GMC Western States

 Tech Center Number 35 - December 15, 2001

Emery Stora


Michelle and I really enjoyed our trip to the Mariposa Rally, especially meeting old friends and making some new ones. The Condos and Truberts put on a great roundup. The volunteers who presented the technical seminars did an excellent job.

We started out with Darren Paget of TZE Plus GMC Motorhome Products whose topic was "Putting Your GMC On a Diet". Darren covered many weight reduction ideas. He has made extensive use of aluminum to replace the particleboard and heavy plywood used in the GMCs and had many, many other good suggestions on traveling lighter.

This was followed by something a bit different. We formed into groups of about 10 people each and went out to look and feel GMCs close up. Each group had a leader and the groups' discussions were wide open -- the people in the group determined the direction taken. A wide variety of topics were covered and many questions and concerns of the participants were answered. I received excellent feedback on this, and we'll probably do it again at a future rally.

We missed Henry Davis' presentation on Four Bag Rear Suspension Systems, as he could not attend due to repercussions of the Sept. 11 events in NYC. We'll try to get Henry on the program at Prescott, AZ in May.

Art Woodell, the vendor for Digipanel gauges, gave a presentation on his product and also on general monitoring of one's engine and driveline. This was well received and Art answered many questions from the audience.

Kerry Tandy had lined up a presentation by Rod Osburn of Swepco (Southwestern Petroleum Corp.), but Rod was unable to attend at the last minute. Kerry filled in admirably on this topic and presented much of the information that Rod had intended to cover. Kerry also had quite a selection of greases, oils and fuel additives available for purchase at the roundup.

Jim Kanamoto of Applied Air Filters is the supplier of 3.55:1 and 3.70:1 final drives for the GMC motorhome. He gave a detailed presentation on the various final drives available, both from him and from other suppliers, and discussed the benefits of lower gearing, especially when pulling a toad or driving in hilly or mountainous areas.

Sara Robinson, a fairly recent owner of a GMC, gave an excellent presentation, "Traveling in Style," a Designer's View of GMC Interiors. Sara covered interior remodeling, use of fabrics, carpeting, color selection and coordination and the use of items such as dishes, throw rugs, and other accessories to adapt the GMC to match your own personality while still remaining practical and useable for travel and camping.

Our last presentation was by Bill Boze of Jorgensen & Co. of Modesto, CA who discussed fire extinguishers, their selection and proper use. He had planned a demo on actually putting out a fire but due to the dry climate, the State of California was prohibiting any use of fire at that time. Bill also checked fire extinguishers for anyone who wanted that free service, and he recharged many of them for a small fee.

Our wrap-up to the tech sessions was an "Ask The Experts" panel. I want to thank Chuck Aulgur, Jim Bounds, Ed Burner, and Bob Lamey for their participation. As usual our panel displayed a wide range of knowledge on the GMC motorhome and its parts and operation. If anyone has a question or a problem, this is definitely the place to ask about it.

We also had a Vendor's Area all week in which the vendors worked on GMCs and gave lots of demonstrations on "how to" subjects. Art Woodell displayed his Digipanels, Mac McNeal was there with his displays of dashes and gauges and did some installations, Jim Bounds of GMC Co-Op came all the way from Florida with three workers and did a lot of mechanical work. He even brought his tire truing machine. Darren Paget came down from Calgary, Alberta, Canada with his TZE products, Bob Lamey was also there and really wanted to just be on vacation, but he did several emergency repairs for attendees. Jim Kanamoto of Applied Air Filters installed his 3.55 and 3.70 final drive units.

We all learned a lot watching the vendors do their work, and they were very free with advice and suggestions for those who wanted to learn how to do things themselves. We thank them all for coming to our roundup. We want to make this a continuing event as many members have indicated that this is definitely one of the reasons they are coming to the roundups.


Some time ago I had a request to provide information on how to build cornering lights for the GMC. Most luxury sedans and SUVs have them. They are handy for lighting up the area to the side of the vehicle so that the driver can see to the side when turning and perhaps avoiding hitting a curb or a ditch. They can also warn a driver coming up from behind that you intend to turn to that side.

The lights themselves can be done very simply. Either use a small 12-volt floodlight, such as a 4" chrome plated light, mounted to the bumper or go for something fancier such as a Cadillac or Lincoln built in cornering light and cut a hole in the GMC front fender to mount it. You should be able to find many of these in the junkyards (the GMCer Male Mall) or you can buy new ones from the dealers. If you decide to use bumper mounted floodlights, pay close attention to the lens design. You want to use ones that have a fluted or frosted design so as to diffuse the light and spread it over a wide area. Do not use spotlights or "road" lights that project a long, narrow beam. Instead use fog, backup, utility and implement lights. For example, Northern Hydraulics makes halogen tractor lamps (#16336) that are 100,000 candlepower rectangular sealed beams. The base is adjustable, up and down, and sideways.

