GMC Western States

 Tech Center Number 10 - December 1994


TECH TIPS Written, Selected and Edited By Bob DeSaussure, Technical V.P.


Brake Fluid

Because of contamination with water it is advisable to drain, flush and refill brake lines every 2 to 3 years. D.O.T. is the specified brake fluid but improved fluids such as D.O.T. 4 (and perhaps D.O.T. 3 Ford Heavy Duty #C6A219542) should probably be the fluid of choice. Brake shoes, drums and rotors may reach temperatures of up to 750ø under heavy usage and some of this heat is transferred to the brake cylinders and into the brake fluid. Water contaminated brake fluid can boil and interfere with normal brake operation.
Because of a severe operating environment found in our GMCs, brake fluid boiling specs are important. Specifications Dry/Wet (Wet=contaminated with some water) are:
D.O.T. 3 Min. Specs Boiling Pt.401ø/284.ø
D.O.T. 4 Min. Specs Boiling Pt.446ø/311.ø
D.O.T. 5 Min. Specs Boiling Pt.500ø/356.ø
Ford Heavy Duty D.O.T. 3 Stated Boiling Pt. 5500/(390ø)
The 390ø wet figure on Ford fluid is my linear extrapolation. According to Govt. Laboratories, brake fluid picks up and saturates with approximately 3% water within 3 years at which time all brake fluids boil at approximately 300ø.
Use of D.O.T. 5 is definitely not recommended by Ford and Chrysler. Requires frequent changes to maintain integrity.


Engine Cooling

Radiators should be drained and flushed every 2 years for proper maintenance. Use of an additive such as Barr's Radiator Soluble Oil and Rust Inhibitor (1/2 cup added to overflow reservoir), will help lubricate the pump and inhibit rust.
To increase air flow over the engine, you may want to consider using G/M Diesel Engine Fan G.M. #1064AD22500652. It is a 7 blade 18 3/4" diameter fan, and a direct fit to your hub. It has very little more noise at lockup, but a greatly increased airflow.


CV Boots

Contrary to previous experience with others, I have found that SpeediBoot 03609 by Motomite Mfg., a split boot similar to the Volkswagen C/V joint used for years, will satisfactorily perform its function while eliminating the need to remove the drive line. The joint must be cleaned up prior to installation. It is a good idea to seal clamped ends with Permaseal. This may be a useful item to keep on board with your spares. Approx. $25 each: Call 215-997-1800, Colmar, PA for nearest dealer.


Timing Chain Replacement

Original equipment timing chains can show excessive wear at 80 to 90 thousand miles. You may wish to upgrade to G/M Performance Double Roller Chain as used on factory racing equipment. GM part numbers: Chain; 401584, Engine Gear 382880, Cam Gear 3812563. The cost is not much more and the steel gears are not noticeably noisier.


Backup Electric Fuel Pump

I have found that the use of a Carter Flow Thru Pump (P7401A) can be used as a backup to your mechanical pump for emergency use or to assist in vapor lock conditions. Install before the crossover valve on the reserve tank hose and wire it across the crossover switch. It will operate when the reserve tank is selected. Reasonable and easy to install, it causes little restriction when not operating, as opposed to other vane type pumps. Adding an oil pressure actuated mm off switch is advised for safety reasons.



For those of you who like to experiment with new goodies, the Optima battery, a small, sealed lead acid battery, is available as a vehicle starting battery (engine battery) with 800Amp. cranking capacity. Featuring a spiral wound assembly of high purity lead grids interleaved with a porous glass separator, it is similar to some of those that power lap top computers. It is advertised to be maintenance free, vibration resistant, has a long life and is supposedly leak and corrosion free and explosion proof. Call Optima Batteries, Denver, CO (303) 744 5299 for nearest distributor and more information. Approximate Retail Price: $120.00. A deep cycle design for House Batteries is reported to be in the future.


Bash Heater Tip

Now that winter is upon us, check the Dash Heater before you must depend upon its use. Operation of the Heater Blower is a must. Power is applied to the blower motor through power resistors for low speed ranges. In the "High" blower position, power is applied directly from the Engine Alternator via a Relay (located on the fire wall next to the blower motor). Assuming the blower is functional, now we are ready for the heat. The engine water is routed through the heater radiator via a vacuum operated inline heater valve. The valve is located between the front two hood covers and is in line with the heater hose. The vacuum operated valve sometimes does not operate properly. An easy way to determine if the water is flowing properly is to feel the hose temperature during the engine warm-up period. With the controls in the HEATER/HOT position, verify that the heater hose is the same temperature on both sides of the valve. Remove and replace if necessary to obtain proper heating.


