Page 19 - Spring 2018 GMC Vintage RVing

For additional technical session presentations and papers, go to gmcmi.com/technical-presentations-papers/60 mph or 65 mph, do not shift to second, use your brakes to slow the coach down. 40 mphto 45 mph is a good speed to shift to second. Obviously, use common sense, if you are going too fast and not able to get slowed down with brakes then by all means shift to second –safety first. The reason for this “no-no” is thatthe loads on the transmission are reverse ofwhat is normally applied to the transmission. Furthermore, the pump is not efficient during extreme deceleration. The loading is coming from the rear of the transmission (which only has bushings) rather than the front (where bearings are used). Manny suggested that if you do not have an external transmission cooler – get one! Manny wants the transmission to run as coolas possible. If you have headers and have not moved the transmission lines, then they shouldbe moved out toward the frame and away from the headers that generate lots of heat. While you are moving the lines, change to 3/8” lines. When going up hill and downshifting is needed (speed down to 40 or 45) shift down manually, do not allow the transmission to automatically shift down. The pressures on the clutches will be higher with a manual down shift. Paying attention to these “No-Nos” will provide many miles of good performance from the 425 transmission.>> Things We Have SeenCome Into Our Shops| Jim Kanomata and Miguel MendezJim and Miquel have seen many interesting things that have been done to coaches. They shared unusual situations that require extra time and effort to service. One of the main issues is wires that have been added for whatever purpose and not labeled or without any diagram provided.In one case, there were a bunch of added wired and all were blue. Fuel system changes to combat vapor lock provides extra complication and if not documented can cause addition time and effort for someone to figure out resulting in higher cost for repair. Fuel lines should be ethanol tolerant(most new products are). House plumbing should use the correct type of pipes and potable safe antifreeze should be used to winterize the plumbing system. Make sure proper maintenance is used for keeping batteries in top shape. Miquel has had cases where the owner says they just put in a new battery not long ago and are now having problems. Checking date codes showed that the battery was 12 years old. The owner responds, “has it been that long?”. Batteries can keep you from enjoying your next trip. Overweight coaches can cause multiple issues so trim down the extraneous items to lighten the load. Attention to some of these details can add value and enhance the enjoyment of the GMC Motorhome.the engine in the vehicle makes a degree wheela non-starter in most cases. To be conservative, Randy chose to advance the timing by 2 degrees. By the way, there are new products available now that allow cam advance or retard of 1, 2, 3, or 4 degrees rather than the standard 4 degree advance or retard. Gasket installation is the other very difficult process which was discussed with tips and techniques given. Understanding what is involved in replacing the cam chain is an important step in making the decision to do this project yourself or get more professional help in making the change.>> Heat / AC Dash Control and Vacuum Controls | John Watkins>> Camshaft Timing | Randy Van Winkle & George BeckmanThe GMC gurus say thata coach with more than90,000 miles, the timingchain should be replaced.With 140,000 miles on hiscoach, Randy decided itmight be time to replace thetiming chain. After recruitinghis friend, George, they setabout making this change. Beingshade tree mechanics, the result wasa bit of a comedy of errors. With excellent advice and guidance from Dick Paterson and Jim Hupy, they succeeded in getting the job done. This discussion centered on the processof changing the chain and provided several tips on making the job a success. Key items included making a decision on the amount of advance to give the new chain. Four degrees advance is the standard recommendation but those who have worked with engines say that you never know what the real setting is for a factory installed chain – too many variables for there to be a standard. The official way to do a chain is using a cam degree wheel. Changing the chain withThis presentation was designed to help the audience understand how thedash heater and AC controls were intended to work fromthe factory, then how to diagnose problems withThank you to all of our tech session presenters!AttendeeCommentWish I could of stayed longer. So much great information. member one year,first Conventionthe system. The 9-pin vacuum selector valves (’77 and ’78 – earlymodels are similar) used on the GMC Motorhomeis fairly common among GM cars/trucks during the 70sand 80s and even later. Within the selector valves is a disk that isprogrammed for how the system operates. John has worked on various methods to make new properly programmed disks but as yet, has not been successful. John spent considerable amount of time showing how to clean and lubricate the system which in most cases will correct most problems. The discussion moved on to cover the electrical systems of the heater/AC controls with tips and techniques for diagnosis and repair of these components. A well maintained heater/AC control system will keep the heater and air conditioning components working as designed from the factory, providing a more comfortable experience.SPRING 2018 GMC MOTORHOMES INTERNATIONAL 19