GMC Western States

 Tech Center Number 1 - February 1992


The first issue of Tech Center will cover service. We have many new GMC owners in the club who have not had the benefit of a basic service schedule. It's also a good idea for the "veterans" to brush up once in a while. The whole idea behind service or preventative maintenance, is to catch small problems before they become big ones and leave you stranded somewhere. Speaking of being stranded, make sure that whatever towing coverage you have covers motorhomes. Too many members have had to pay several hundred dollars because their auto policy did not cover their motorhome, and they found that out in the middle of nowhere.
We are going to cover the factory recommended service and several other items which are not mentioned by the factory, but are of critical importance.
It's a good idea to have a complete set of service manuals for your coach, they can really be of help when you need service and cannot find a mechanic who is familiar with the GMC. They are also a must if you plan to do any work yourself. Manuals are available from Cinnabar Engineering at 415-948-2618, Monday-Friday 8: 00am-5: 00pm Pacific time.

Every 3,000 miles or 3 months:

Oil and filter change and chassis lubrication, changing the filter every other oil change puts a quart of dirty oil back into the system. The $5.00 it costs for an oil filter is cheap insurance against oil contamination.
While you (or your mechanic) are under the coach, check the oil lines which run from the oil filter bracket to the radiator. The lines carry high pressure oil to the oil cooler inside the radiator. These lines will wear with age and eventually fail. If they fail while you are driving you will lose oil pressure and there is a great risk of fire if the hot oil contacts the even hotter exhaust manifold. If the lines have never been replaced, or if you don't know if they have been, it's worth the effort to do it. Any good hose/fitting shop can duplicate the lines. In some cases the fittings on your old lines can be re-used. If the lines look good, make sure they are not rubbing on the frame or the engine. Make a point of checking them each time you change the oil.
A second item to check while you are looking at the oil filter is the filter bracket itself. It tends to loosen up after several tighten and loosen cycles of the oil filter. If its gets loose enough you will have a messy oil leak and possibly loss of oil pressure. Once the oil filter is removed you can tighten the nut which holds it to the engine block. If the gasket between the bracket and he engine is worn out, take the gasket from the oil filter you just removed and use it as a replacement, it's the same size.
Also check the lower radiator hose and the hoses running to and from the power steering pump. Look for cracks or places where they are rubbing against something else. Now is the time to correct any minor problems you might find.
Check all your fluid levels including; transmission, power steering, windshield washer, batteries (both engine and living area), brake fluid, coolant, and generator oil level. This is also the time to poke around the engine compartment and the under carriage to check for any loose wires, holes, or vacuum hoses which might be cracked.
After checking the engine, turn your attention to your air suspension system. Drain the water from your air compressor wet tank. Check to make sure the drain valve is not clogged and leaking. If it is, replace it, this will keep your compressor from running constantly leading to early failure. If your air system still has the original brass check valve on the air tank, it needs to be replaced. The all brass valve won't seal properly causing the system to leak once it has been turned off. Replace the valve with an updated type which is available from most of the after-market GMC Motorhome parts suppliers.

Every 6,000 miles or 6 months:

Repeat the 3,000 mile service, plus, rotate the tires, while you have the tires off, check the tires for any uneven wear pattern. Check the front disc brake pads. You don't have to remove any parts, there is a little cutout on the back of the brake caliper where you can look in and see how the pads are wearing. The pads have a groove cut down the middle of them. If you look in the cutout and can still see the groove then they are OK. If you cannot see the groove, they will be somewhere between one eighth and one sixteenth inch thick and need to be replaced.
Check the rear drums for any weights that might be loose or have fallen off. A lost weight can cause an annoying imbalance that can drive you crazy if you keep taking the tires and wheels to get balanced but still have a vibration. A good welding shop can reattach the weight.
While your checking the brake pads, check the steering linkage, grab the steering rods and try to move them. They should be tight with no noticeable play. Check the drive axle constant velocity joint boots. The axle connects the final drive unit with the front wheel on each side. The boots should not be torn. They contain vital lubricant for the joint, if they are torn they should be replaced immediately. If you are on the road and notice a split boot, most auto parts houses carry a temporary sealer you can use. The boot needs to be replaced as soon as possible.

