GMC Western States

 Tech Center Number 29 - June 15, 2000


By Chuck Aulgur

We again had some great technical sessions in Ukiah. We learned everything you need to know about toilets, holding tanks, and waste technology from MicroPhor Company. Duane gave his usual good session on "What Every New GMC Owner Should Know", plus he had a lot of follow up discussion. The session on "Bring Your Written Question To The Panel Of Experts" generated a lot of good comments, and a strong request to continue this type of format at future roundups. Several people said it was the best tech session they had ever attended. Having the questions written down gives the panel members a chance to read and understand the questions and determine which panel member can best answer the question. We had a California Highway Patrol Officer give a very informative session on the "Rules of the Road", plus she stayed around to answer any and all questions from the audience. Gene Fisher and Al Chernoff used their laptop computer, connected to a big screen TV, to show all the GMC info that is available on the net. The tech sessions ended on a good note with a successful pressurized leak test on Al Single-ton's and Al Top's coaches. We found lots of leaks so we had a good opportunity to demonstrate how to seal them.

Clarification on Propane Cylinders
In the June 2000 issue of Highways Magazine, on page 70, there is a question-and-answer section where the person answering the question on propane cylinders made a statement about propane cylinders having to be replaced by the year 2002. The question has to do only with portable type cylinders used in RV trailers. These are considered portable tanks as they can be removed for filling.
The need for replacement does not apply to propane tanks used in motorhomes (our GMCs included). Motorhome tanks are not removable and are designed to ASME Standards different from those used on RV trailers

Entrance Door Adjustment
When I walk around and talk to people at the various club rallies, I notice that a lot of entrance doors do not close properly. The maintenance manual does not show anything about how to adjust door closure. On the two 76 GMCs we purchased, the entrance door would not close far enough to engage the primary latch position, even with the striker plate adjusted all the way to the outboard position. You can check the proper door latching by closing the door slowly and observing both the secondary and primary latch functioning. You should hear two clicks. If it is not closing on the primary latch, loosen the four bolts attaching the door striker and move the striker plate outward. The striker plate has about 1/4-inch adjustment capability as it came from the factory. If the door still does not close on the primary latch, you need to make a small modification. Remove the four bolts that attach the striker plate. You will see the bolts go through oversize holes in the body doorframe. There is a floating nut plate behind the two outboard bolts. The two inboard bolts use standard washers and nuts that are accessible from inside the coach next to the inside wall. Using a rotary file or metal router bit, enlarge the holes in the frame about ¼-inch in the outboard direction, being careful not to damage the threads in the nut plate. Now you should be able to reinstall the striker plate and move it out far enough for the door to close on the primary latch. You may need to remove some material from the outboard side of the striker plate if it is interfering with the door closure. Some of the striker studs (what the door latches on) are loose or are dislocated so badly that the stud has to be drilled out, repositioned and welded to the striker plate.
I have also noticed a lot of the entrance doors do not fully close at the top and/or bottom. When you see doors that are bent out at the top and bottom, it is usually on a coach that has had the door "stop strap" removed. And you can usually see a fairly deep dent in the body rub rail where strong winds have caught the door and slammed it against the rub rail. It may not seem possible, but many people have observed their door being bent in this way. GM put the strap on the door for a very good reason.

Engine Oil Leak
Bill Harvey had a recent experience with an oil leak coming from the front of his engine. He was not surprised as he has over 150,000 miles on his engine. Even though oil is relatively inexpensive, he decided he had better see if he could find the source of the leak. He had a good clue as the oil was dripping off the cross member under the front of his engine. Further inspection showed it was leaking from the engine front cover seal around the harmonic balancer shaft. Reviewing his trusty maintenance manual showed it was not a big job to replace the seal. His biggest task was finding how to loosen the power steering pump to remove the v-belts and moving the fan shroud forward to gain access to the harmonic balancer. The maintenance manual showed the type of puller needed to remove the harmonic balancer. After it was removed, the seal was easy to pop out with a screwdriver, and it was very easy to install a new seal.

Fig. 1. Harmonic Balancer

Careful inspection of the shaft area on the harmonic balancer showed a very faint groove where the seal had been rubbing. Bill was aware there was a kit available to repair the shaft, and a trip to his trusty NAPA store took care of the problem. Their P/N 999199 fits all the Toronado engines of our era. Their kit contained good directions, along with a tool to drive the sleeve on the shaft. The discolored area near the top of the shaft on the harmonic balancer, shown in Fig.1, is where the seal had been rubbing. Also shown lying on the harmonic balancer is the sleeve that Bill used to repair the worn shaft.
Note the timing line on the harmonic balancer has been highlighted with white paint. If you haven't done this previously, now is a good time to do it. Also, if you have more than 90,000 miles on your timing chain and /or your water pump, now is a good time to put on new ones. The task is more than half-done with the harmonic balancer already off.

