GMC Western States

 Tech Center Number 5 - May 1993

Thanks to Lee McKay for his Window Channel Replacement contribution in the last issue of Tech Center. Thanks to Egon Elssner for his generous offer to summarize the seminars presented at the last Roundup.




GMC steering stability is very good if all the front end suspension and steering components are dimensionally accurate, excessively worn parts replaced and the whole assembly accurately aligned for proper camber, caster and toe in/out. Bob gave procedures for checking wear, and checking the configuration of parts that make up the front end, including the steering box and steering coupling shaft. He stressed the need for accurate rear and front ride height in relation to steering geometry. He also gave us his preference in after market shock absorbers and commented on other after market components for improved handling.


THE IGNITION SYSTEM Duane Simmons and Bob Lamey

Ignition system fundamentals were presented for both the early model 'points' and later model "HEP" systems. The reasons for, and the details of both the mechanical advance and vacuum advance were given. Problems relating to ignition wires, points and the HEI high voltage coil, module, magnet, star wheel and pickup coil were covered in detail. Distributor rebuilding and availability of parts was also discussed. Data for all four OEM distributors and a 'Custom' distributor were tabulated and compared.



The original GMC exhaust system was always marginal in performance. Recently, most OEM parts have not even been available. Ed introduced his seminar with a rather theoretical discussion of what makes up a good exhaust system,

one that can add an additional 10% to engine power output. Starting with a Thorley header, custom made for the GMC, he detailed his ideas for the ideal exhaust system which includes Iow resistance flow mufflers and pipes. He shared his experience and emphasized the need for shielding of critical non-exhaust parts from the extreme heat radiated by headers, mufflers and pipes.


Jeff Baker of Thompson Lacquer Co. and
Jeff Laliberte of Fleet Finish

This was primarily a question and answer session with Jeff Baker fielding most of the technical questions related to paint, primers and coach preparation. Jeff Laliberte suggested how to relate to paint shops in developing a paint scheme, choosing colors, and writing up a contract. He gave some tips that help reduce final cost.



Cracks have been found in a surprising percentage of front body supports. After Bob Lamey reported at Buellton that he had found two cracked supports out of the two he inspected. I checked mine and found 1.5 inch crack, Bob found another crack while he was repairing the first one. I suggest that everyone who has the newer type supports should inspect them or have them inspected.
These two welded-in supports connect the front body to the chassis beside the engine. Older GMC's had a tripod type support which appears to be trouble free. The newer GMC's {post 1975), have a stamped aluminum triangular member with a 2 to 2.5 inch flange. The cracks start in the flange, usually from the pointed edge.
Dennis Richardson at the GMC RV shop has found several cracked supports including one which had broken completely through. The sooner you find the cracks, the cheaper it is to get them welded together. If no cracks are found, file and sand along the edges to remove any sharpness or irregularities.



At the Mount Hood Roundup, Wes Caughlan warned us to get rid of the Michelin XCA load range-D tires. Since then I have seen several coaches still running the tires. Owners still running these tires risk expensive damage to their coaches. When the fires blow out they damage the lower fiberglass body parts. Even though the fires are warranted for four years, they may fail much sooner than that. Eugene Slanter had two of these tires blow out through the tread while the fires were mounted on his spare fire carrier!. The tires had less than 1500 miles and were less than one-year old. The age of the tires can be determined by checking the three-digit code on the sidewall. The first two numbers indicate the week the fires were manufactured, and the third indicates the year (for example 345 would be week 34 in 1985).



Have you thought about what to do if your coach breaks down on the road in the middle of nowhere? Here are some tips to take with you, hopefully you'll never need them, but they can get you out of trouble. First, an essential item to carry is good towing coverage. Good Sam ERS, and the policy offered by FMCA are excellent lifesavers! These towing policies will properly tow your coach to the nearest qualified repair facility. Check your policy to ensure that there is no mileage restriction, the nearest qualified facility might be one hundred fifty miles away. The second item to have is a CB radio or cellular phone. Make sure that they are in good working condition before you leave, the roadside at 10:00 P.M. is no place to find your CB does not work. In most states the highway patrol monitors CB channel 9 and if you dial 9-1-1 on a cellular phone it rings in the highway patrol office. The third is a good spare tire. Check its condition and tire pressure after checking your CB radio. Once again, the roadside is no place to find out your spare is flat! The fourth item to carry is a good tool kit, nothing fancy, but you should have the tools to change a tire, an air bag, air fittings, belts, and hoses. Even if you don't do the work yourself, you can have someone else do it while you guide them.
If you have a tire blow out (usually the rear tire blows), you will lose ride height on the affected side. If the coach drops to its lowest height, one of the only jacks which will lift it is the OEM jack which came with the coach. If you don't have the original jack, make sure the one you are carrying will fit underneath to lift it from its lowest position. If the air line running to the air bag breaks or is broken by flying tire tread you will need to repair it in order to re-inflate the air bag. Carry three or four feet of 1/4' air line (truck air brake hose) and a couple of union fittings (used to splice two pieces together). You will have to splice a new piece on to the section coming out from the side of the coach and run it to the air bag.
If an air bag blows or gets punctured by flying fire debris, it will feel just like a tire blow out. Again you will have to raise the coach to remove the air bag. Some people carry a spare air bag. Wes Caughlan advises that air bags come with a one-year warranty and some are leakers. If you carry a bag for more than a year before you use it, and find that it leaks, you'll be out of luck for a replacement. Alternatives include checking the air bag for leaks as soon as you get it. Fill it up and put it in a bathtub to check for leaks. Another alternative is to call one of the many GMC parts suppliers and have a new bag shipped overnight to you, if you are in a place where you can stay. The final alternative is to carry a threaded rod cut to about eighteen inches, put two thick washers and two nuts on each end. You can put the rod in place of the air bag and use the nuts to adjust the ride height, place the suspension control on HOLD for the affected side. You'll have a rough ride because the ak bag was your suspension spring, but it will get you to a campground or home.
Many auto parts stores and catalogs carry universal replacement fan belts. They look like a piece of tubing with a metal fitting to hook the ends together. You simply remove the broken fan belt, approximate the size and cut the tubing, loop it around the pulleys and join it with the fitting. These belts are good for temporary use only, but they will get you to a place where you can get a replacement. You can also carry a spare set with you, the next time you have your belts replaced as preventative maintenance, carry the old ones with you as spares. The same goes for radiator hoses. Hoses are more critical than belts because the GMC requires a specially molded radiator hose. The universal hoses available won't do the job properly, it's best to carry a spare set. Another tidbit of information to carry with you is that the GMC radiator core is the same as a Chevy C-10 truck radiator core. Find a shop who can remove the tanks and attach them to a new core and you'll be on the road again. Carry a heater hose union, the ones that are sold for the do-it-yourself flush kits work just fine. If your heater hose springs a leak you can cut it and put the union in line to fix it. If your living area water heater preheat starts leaking, you can cut the hoses up front by the engine and using the union, connect them back together thus eliminating the water heater from the loop.
More next issue!


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