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 I have read many a articles of GMC modifications and wondered when I would be able to describe my fun with such a project. I (we) bought our 1978 Palm Beach GMC in the summer of 1989 and proceeded to start upgrading shortly thereafter. Since I was still working full time the progress was slow, besides, we wanted to use our new pride and joy at the same time. For the sake of interest and clarity I will describe our effort in three stages. First, a general description of upgrading our GMC. Second, a more detail description of changes and additions made and the reasons for them and finally, I will list the price that I paid for parts that I bought. I will also describe some original designs that resulted from problems encountered in completing our GMC. When added up, the total outlay was over $ 40,000 (including the purchase of the coach) - certainly a lot more than I thought I would spend.

Upgrading our GMC


In retrospect, I would have saved money and time if I had information such as the Intíl GMC Newsletter, your magazine and the letters that Cinnebar issues. I started somewhat fearful, since my first task was the replacement of most undercarriage components. I chose to do most of the replacement because we intend to keep our GMC for a long time, thus knowing that we have a relative new unit would help our peace of mind. Replacement included both knuckles (one was loose and the other was the wrong type), upper & lower ball joints, all brake parts including brake booster, cylinder and lines, bell crank, transmission and motor oil lines, CV boots, radiator and tires. The reason that I replaced the radiator, which appeared to be new, was the fact that on our first trip to Florida, the engine was running hot. After consulting people that we met at our first GMC rally attendance, we learned that the radiator had the wrong number of fins, thus was the wrong radiator.

One of the first mistakes that I made was to assume that the shop that assembled the bearings into the knuckle knew what they were doing. This shop is known in the area as a GMC repair shop!? makes you wonder about units that are repaired here on a regular basis? The reason I learned this, was the fact that three years later both knuckles and bearings had to be replaced due to a catastrophic failure on one side. Luckily, I have since attended some of the GMC rallies, subscribe to information and best of all got to know people in this business who really know what they are doing and/or talking about. In addition to replacement of undercarriage parts I installed an inline transmission filter, replaced the carburetor with a new one, replaced my noisy 3.07 final drive with a 3.21 drive and replaced the exhaust system with headers and a 3" exhaust line. This change and the 3.21 drive improved the power of my 403 engine considerably, therefore this is a modification that I would highly recommend. Of course, even this drive was replaced for more power, a 3.62 final drive.

An additional fuel relief line was added by separating the second tank and mounting the opening in the fueling cavity. This improved the filling time, but did not rid the problem entirely. I also replaced my crumbling steering wheel and the water temperature sender with a NAPA unit as recommended. Finally, I bought a complete set of tire monitors for the GMC and my tow car, mostly for peace of mind. I will write a separate report on my experience with these senders sometime next year. After replacing all of the brake components I now have a unit that responds very nicely, even in emergency situations with a vehicle in tow.

On the body, additions of side vents, door steps, new stripes a 14 ft. awning and 3 window awnings. The bumpers were powder coat painted and finally an Imron paint job completed the unit  in the summer of 1996. The most difficult part was the body work to close the side air ducts that were  required for the original 2-way refrigerator after we replaced it with a  new 3-way refrigerator.


After taking early retirement, I decided to totally replace the inside of our unit. By this time we really disliked the green colored inside (and outside) so we gutted the whole unit. It was here that I realized the heavy weight of the panels and cabinets. The pressed wood covered with Formica did look nice but added a lot of weight to the GMC. The replacement material that I used was light Swedish oak that was 70% lighter which eliminated at least 200 lbs. This also was my greatest challenge since I decided to build my own furniture including all cabinets, tables, walls and bed.


I also added a roll-out cabinet to increase food storage and lengthened the counter top by 1 foot. This reduced the closet but still leaves us with plenty of storage for clothes since I also now have a large drawer under the permanent bed. I used Corian like material to give the kitchen a classy look.

We also shifted the new 3-way refrigerator towards the door which made room for a roll-out cabinet and storage behind the refrigerator.


To my surprise and delight all turned out quite beautiful, after a few tries. Before any reassembly was possible the coach was completely rewired for 12 volt, 115AC lines, cable and TV connections. The big box in the closet was removed and both fuse panels relocated to the bottom of the closet. The converter was relocated under the back of the sink and the heavy 50 amp wire was relocated into the generator cabin next to the battery. The house battery was replaced by two golf cart batteries. All walls and ceiling panels were replaced and additional insulation was added. Since we reworked the inside with the comfort for two people, several changes of the original layout were made. The back dinette was changed into a permanent bed, the front couch/double row beds were replaced by a couch that opens into a double bed.



The front dinette was changed into two captain chairs and a custom cocktail/dinette table.

We did keep the two front chairs but had did replace them later.


Detail of Changes - Outside Body & Layout Changes:

1. LP tank cavity - replaced the LP tank with a smaller unit and added water intake into the cavity. We replaced the LP tank because it would take us typically 2 years to empty one tank filling. A new LP line for the refrigerator was routed on the inside, under the hot water tank. A steel plate was put under the LP line to protect it from an unlikely air bag explosion. The water intake is now by the LP tank and a valve was put in place so that the water tank could be filled with the hose still hooked up. Part of the intake is a clear plastic tube that allows outside viewing of the water level in the tank. A cable connector and a small shelf were added above the tank for storage of small items. Additional storage is available next to the smaller tank.

2. Generator cavity - the 50 amp cable is stored in the area and the house battery was replaced with two golf cart batteries.

3. Motor Cavity - Replaced the transmission lines and added a line filter, offered by Camper World. I had to relocate the charcoal fuel line filter to the driver side front to make room for placing captain chairs on the inside. The old water tank was cut down and now serves as the storage for the battery and other things. The new, smaller tank is in the same location.


As mentioned earlier, side vents (from a Saab 6000) were added and a duct into the motor cavity connected to a fan was installed, I used the booster fan from the furnace for this purpose. This fan is used whenever the motor is stopped for a short time to help reduce the motor cavity temperature. I also routed a duct from the front to the carburetor intake.


4. Roof area - I am very happy with a rubber sheet cover that was added on the roof, sold by Camper World, to reduce noise and improve cold/heat transfer. The only problem with this is that the colors offered are limited. The roof radio and CB antennas were combined and mounted on the passenger side mirror.

5. I also removed the hoses to the hot water tank and used the connections for a heater and fan, located under the front dinette chair for use during driving on cold days.

6. This keeps the living area surprisingly hot while driving. Under the front couch/bed a large drawer was installed for bedding storage.

7. Replacement circuits - in the process of fixing things, I became aware that not all parts are available any more. The following circuits have been replaced or provide additional functions:

a) Low fuel alarm circuit, I designed my own unit with up to date parts and added a test circuit for the Tel-Tale light, including the low radiator water circuit. This way whenever I start my motor both bulbs light up for about 10 seconds thus assuring a working circuit. 

b) A/C Delay circuit, this is a replacement for a circuit that is not available any more (at a reasonable cost). While this unit can be bypassed I believe it is better to have that circuit installed.

c) Fuel Solenoid circuit, on one of my trips my fuel switch solenoid locked up and I ended up out of gas because while I was watching the secondary tank fuel level, I was still using the primary tank fuel. This circuit checks the main tank fuel level while the secondary tank is switched on. Thus, if the solenoid does not switch to the secondary tank at least you will be alarmed when the primary tank reaches the low level.

d) I had to repair my low radiator water warning circuit and since I could not buy a new one I designed my own.

I  designed a temperature monitoring circuit for the brakes and CV joints. Since I had a CV boot failure, due to loose bearing, I want to see if I can avoid a similar recurrence.