Safety Gauges
Modern day cars have several computers that control and monitor various functions as we drive down the road. If something malfunctions, the computer gives a warning such as "check engine" or other indicators to let us know something is wrong.
We don't have this type of monitoring system on our classic GMCs, but we can add gauges that will do a similar function. There is a small company called "Westberg Mfg. Inc." located in Sonoma, CA that makes numerous "Westach" gauges for automotive, aircraft, and marine applications. They make a series of gauges that have a built-in super bright LED warning light that can easily be adjusted to come on at whatever temperature or pressure you want to be notified. There is also a pin on the back of the gauges that can be used to operate an auxiliary light or buzzer.
I have four of these gauges on our GMC: one for engine oil pressure, one for engine oil temperature, one for engine coolant, and one for transmission oil temperature. I have all these gauges connected to a common bright red light located in the dash in front of the driver and also to a common loud buzzer. I have the "set point" on the three temperature gauges just a few degrees above their respective normal maximum operating temperatures. The oil pressure "set point" is on the low side, a few psi below the normal minimum operating pressure.
I don't have to monitor my gauges as I drive, because I know they are monitoring the four critical functions. If the buzzer goes off, all I have to do is glance at the gauges to see which one has its LED turned on and make a judgment as to what I need to do to possibly save my engine or transmission.
Wesberg is a small company, and they are very amenable to work with. They will make about any type of gauge you want including dual function gauges. They even installed the temperature-sending unit inside the drain plug for my engine. I have one of their dual function gauges to monitor air bag pressure and another one to monitor engine vacuum and RPM. If you are interested in automatic monitoring of critical functions on your vehicles, you can call them at 1-800-400 7024, and they will send you their catalog.

Air Fuel Ratio Gauge
Both Chuck Garton and I have a Westac analog air fuel (A/F) gauge to monitor the operation of the fuel induction system. An O2 (oxygen) sensor located in the exhaust system drives this gauge. It reads from 17 (lean) on the left to 12 (rich) on the right with the ideal ratio of 14.7 marked near the center.
The A/F gauge operates the same on both of our GMCs. Under normal driving with a light load and high engine vacuum, the gauge needle stays all the way to the left, indicating a lean fuel mixture. When the engine load is increased sufficiently to cause the vacuum to drop below 7 inches, the gauge needle slowly swings all the way to the right due to the power valve in the carburetor enriching the fuel/air ratio. The needle stays all the way to the right as long as engine vacuum stays below 7 inches. When the engine load is decreased sufficiently to cause the vacuum to increase above 7 inches, the carburetor power valves closes, and the A/F gauge needle swings all the way left, again indicating a lean fuel mixture.
These A/F gauges have not been calibrated, so the mixture reading is not precise. However, when used in conjunction with a good engine vacuum gauge, they can be very beneficial in showing proper carburetor operation.
Chuck Garton had a recent experience where his A/F gauge indicated there was something strange going on in his engine. When his vacuum dropped below 7 inches, the gauge properly indicated fuel enrichment by moving all the way to full rich. But, after a short time at low vacuum, the needle slowly moved left to full lean indicating the carburetor was not working properly (not supplying enough gas). He first checked his carburetor and found it to be okay. He next replaced his O2 sensor that drives the A/F gauge. However, the symptoms stayed the same. He recalled reading a recent article by Wes Caughlin discussing problems with cracked intake manifolds causing lean fuel mixtures. Inspecting his manifold revealed extensive cracking in the area under the carburetor. He replaced his intake manifold and his A/F gauge showed everything was back to normal.
When he applied a heavy load to his engine, the hot exhaust gasses heated the area under the carburetor causing the manifold crack to get larger. This allowed the exhaust to dilute the air/fuel mixture, which caused the lean fuel condition that was properly indicated by his trusty A/F gauge. If this condition had gone on undetected for a long time, it could have resulted in burnt exhaust valves. Thus, his A/F gauge may have saved him some big bucks ($).

Parking Brake Adjustment
You hear a lot of people complaining about how poorly their parking brake works on their GMCs. Some say: "It is absolutely no good, so I disconnected it". I don't know how people get along without a parking brake unless they carry along blocks and have their passenger get out and chock the wheels whenever they park on a steep incline. If you have to rely on the "parking paw" to hold your coach on a steep hill, you may not be able to get it out of park. If you try to take it out of park with a high load on the parking paw, you may damage the transmission.
The parking brake on our GMCs works adequately if they are properly adjusted. First, your rear brakes must be in good working condition, and they must be properly adjusted to keep the brake shoes in near contact with the brake drums. The automatic adjusters do not work very well on our GMCs because of our limited usage and braking while backing. There are two modifications discussed later that will considerably improve the barking brake function.
The best way I have found to adjust the parking brakes is to jack up the rear of the coach and place it on jack stands, so all four wheels are off the ground. Release the parking brake and turn the brake parking handle knob counterclockwise until it is all the way out. Loosen the adjustment nuts at the "Y" equalizer on the outside of the frame rail where the single cable coming from the front connects to the dual cables going to each set of rear brakes, as shown in Fig.2. Make sure there is no tension on these cables. Manually pull and release the cable going to each rear brake and verify the cable slides freely within its housing. If not, they need to be replaced.

