Transmission" under the outside dome. Bebe disassembled a GMC 425 transmission showing the various parts, discussing the problems he found as he took it apart and explaining the reason for its failure. This was a great chance for rally attendees to see the inside of a transmission and how the various clutch packs, bands, gears, chain, valve body and other parts actually go together within the case. This was very well received by all present, and Bebe fielded a lot of good questions from his audience. We want to thank him for all the time that he spent on this project as it certainly added to the content of our rally.

Our tech sessions ended on the afternoon of the last day of sessions with an "Ask the Experts" panel. I want to thank Frank Condos, Chuck Garton and Alex Sirum for their participation on this panel. They fielded a lot of questions; however, no one seemed to stump the panel. Our members appreciate the chance to ask about a problem or get good advice on a variety of things. As usual, our panel displayed a wide range of knowledge on the GMC motorhome and its parts and operation. We will likely have this again at the next rally so start preparing your questions now to take advantage of the knowledge and experience of those members who help out on this panel.

I am working on the tech sessions for our September 2002 rally at Cody, Wyoming. If anyone has suggestions for a topic to be covered please let me know. We want to meet the needs of our members so we need your input to do this properly.



You have two separate battery systems in your GMC. First, you have a battery that is just for starting and running the engine, running lights, gauges, panel lights, ignition coil, etc. Second, you have what we often refer to as the house battery. This is either a single 12-volt battery or two 6-volt golf cart batteries in series, or sometimes (but not recommended) two 12-volt batteries in parallel.

The engine battery is designed for heavy loads for short times, and it is charged back up quickly by the engine alternator. The house battery is designed for lighter loads for longer times and is often called a "deep discharge" battery. It is meant to be taken down low and then recharged, whereas a battery designed for starting an engine will fail after relatively few deep discharges.

The engine alternator will charge both sets of batteries. It does this by use of an "electronic switch" called a "battery isolator" (some GMCers now use a "combiner" which does the same function but operates a bit differently). The battery isolator has three terminals (some have four and charge three sets of batteries), but the GMC normally has only two sets of batteries to isolate. The alternator wire goes to the center lead and then the charging current of the alternator is split by the isolator so that some current goes to each set of batteries. The isolator contains diodes (think of them as electric check valves) which allow the current to flow from the alternator to each set of batteries but the current cannot flow back through the diodes so that a load on one battery bank cannot draw current from the other battery bank. The diodes used in the isolator do have a small voltage drop of up to perhaps 1/2 volt. The use of a combiner eliminates this small voltage drop and that allows the alternator to charge the battery to a bit higher voltage.

In the event that you have a dead battery (either the engine starting battery or the house battery), you can "boost" the dead battery from the other set by pushing the "boost" switch on the dash. This supplies power to a coil in the boost solenoid that jumpers the two heavy wires going to the solenoid. One wire leads to the engine battery and one leads to the house battery.

The house battery is charged when you are plugged in or are running the Onan by the battery charger. The original GMC charger, affectionately (?) called the "buzz box" because of the noise it makes, is connected to the house battery by the wire going from the charger to the 12-volt "house" fuse panel. It will not charge the engine battery because of the action of the isolator so it only recharges the house battery bank.

Now, here's an exception. If the buzz box is plugged in and you were to hold down the boost switch, then the buzz box would also charge the engine battery. Some of us carry a small jumper wire that can be used the same as holding the boost switch down. The boost solenoid, which is located behind the front hood panel on the passenger side, has four terminals: two large ones and two small ones. One of each of the large ones goes to cables that lead to the two battery banks. One of the small terminals has a short wire that goes to ground. The other small terminal has a wire that leads from the boost switch. If you take a short wire with alligator clips at each end and clip one end to the terminal that the boost switch goes to and clip the other end to a "hot" lead, such as the positive battery post or to the large cable which goes to the side of the solenoid from the house battery, you can actuate the solenoid just as if you had pushed the boost switch. If your house battery were to be completely dead you might have to connect the alligator to the side going to the engine battery in order to get the solenoid to work. You can leave this jumper in place to charge the engine battery from the buzz box. Just don't leave it there all the time as you don't normally want these batteries connected together. Note: most factory built GMCs had a momentary contact boost switch whereas some of the converted Transmodes such as the Coachman Royales may have used an on-off switch instead of the momentary boost switch. If you have this type of switch, you should take care not to leave it in the boost position.

In case you should have a failed alternator while on a trip, or a bad engine battery, you can use this jumper to actuate the solenoid and then start your Onan and allow the house battery to provide power to your engine and lights. You would leave this jumper in place until you get home or to someplace where you can get your alternator repaired. Instead of using a jumper, once you have the engine running you could also disconnect the two outside wires from the isolator and connect them together. This would keep your engine running and your engine battery charged from the Onan and the buzz box. Just don't shut off the engine and try to start it again with only this wire connected as your starter could draw too much current through this small wire. Use the boost switch to start.

If you purchased a "smart battery charger" such as the StatPower 40+, there are separate leads to charge sets of batteries. The 40+ can charge up to three sets of batteries. The charger has internal circuits so that the batteries stay isolated. It would be necessary to run an additional heavy wire (6 gauge will work) from the charger to the engine battery. This would allow the charger to keep both sets of batteries charged up when plugged into 120 volts or when running the Onan, but the different sets of batteries will not interfere with each other and will not discharge each other. If you have one of these chargers with a lead from it to the engine battery, then the Onan could keep your engine battery charged even when running with a bad alternator as long as you run the Onan. The nice thing about using a "smart" charger is that they automatically taper down the charge so that it will not boil the water out of the batteries even when the charger is left on in storage. There are also accessories available such as a "status" panel and a temperature probe that will automatically adjust the charge rate depending on the temperature of the battery being charged. This prevents overheating of the battery, which could result in warped plates and/or gassing.

