Before I go on a 3 state killing spree. I always thought a sender was a variable resisitor that used straight 12V for it's power source. As the resistance changed the needle showed accordingly. This is a GM (need you guess on what) 0-90 ohm. The reason I ask is I hooked a new 12V power source to it and it popped the fuse and I thought I smoked the sender. Is it a resistor to ground? I am confused to say the least. Is it resistored down to say 8.5 Volts internally in the fuse panel? The reason I ran a new hot wire was because I was reading low and the intank pump wouldn't work. Now I can get the pump to work but it is causing the fuel gauge to read open or full all the time. Sorry bout the rant but it's time for this problem to be a memory.
I am an electrical engineer and could probably help you with this problem but there are too many variables involved to venture a guess at this point. There are many ways to build a circuit using a variable resistor, without a schematic it's impossible to tell how this circuit is wired. However, if you are reading less than 12v it is very likely that this variable resistor is used to bias a transistor circuit. If you could get the schematic to me I will look it over and tell how the circuit is supposed to work....
See, that is a problem in itself. There are no schematics.I own the complete set of manuals and wiring diagrams for 1987 Chevy light truck thru heavy duty. This is something that used factory parts and harnesses but the coach mfg. had the tank added etc. so it doesn't show in the books. I know it sux.
There are only three wires. Blk is ground then there are 2 wires that go to the tank itself. A tan wire approx. 14 gauge and a pink wire approx.16 gauge? I had heard thachevy only uses tan wires for the senders but I think that may be wrong. I will ring out the tan wire tomorrow and see what happens.
I'm not knowledgeable on this. It has been my guess that the 12v that comes off the the switche goes thru the gas gage and to the sending unit which is like a rheostat. Is that near to the truth?
3 wires?? all my Fords send one to the tank. Ford Chevy and I think Chrysler work on the same principle but with different resistance ratings.
one is for the intank pump...see that is what I was troubleshooting to start with...this just happened. I had to repair the harness and when I removed it I got my wires crossed I guess.
The power goes to the gauge, the ground goes to the sender in the tank. Two poles on gauge, power and ground. Not a good idea to send power to the tank! Might be wrong, but thats how my Fords work. Just tryin to help, as I almost blowed myself up once, spark to the tank!!!...OLDBEET
<font color="green"> If you dig through this, there is some useable info. Even if you don't use their kit, the fuse block is a stocker out of something..... http://www.painlessperformance.com/manuals/90507m/90507mhome.htm </font>
http://www.vdona.com/Tech%20Support/Automotive/instructionsheetspdffiles/0%20515%20012%20178%20--%20Troubleshooting%20Guide.pdf
Root, One wire should be power to the electric pump, the other for the sending unit. Take the wires off the sending unit. Ground them one at a time. When you ground the correct wire for the sending unit, the gage will go way past full. Putting 12v to the sending unit may or may not have fried it. You're just lucky it didn't go BOOM!
Don't put the hot wire straight to the sender this will probably fry it. The wire from the gauge goes to the sender.
The hot 12v wire goes to the gauge and the other to the sender.The sender acts as the rheostat.Make sure you have a good ground from tank to frame,don't always count on the tank straps to complete the ground.You're lucky you didn't go BOOM when you hooked up that hot lead to the tank.That sender probably was like a toaster on the inside.Hope it didn't burn it up. Hope that helps? DAN
No worries fiends. The pump and sender were out of the tank laying on the ground when I hooked it up. I took the sender apart last night after posting this , cleaned the armature and checked the resistance with the sender disconnected. I had the right range from 0-90 ohms. So I assume it is still good? On one of the links Rashy sent me, it says to ground the tan wire and it should read empty. I am gonna try that today when I get home.
Root, the gas gauge is really just an ohm-meter, the sender is what supplies the variable ohms. So you should never have any voltage on the sender wire. The gauge will need a small 12v power to operate which is a different connection than the signal, but the signal is only resistance through the sender, not voltage. I think if I read correctly that your tan wire is 12v power for the in-tank pump. The pink wire should be variable resistance from the sender going to the signal input to the gauge. Black would be tank ground, as the pink wire goes through the variable resistance of the sender and needs to have chassis ground to complete the circuit.
Nope the tan is for the sender, the pink is for the pump and the black is chassis ground best I can tell. I got a buddy that is an electrician that is gonna gimme a hand after work. Hopefully I will have good news tonight.
Check your ground!!! I rebuilt a 90 chevy truck a while back and the gauge ate my lunch for a while. make sure where the black ground wire is attached to the frame it is clean. I had to use my grinder to clean mine enough. Also where the wiring attaches on the inside of the tank at the top of the sending unit make sure it isn't burnt looking or damaged in any way. cheverolet has a real problem with fuel pumps and sending units. any rust is a bad thing on the sending unit.
Roothawg - I'm sure you have it all figured out by now but I'll ad my two cents. To keep it easy for my simple mind, I always view the fuel gauge as the focal point. It needs both a pos(+) and neg(-) to operate. The pos.(+) comes from an ignition source and the neg(-) actually comes from the SENDER. If take the sender out of the picture and just ground the sender wire, you'll see the gauge go to FULL. So the sender is just a rheostat that controls how much neg(-) gets to the gauge....which determines what the gauge reads. Hope that helps. Reggie
Yeah, I think I had my theory all backwards last night. Now, I think my little brain can follow. Thanks guys. I swear... if this thing ever gets fixed, it will be banned from the hamb forever.
Well, I hate to say it but I fried the gas gauge. I hit several salvage yards and to add salt to the wound they all say "man I never seen one like that, that must be a special one". I guess I'll throw a VDO in there. The pump works now though.
Man I don't know... I am assuming I can just get a GM 0-90 ohm gauge. Parr has a VDO for 22 bux. Guess I'll get one tomorrow. The freaking gauge goes all the way to full with the key on but when you turn the key off it just stays there. Even after I pulled it out.
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