Fig.2. Adjusting Parking Brakes

Adjust the rear brake shoes on all four wheels by turning the brake adjustment screw and expanding the brake shoes until each wheel can just be turned by hand. With the vehicle engine running, apply the vehicle road brakes as hard as you can. Recheck and/or readjust each rear brake shoes so they all have about the same drag. Back off the brake adjustment screw on each wheel (you have to release the automatic adjustment lever with a small screwdriver) until the wheel just starts to turn freely with only a slight drag. Reapply the vehicle road brakes, and then recheck the rear wheels to make sure they still turn freely. Now you are ready to adjust the parking brakes.
Tighten the adjustment nut (at the "Y" connection) a few turns, alternating from side-to-side, until you start to feel some drag on each wheel. If the drag is not uniform, you need to equalize the cable pull on each wheel by sliding the cable within the intermediate guide (at the "Y" connection) until you have equal drag on each wheel. It is sometimes difficult to slide the cable within the guide as the cable tends to get bent into the shape of the guide. It is very important to do this step so you will have equal pull on each wheel and remove all the slack in the parking mechanism. From this point on, you should be able to take any additional slack out of the system by adjusting the knob on the parking brake handle. This knob should be kept adjusted to the point you are just able to pull the parking brake to the "locked" position. One of the shortcomings of the GMC parking system is the limited amount of cable travel provided by the brake handle mechanism. Thus, you have to keep the rear brakes properly adjusted to have adequate parking brakes.

Parking Brake Modifications
The parking brake function and adjustment can be improved by making two simple modifications. The first one is to replace the "hook" guide where the cable goes through the frame rail (shown in Fig.3).

Fig. 3. Hook Guide for Brake Cable

Go to the Home Depot section where they sell hardware for sliding glass doors and purchase two of their larger ball-bearing rollers (about 1.5" in diameter) that have a groove around the outside diameter. Loosen the brake cables and place the roller on top of the bracket that holds the "hook" guide. Align the roller on the bracket so the cable will go around the roller, and through the slot in the frame rail without dragging on the frame. Mark the location of the roller center hole on the bracket and remove the cable. Drill a hole in the bracket the same size as the roller center hole. Pack the roller-bearing with grease and install a large "fender washer" on top of the roller. Bolt the roller to the bracket using a locking nut. Install the cable into the groove on the roller and you now have one of the modifications complete.
The second improvement involves replacing the guide at the "Y" connection on the outside of the vehicle frame where the parking brake adjustment is made. While you are in Home Depot purchasing the above discussed rollers, go to their garage door section and purchase two of their ball-bearing pulleys used to guide the return spring cables on overhead garage doors. They are about the same diameter as the spacing between the two cables going to the rear brakes. Using 1/8"x1.0" wide steel bar, make a "U" shaped bracket about five inches long. Drill a hole in the center of the "U" to fit the threaded area on the end of the front cable. The "U" legs need to be far enough apart to get a box-end wrench between the legs so the adjustment nut can be turned. Drill matching holes near the end of the "U" legs to match the center hole of the pulley. You will need to put a small "dog-leg" in the legs of the "U" as the thickness of the pulley is less then the width needed for the adjustment wrench. Saw off about half of the threaded section on the front cable. If the shortened threads do not provide sufficient adjustment capability, there is another cable adjustment under the coach that can provide additional adjustment. Remove the OEM guides at the "Y" connection on each side of the vehicle and replace them with the new pulleys. Place the "U" bracket over the pulley and bolt the pulley to the bracket using a locking nut. Insert the threaded end of the front cable through the hole in the "U" and install the adjustment nut. You need a nut on each side of the bracket so they can be "locked" same as with the OEM guide.
You can now adjust the rear parking brakes using the procedure discussed previously. However, you won't need to go through the tedious procedure of trying to balance the pull going to each of the rear brakes; the pulleys do that for you.
The material for the two above modifications will cost you less than $20, and you will realize a significant improvement in your parking brake function. Remember that the proper way to "set" your parking brake is to first apply and hold the foot brakes, then apply the parking brake. The parking brake will develop significant more braking capability if the rear shoes are already expanded tightly against the drums when the parking brake is applied.

 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.





 Please send your comments and ideas for the Tech Center to:
Chuck Aulgur, Technical Vice President,
9805 Ogram Dr., La Mesa, CA 91941
Phone: (619) 465-9875



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