Many of the original house batteries were a single 12-volt truck battery (model 4D). This was a large battery but was basically designed to be an engine battery for a truck. The problem with it was that if it were discharged down low and then recharged, it often lost some of its capacity. More modern batteries are available for use as house batteries. They are usually called "deep discharge" batteries. Deep discharge batteries are available but golf cart batteries are also deep discharge batteries and are often much cheaper to purchase. Many have replaced the original single large battery with two 6-volt golf cart batteries wired in series. The "house" hot wire is hooked to the positive post of the first 6-volt battery and that battery's negative post is connected to the positive post of the second battery. The negative post of the second battery is attached to the "house" ground. This arrangement gives you a 12-volt supply. Others have wired two 12-volt batteries in parallel but this can lead to problems with one battery discharging the other if there should be a "bad" or weak cell. This will not occur with 6-volt batteries placed in series.

This is just the basics about batteries. If you need more information, there are many books on the subject pertaining to battery use in a RV. There are also several good sites on the Internet. Let me know if you need some sources.

Egon H. Elssner

Using a Laser Pointer for Checking
Wheel Alignment

Ever wonder if all your wheels are going in the same direction?

Here is an easily made, easily used "gadget" that may be helpful. It consists of an ordinary laser pointer, such as those used by lecturers to point to visual aids, held in a plastic ring in a way that lines up the laser with the plane of the wheel. This gadget is placed directly on the exposed part of the wheel hub, the raised, accurately machined part that centers and supports the wheel. If the laser is accurately lined up in the plastic ring, then the laser will point in the direction of wheel travel.

The ring is made from a section of 4" ABS plastic sewer pipe (4.5" OD and 4.0" ID) that fits onto this ridge beautifully. A small shirt-pocket sized laser pointer is fitted to this ring approximately equidistant from the two flat surfaces.

The "Gadget" - plastic ring with laser pointer

Cut a piece of sewer pipe a bit over 3" long. Square the ends and make the two sides as parallel as possible. Finding a "machine cut" end makes life easier, or you can file and use a piece of sandpaper on a flat plate to square up one end. To make the other end parallel, I use a drill press fitted with a 1/2" end mill. Lock the spindle and using very, very light cuts, gradually cut the rough end parallel. If you like, turn it over and take a light cut off the first section to really true it up. Caution: the cuts must be very light and the pipe must be held very tightly to avoid loss of control, a damaged pipe and possible bodily harm. This pipe is then center marked and drilled to hold the laser pointer. I used a brass body Radio Shack "Slimline Laser Pointer" part # 63 1046. Unfortunately, the lasing element in this pointer is not accurately aligned in the housing, so it is necessary to fit the back end of the pointer with an adjustable "sliding sideways" support with screw slots (a piece of 3" ABS pipe coupling, approx. 4" OD, works fine) to allow beam alignment. Alignment is simple: place the device on a stable table and point the beam onto a wall or target then mark. Turn it upside down and repeat. When properly adjusted, the beam will hit the mark with either side up. Since you will want to carry the device from one side of the coach to the other without losing orientation, it is helpful to fit a sheet metal screw to what would be called the bottom of the device. Since the pointer is a Class IIIa laser, it can do serious irreversible eye damage if the beam impinges on someone's retina. You or your assistant must take care when the laser is on!

Gadget on wheel hub

To use simply hold the gadget on the hub ridge (clean off grease and dirt), aim and activate the laser. Once you do this, you will instantly see the need for a target setup or setups. There are many possibilities. I use one arrangement that is made of two pieces of cardboard joined by a 2x4 and clamps (one side has a vertical line as a mark and the other side carries a sliding cardboard mark). An assistant moves the target assembly and makes measurements. Measurements need to be at center wheel height to eliminate error due to camber. I have also used a garage door about 100 feet away on an overcast day for a quick check of rear wheel alignment. This is useful if you suspect that something may have been bent.

As a start, with the gadget on one of the rear wheel hubs, mark the position of the laser beam when directed toward a piece of tape on the floor just behind the rear wheel. Do this on both sides, and with a tape measure you will have a number for your target setup. Note that this number is the hub to hub spacing plus the thickness of the gadget. Repeat for the front wheels. With a piece of wood held against the frame close to a wheel, you can also get a rear wheel hub to frame measurement to set up a target arrangement to be held against the frame just behind the front wheel or carried on the front bumper that will give an indication of whether the rear wheels are running true with the frame. For some innovative target designs and using some trigonometry, it would also be possible to determine camber.

Toe in or toe out for a single wheel can be figured by simple proportion. 1/16" (0.0625") toe in or out on a 30" (2.5 feet) dia. tire corresponds to 5/8"([0.625") displacement at 300" (25 feet). For alignment specs refer to the GM X7525 or X7725 Maintenance Manuals.

Always drive and leave the coach in a wheels straight ahead situation before making measurements. Measurements may not be exactly reproducible due to pin wear, control arm flexing and stresses introduced by steering and braking maneuvers prior to measurement. If the measurements seem unreasonable, it is probably best to consult a knowledgeable shop that does GMC wheel alignment correctly.

 This material is based on my personal experience and the personal experience of other club members. It is our viewpoint and does not represent authorized data pertaining to the GMC Motorhome. It is the responsibility of the readers to make their own judgment as to the validity of this material in relation to any repairs and/or modifications to their own vehicles.

 3128 Vista Sandia, Santa Fe, NM  87501-8526

 (505) 989-8157

 Please send your comments and ideas for the Tech Center to:
Emery Stora, Technical Vice President,
3128 Vista Sandia, Santa Fe, NM  87501-8526
Phone: (505) 989-8157



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