If the cornering lights were to be hooked up to the turn signal lights, they would flash on and off. The harder part is converting the on and off electrical signal that goes to the right and left hand turn signals so that there is a steady, non-pulsing electrical current to the turning lights. There are several methods of coming up with a steady current to the cornering lights, ranging from a "brute force" method using a very large capacitor to using a programmed computer chip.

I don't recommend the "brute force" method as it involves using a 50,000 or greater microfarad capacitor wired into a circuit with a diode to the turn signal wire and going to a 12-volt relay. This capacitor is quite large, typically about 2 inches in diameter and perhaps 4 or 5 inches long. The capacitor will charge up when the turn signal turns on and keeps power to the relay long enough so that when the turn signal flashes off, the cornering light stays lit. I won't even bother drawing a schematic of this one. If someone really wants it, let me know and I'll mail it to him or her.

Probably the easiest way to build your own cornering lights is to use a special relay called a "cornering relay". This type of relay comes on when the turn signal is actuated. It has an internal heater that holds the relay on as long as it has the steady pulsing from the turn signal light lead. When you center the wheel or when you manually switch off the turn signal with the turn signal lever, it will quickly cool off and automatically turn off the cornering light. Besides just coming on when the turn signal lever is moved, the cornering lights will both come on when the emergency four-way flasher is turned on. This will provide even more visibility and attract attention to your vehicle. Fig. 1 shows a circuit diagram for this approach.

Fig.1 Cornering Light Wiring Diagram (Left Side Only)

The parts needed for this setup are a fuse and a switch, wires, two cornering relays and two heavy duty light relays plus wire connectors and possibly terminal strips depending on how you wish to wire it up. The cornering light relays can be found at the dealers, at the junkyard or at most auto parts stores. If you are getting them at the junkyard look for a wrecked Lincoln. This one carries the designation Sylvania R-104 or Motorcraft D74B-15A217-AA. A similar one can be found on large fancy GM vehicles. If you get the cornering lights off a junked car it should also have the relays. A Ford, Lincoln or Mercury dealer will have new ones available. The Ford part number is Motorcraft E35Y-15A217-A.

The light relays such as those made by Bosch or other manufacturers are available at most auto supply stores and at Radio Shack (RS-275-226).

It would probably be easiest to mount all the parts, 4 relays, fuse holder and a terminal strip on a 4" x 6" aluminum plate that you can mount under the front hood and ground to the chassis. The switch can be mounted by the dash so that the driver can use it to override the cornering lights.

The main power lead to the lights should be directly off a battery lead or, if you want the lights to work only at night, off a headlight lead. Do not use a lead that is on only with the ignition switch as the lights will draw a significant amount of power, and you don't want to have this much current going through the ignition switch. Use at least 16-gauge wire for all wiring and perhaps even heavier wire for the leads from the power source to the lights and from the lights to ground.

When you turn left, the left cornering light should stay on until the turn signal cancels. When you turn right, the right cornering light should stay on until the turn signal cancels. When you turn on the four-way flashers, both lights should come on. Now, when you go to park in a campsite at night, you can put on your four-way flashers and see both sides of the campsite when you pull in. No more excuses if you hit a stump, picnic table or your co-pilot who is guiding you in.

A third method of wiring up this project is to use a solid state circuit in place of the cornering relay. If you had to buy new cornering relays for the previous circuit, this should cost less. The circuit diagram of this approach is shown on

Fig. 2. One could use an n-channel mosfet to trigger the relay. The 1 M resistor may have to be changed to a value from 100K to 10M depending on the flashing rate of your turn signal. The 1 M should give a time on of about 1 to 30 seconds depending on the mosfet.

Fig. 2 Solid State Design

A fourth method using a programmed chip was submitted to me by Dave Mumert who can be reached at . Dave can supply a preprogrammed chip for anyone who wants to build it himself, and he might be able to also supply a ready to install kit. Dave is from Calgary, Alberta, Canada. His phone number is 403-284-3492. He says, "I have been using PIC microcomputers for a number of projects. They have a small 8-pin device that has 6 I/O pins. I programmed one to act as a timer for the cornering lights. It drives a relay when it sees the signal light go on and holds it on for 1.5 seconds. It starts retiming each time the signal light goes on. It handles both sides with 2 pins left over. I suppose I could add the suspension reminder light if there is any interest." You can contact Dave for a copy of his circuit if you wish.


PLEASE let me know if there is any particular topic that you would like covered in a future technical write-up. I would be happy to research any GMC related topic that might be of interest.

Enjoy your travels. We hope to see many of you at the next roundup in Prescott, AZ in May.

Window Repair Work at the Mariposa Roundup
Front Wheel Work at the Mariposa Roundup

 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.

 3128 Vista Sandia, Santa Fe, NM  87501-8526

 (505) 989-8157

 Please send your comments and ideas for the Tech Center to:
Emery Stora, Technical Vice President,
3128 Vista Sandia, Santa Fe, NM  87501-8526
Phone: (505) 989-8157



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