Evening Fellow Fiero Fanatics!

I just finished reading 18 threads that turned up under search "Fuel Guage" and I did find a couple that are similar to my problem. I'm just looking for some re assurance that I am on the right track. I hate to drop the tank if there is another place I should be looking.

Here's the problem with some history!

Awhile back after the 4.9 swap, the fuel pump went out. At that time, the fuel gauge was working semi accurately - the basic "I'd better be looking for gas at a quarter tank becuase you never know when it's going to run out". So when I dropped the tank to replace the pump, I decided to do the "sending unit rehab". After I put it back together and turned on the key. Gauge went from 1/4 tank to pegged above full.

Today, I tore apart the dash and checked out the gauge itself because even with the pink wire disconnected, the gauge was pegged above the full mark. I thought that it may have been stuck. The guage checked out and when I put it back together, I set the needle to empty. Then I turned on the key and the gauge pegged above full. Key off, it would come back to 1/2 or less! So I'm thinking I have a problem with the sending unit in the tank? Does this sound about right? I have traced and checked all the wires - everything looks good and it was "semi" working before I messed with the sending unit.

Here it is with the key off:


And with the key on:


Thanks in advance!

Pat

Disconnect the fuel tank connector and see what it does open circuit. <- This was a stupid answer . . . I guess it's just been a long weekend, please forgive.

[This message has been edited by 86FieroSEv6 (edited 12-19-2010).]

Looks to me as if the sending unit wire is grounded out and reading infinite OHMS. Disconnect the pink wire from the gauge, and connect it in series with your multimeter and a known good ground, see what the resistance is showing in the wire.

quote Originally posted by Hulki U. My-BFF: Looks to me as if the sending unit wire is grounded out and reading infinite OHMS. Disconnect the pink wire from the gauge, and connect it in series with your multimeter and a known good ground, see what the resistance is showing in the wire.

Respectfully I differ

Grounded out would be 0 ohms

Open would be infinite ohms.

The sending unit in the tank is supposed to be at a resistance of 0 ohms when empty, 90 ohms when full. Unplug the yellow connector and read the resistance of the sending unit in the tank. You will read this across the pink - to black wire. The resistance should read between 0 and 90 ohms. If the meter reads infinite (way more than 90 ohms) then the trouble is in the sending unit or wiring inside the tank.

The full pegged right you are getting would be an indication of an open. One more test you can try before you drop the tank is to connect a short between the pink wire and ground. With a short between the pink wire and ground you should read empty (key on)

Also with the yellow plug disconnected so nothing is connected to the pink wire, the gauge should read past full (like yours is) with the key on.

The way the gauges work on 80's GM cars is that with the key OFF what the gauge does means nothing. Some will stop right where they are at. Some will move somewhere and stop. Some might move part way to somewhere and stop. It doesn't matter if the key is off. It means nothing.

[This message has been edited by phonedawgz (edited 12-19-2010).]

Thanks everyone for your replies.

Phonedawgz,

So are you saying I can put a jumper between pink and ground, turn on the key and the gauge should read empty? Just want to make sure I have it right before I smoke something! Thanks again for your reply.

Pat

Yes - unplug the yellow connector on the firewall right behind the passengers seat (IN the engine compartment)



This is the tank side of this connector.

There are three wires on it.

Pink - Fuel Gauge
Tan/White - Fuel Pump Power
Black - Ground.

At full the fuel tank sender should have 90ohms resistance to ground.
At empty the fuel tank sender should have 0 ohms resistance to ground

So unplug the connector, and on the CAR (not tank) side of the connector connect a small jumper from Pink to Black and with the key on, the gauge should read empty. Take the jumper out and the gauge should move up to past full.

I agree with Phonedawgz. You have on open circuit from the gauge plug (C3-7, PNK) to the sensor plug (C502-B, PNK) or in the tank itself. I would first put a meter on the C3 plug (pin7) and check to ground and see if you have a resistance value beteen 0-90 ohms. That will tell you if you are getting a signal from the sending unit or not. I suspect it is open (reading infinity< or some other extremely high value). If this open, you have found the root of the problem and just have to figure out if the sending unit itself is bad or perhaps the wiring between it and the gauge. Checking the plug coming from the tank (C502) between pin-B (PNK) and pin-A(BLK) should give you that resistance between 0-90ohms. If it is not there the sending unit is bad. If you want to just check the gauge, you will have to find a way to insert a resistor of a know value (ideally around 45ohms) between the C3-7 (instrument panel side) and ground. 45 ohms should give you around 1/2 full. Using this method you will keep from frying anything as long as you connect to the right pin on the C3 dash plug. Good luck.

Thanks so much Phonedawgz and olchap. I'll dig into it this weekend.

Pat

Butting in...
Not trying to put a monkey wrench in the assistance here, but 0 ohms would put the needle at the "F", correct? Katatak's needle is pegged to the far right, indicating far less than 0 ohms. Would it not be receiving power on the sending unit's pink wire?
I am just trying to make sure I understand what I was seeing.
Thanks.

-Dave

adding:
I have seen this after dropping my gas tank due to fuel delivery issues:



That was a new (less than 100 miles / 1 year), Fiero store purchased, sending unit.

[This message has been edited by IFLYR22 (edited 12-22-2010).]

quote Originally posted by IFLYR22: Butting in... Not trying to put a monkey wrench in the assistance here, but 0 ohms would put the needle at the "F", correct? Katatak's needle is pegged to the far right, indicating far less than 0 ohms. Would it not be receiving power on the sending unit's pink wire? I am just trying to make sure I understand what I was seeing. Thanks. -Dave adding: I have seen this after dropping my gas tank due to fuel delivery issues: That was a new (less than 100 miles / 1 year), Fiero store purchased, sending unit.

No, the guys have it right, full swing to the right = high resistance or open circuit. 0 resistance = empty tank. He didn't mention it but I believe he missed performing a full, and empty test of the sending unit after working on it. It sounds like an easy issue to address but those old parts particularly the resistance board is very picky and after tampering with it you must test it before reinstalling it or there's a good chance you'll have to drop the tank again.

Sounds like the tank is going to have to come out again. If it is, may i suggest you do this and fix it once and for all. https://www.fiero.nl/forum/A...090219-2-078597.html Enjoy

[This message has been edited by DLCLK87GT (edited 12-22-2010).]

I hear you on the seemingly backwardness of the gauge. The Oil Pressure sender works the same. 0 ohms is 0 psi and 90 ohms is supposed to be the max standard gauge deflection. Disconnect the OP sender wire and the OP gauge will read PAST full.

The resistance is also printed on the electrical prints in my Haynes manual. I would think it's also printed on the GM service manual prints.

quote Originally posted by Joseph Upson: No, the guys have it right, full swing to the right = high resistance or open circuit. 0 resistance = empty tank. He didn't mention it but I believe he missed performing a full, and empty test of the sending unit after working on it. It sounds like an easy issue to address but those old parts particularly the resistance board is very picky and after tampering with it you must test it before reinstalling it or there's a good chance you'll have to drop the tank again.

Thank you for clearing this up. I was not totally understanding what I was reading, so I asked.

-Dave