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98 GMC C3500 HD airbag suspension????

Just bought a 1998 GMC C3500 HD diesel, auto, 19.5 dually wheels. The rear suspension has me a little confusion. The spring in front of the rear axle looks normal but the back half is bent down and then back. There is an airbag at this point that goes up to the frame. Like a coil spring. OK, that I get. Right now the airbags are collapsed and the frame is down on the axle housing. There are two steel tanks (4" x 15" long) under each frame rail at the rear and one steel tank (6" x 18") running across behind the rear axle. Under this larger tank is a unit (pump??)with a few wires and some sort of cable that looks like the emergency release on the tongue of a trailer for brakes. But the end is just connected to the frame.

Question is: where does the air come from? Is there a pump somewhere I cannot find? Is there a tire valve somewhere that I can put in air? Does it take air or some sort of gas?

This truck was used by the county Fire/Rescue service with one of those big ambulance boxes on the back. It looks like when they took the box off they just cut the wires going everywhere. There is a bracket on the engine, above the alternator, where they have removed what could have been another alternator or maybe the airpump I am looking for?

Would appreciate any particulars or just an overview on how this system is supposed to work. Sure rides rough with the frame sitting on the axle. Also need to located a factory service manual. Any sites online to buy one?

Thanks for your time, Chief

Reply to
Chief McGee
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By description it sounds as though the unit was equipped with a Granning rear air suspension system. This is a fairly common air ride system used on the GMC 3500HD or Ford F-450 chassis. Granning is still in business today, located in Brookston, IN if I am not mistaken. You should be able to find the on the Internet. The smaller tanks you describe as known as ping tanks which serve to enhance ride quality. The larger tank would be the reservoir tank. I believe what you are seeing on the bottom of this tank is the electric moisture ejector. The air source for this system is typically a

12-volt compressor, often frame mounted mid-way between the cab and rear axle. I believe the Granning often installed the compressor inside of a protective box. Some ambulance manufacturers will put the compressor in a compartment of the ambulance module. That may be the case here and obviously you didn't get that portion of the vehicle. It would be extremely rare that the unit would have had an engine driven compressor option. I am guessing that somebody removed the additional alternator that may have been installed to increase the amperage required to run the ambulance and it accessories. This should not be any type of Schrader valve that would allow you to fill the system from an external air source. You are right, until you get it repaired, it will ride horribly. I would limit driving the unit until you can get the system up and running. Hope this helps.

Reply to
Gary Picha

This got me on the right track. I found the company that built the truck originally. They are working with me. Thanks

Reply to
Chief McGee

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