R Model Fuel Tanks

[stev]

So, I recently bought my first Mack, a 1988 r688t, and am trying to address a few issues on it,

My first is the fuel gauge. It reads empty, all the time. Several knowledgable people have told me that this is common, and just to use a stick to measure the fuel.

If this is really common, the reason must be understood. Can anyone tell me what is likely causing this failure, and how i can fix it?

Thanx, this is only the beginning of several issues i have, hopefully I can get some assistance here to resolve them. . . . . .

[HK Trucking]

Open up the dash and check for current to the gauge with a test light. Key switch must be on. No current there means a blown circuit breaker or defective wiring on the "hot" side of the gauge, which must be fixed. If there is current to one terminal of the gauge, then ground out the other terminal on the gauge. If the gauge now reads empty, the gauge itself is bad. If the gauge now reads full, then there is a loss of continuity in the wiring between the dash gauge and the tank unit, or the tank unit is bad, or the tank has lost it's ground. Take the wire off the sending unit on the tank. With the key switch "on" ground said wire to a good ground on the truck frame. If the dash fuel gauge now reads full, then the tank unit is bad, or the tank has lost it's ground.

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[stev]

Thanks for the reply. It turned out to be no power from the ignition switch. I opened up the dash, and found quite a selection of unused wires back there. Some clearly had never been used, and some were just flopping around. A small number had been cut. I found one of those that was hot when the key was turned, but not when the key was out, and just connected it to the source side of the gauge, and it works now. I found this illustration:

http://www.tpub.com/content/firetrucksandequipment/TM-5-4210-230-14P-1/css/TM-5-4210-230-14P-1_922.htm

particularly useful.

Thanks for the quick answer. I assume that the draw for the gauge is small enough that i don't need to worry about which circuit this is on, do you think I should be worried about it for a different reason? Some diagrams for this on the Internet refer to a voltage regulator, but the above one does not, so I assumed that I was ok. I really need to get a service manual for is beast. I bought an owners manual off eBay awhile ago.