Every 12,000 miles or 12 months:

Repeat the 3,000 and the 6,000 mile service plus, change the transmission and final drive fluid and clean the outside of the radiator with a garden hose flushing toward the front of the coach. Replace the engine air filter, fuel filter, and spark plugs. Check the spark plug wires for cracking or fraying. Carefully check the distributor cap for cracks, carbon tracking, or terminal corrosion. Replace it if you find any of these conditions. If you have a 1973-1974 coach with conventional ignition, replace the points and condenser as well as the distributor cap and rotor. You will need to check the timing after replacing the points. Check the fan belts for wear and proper tension.
Remove the rear brake drums and check the linings. The basic rule-of-thumb is if the lining material is as thin or thinner than the backing it's attached to, then they need to be replaced. After re-assembling the drums. Take the coach on a road test. Adjust the rear brakes by stopping firmly a couple of times while rolling backwards at about 5mph. A nice empty shopping center or church parking lot is perfect for this. Once the brakes are adjusted, test the holding ability of he parking brake on a small incline. You can make minor adjustments by turning the knob on the end of the parking brake lever. The parking brake is important because the coach can get locked in park if you stop on an incline and it rolls backward while the transmission gearshift is in park. It's good driving practice to set the parking brake before you put the transmission in park.
This is also the time to check all your interior appliances. Make sure the furnace works properly, check the stove and oven, and check the stove vent, and the refrigerator for proper operation. Check all interior lights and fans for proper operation.

Every 24,000 miles or 24 months:

Repeat the 3,000, 6,000, and 12,000 mile service plus, Replace the front wheel bearings and repack the rear wheel bearings. The front wheel bearing service is best left to an expert if you are not familiar with the process. The bearings need to be pressed off and on the hub. In addition, they must be measured precisely and fitted to the hub. They are sold in matched sets and cannot be mixed. Several club members perform this service as well as all good GMC Motorhome service facilities. This bearing replacement is critical because the bearings are the same ones used in the Oldsmobile Toronado from which most of the Motorhome front end was borrowed. The bearings can carry the weight of the Toronado for 100,000 miles, but when you triple that weight on a Motorhome, the bearings wear out much faster. It' s cheaper to replace them every 24,000 miles than to repack them and risk failure a few thousand miles later.
The rear wheel bearings can be checked, cleaned, and repacked. Make sure you use new grease seals and set the bearing preload properly, always use a new cotter pin when you reassemble the hubs.
Check your air suspension system for cracked hoses, leaks (use a soapy water solution), and missing parts. The fittings should be tight but not so tight that they distort and leak, this is especially true for the plastic AMP fittings. Look at the air bags, they should not be peeling or "hot dogging". Periodic rotation of the air bag can eliminate the "hot dogging" problem.
Don't neglect your generator, now is the time to change the oil and filter, clean the air filter and replace the spark plugs. Lubricate the throttle linkage. Check the unit for any loose wires or hoses. Make sure it will be ready when you need it.

City Water Entry valve Information:

The original equipment city water entry valve and pressure regulator is made by Shur-Flo and is made of brass and plastic assembled using screws and rubber gaskets. It is very easy to take apart, however, Shur-Flo does not carry replacement service parts for it anymore. The model was discontinued about six years ago. The only model they make is a standard all plastic one which uses no common parts and cannot be taken apart. The original valve has a pipe fitting on the side of the unit to connect it to your plumbing system. The interior closet wall (on side-bath models) is right against the valve. If the valve fails and you replace it, the new all-plastic unit has the pipe fitting on the end. You will need to use some adaptors to get your plumbing to connect to the new valve. Save the 1/2"-to-3/ 4" adaptor from the old valve, you will need it to connect to the motorhome plumbing. The closet wall might have to be moved to accommodate the pipe fittings. This appears to be the configuration on the side-bath 26 foot models.

Air Suspension Update:

This is for those of you who have installed the air suspension upgrade kit from Cinnabar Engineering or a similar kit and are having trouble finding a leak. Check the polycarbonate bowl and filter on the air line which runs from the compressor to the wet tank. What appears to be a leaking schrader valve could also be a cracked bowl. If you look carefully you may be able to see hairline cracks where the valve screws into the bowl. If you are on the road, a piece of chewing gum, urethane, or "plastic rubber" sealer put down inside the bottom of the bowl will seal it until you can replace it.


 Tech Info