GFI Problem

Several coaches have been observed to have a GFI (GFCI) "Trip" problem when connecting shore power to their coaches as recently as the Kingsburg Roundup. Many RV parks are upgrading or being built with a GFI circuit for AC shore power. GFI (Ground Fault Interrupt) is a fast acting circuit breaker that helps prevent serious electrical shock. This device measures any unbalanced current flowing in the "hot" and "neutral" (not the "ground" wire). Any unbalance, even with as little as 0.005 Amp. or 5 milliamperes (which can occur, for instance, if someone touches a "hot" line in an accidental contact), trips this fast acting circuit breaker cutting off power within milliseconds, thus minimizing the chance for serious shock.
The Electrical Code requires RV AC wiring to have the neutral wire (white wire) isolated from the chassis "ground" wire (bare or green wire). The electrical resistance between the neutral and the "ground" wire should be very high, perhaps several hundred Kilo ohms or more. A simple resistance (ohmmeter) measurement between the safety/"ground" pin to neutral pin of your shore power connector can help verify proper coach wiring.
In the event that the safety/"ground" and neutral pins are not isolated from each other (not a high resistance), trouble shooting will be necessary to identify potential problems or wiring error. Reversal of the coach power wire (black) and neutral wire (white) at the shore power connector can be extremely hazardous because the coach chassis could become "hot" (120v AC) in the event that the chassis/"ground" wire becomes disconnected or fails.
A replaced Roof Air Conditioner, wired incorrectly, has been found to be a high suspect candidate for wire reversals and chassis/"ground" to "neutral" connection errors leading to tripped GFIs.
Note: The Onan AC Generator is internally connected between safety/"ground" and neutral (white wire) by design. Therefore, the Onan wiring circuits must be isolated from shore power circuits in the event both are operating simultaneously. This usually happens automatically when the shore power (pigtail) is removed from the receptacle connected to the Onan output. (In some Royales the rear Roof Air Conditioner is connected only to the Onan and may be a problem.)
To verify your system, plug your coach in at home to a source protected by a 20 Amp GFI. If unavailable, you can purchase an adapter containing a GFI (approximately $12.50 at hardware stores). Often it is not necessary to turn anything on to trip the GFI if there is something wrong with your coach wiring, but it is best to systematically turn on one appliance at a time to make sure that the appliances are wired correctly and have no internal problems. By the way, this adapter GFI unit is very useful when used with hand power tools for safety reasons.
If in doubt, see a qualified electrician for verification of your coach's AC wiring.


See Through Steering Wheels

Deep Drop Steering wheels are readily available at wrecking yards: '83-'87 Olds Cutlass, Sierra and Omega. The standard wheel (not deluxe) has a center horn button and makes it a direct replacement. See also January '90 Coach Talk by Wes Caughlan. Additional dash clearance can be gained by shimming yoke under dash approximately 3/4". Approximate wheel cost $25.


More on Fan Clutches

Two Hayden fan clutches 2747 heavy duty and the 2749 severe duty have been popular replacements. One of the specifications that make them different are the temperature ratings at which the clutches engage: for the 2747 it is 175 plus or minus 10 deg. F. and for the 2749 it is 185 plus or minus 10 deg. F. If you've tried the 2747 and found that the fan is engaged more than you'd like, you might consider the higher temperature rated 2749. Hayden also says the 2749 has a more durable clutch design and it is the one that they recommend for motor homes.


Electrical Connection Tip

Many 12 volt D.C. electrical problems can be traced to the ground (negative or return) circuit. Most electrical units rely upon current returning to the battery/alternator via the coach frame or body shell. Yet, very little attention is usually given to continuity of the return circuit path. The coach body frame is primarily aluminum which, when used as a return circuit path, can result in high resistance connections due to the corrosion of the aluminum and the production of a very thin film of highly insulating aluminum oxide. Preventive maintenance of these "grounding connectors" is to scrape or sand both the aluminum frame and the ground lug or connector to shiny metal and coat both parts with a protective thin film of petroleum jelly (Vaseline) before reattachment. The same applies to steel frame members. Particular attention should be given to large grounding straps in the engine compartment because they bond the various parts of the frame, engine and body electrically. Same goes for battery cable to frame connector and the cable to battery connectors too.


 Tech Info