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Discussion Starter · #1 ·
Im trying to address an old problem which I never got around to. My fuel guage (SS style) cannot read past 1/4 tank when the tank is actually full. From there it eases its way down from 1/4 to E as the gas is used up. I replaced the sending unit and tested the restistance (90 ohm at full..if i remember correctly). When I disconnect the brown wire, the tank reads past full. Some research on this site makes me beleive that a resistor that is on the guage itself might have went bad. Here are my questions.

1 How do I replace this resistor (I never took out guages so I need details.. does the dash have to come out?

2 Where can I get a replacement resistor?

3 Can I work around this by adding a variable resistor somwhere between the sender and guage (at the brown wire connector), fill up the tank, then adjust the resistance until the guage reads full???


****EDITED****
Looks like option 3 is a no go as I remember when testing the guage at different resistance...
@47 ohms would read 1/8 of a tank
@97 ohms would read 1/3 of a tank

so the minimum resistance I could add is probably under 30 ohms which would not help get to full.

Looks like I need to either change the guage or resistor on the guage. Can someone tell me what is involved with this? Also I will most likely have to change the housing for the cluster as when I took out my dash pad, I noticed that the housing for the guages (the part that gets screwed to the dash itself) was VERY VERY brittle and WILL break when I try to either remove or put back the housing. What is involved with changing the housing? is there a harness that plugs into the housing or is there some soldering involved?

Sorry for confusing anyone..
thanks

thanks

tom
 

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The resistor is on the back of the gage. Looks like a white ceramic block with a colored strip on it. Can you get at it by removing just the dash pad?
 

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I can send you a used one out of a 77/78. Need an address. If you want send it to my listed email but include something like Team Chevelle in the subject line. The spam filter is set on max delete.
 

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Discussion Starter · #7 ·
I put the new resistor in. Seems like I still have the same problem (full reads a little past 1/4). Any other suggestions?

thanks

Tom

PS the new resistor has a blue strip whereas my original has a red strip. Does the color mean anything?
 

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No the color doesn't matter. I did mention by email to make sure that resistor did get good contact. It looked a little worn where the nuts would contact the trace. From the numbers you put up in your original post it looks like a meter problem. You seem to know how to check the gage and verify power and ground. I was hoping that the resistor would make the difference.
 

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There are windings in the sending unit similar to a rheostat on a volume controller if you unplug the brown wire and the gauge goes to full the gauge should be fine. The sending unit windings are probably broken which means pulling the tank. Not fun, I just did it on mine
 

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Here you go.

Problem Diagnosis
This process can be applied to all recent GM cars except those with computerized gauges.

1) Locate the fuel sender feed wire near the fuel tank. You're looking for a single tan colored wire. With the tan wire disconnected at the fuel tank, the fuel gauge should read past full with the ignition on. Wait a few moments as some fuel gauges take a long time to respond. Touch the tan wire from the body to any convenient ground and the gauge should read empty. If not, you have a wiring problem or a bad gauge.

2) If the gauge responds correctly, the gauge and wiring are OK. Next use a multi-meter to measure resistance to ground of the sender wire connection on the top of the fuel sender or the tan wire from the top of the fuel tank. Measurements should track the fuel in tank.

Full - 84-88 ohms
Half - 40 ohms, give or take
Empty - 0-2 ohms

If this doesn't check, then sender or wiring on top of the tank is bad or the sender not adequately grounded. Senders are typically grounded by a black wire which is welded to the sender and attached to the body with a sheet metal screw.

3) If the sender checks OK but gauge and wiring don't, clean the connections, reconnect the sender wiring and separate the Fisher connector (located just outboard of the fuse block under the dash). The gauge should then read past full. Ground the tan wire in the dash side of the Fisher connector and the gauge should read empty. If not, you probably have a bad gauge or possibly a dash wiring problem. Go to Step 5.

4) If the gauge checks OK, then make the same resistance checks to the tan wire in the body side of the Fisher connector. If the readings are different than those at the sender, body wiring has a problem and requires detailed inspection. If they look OK, then the Fisher connector is probably dirty.

5) Clean and reconnect Fisher connector, pull the connector off the back of the gauge and make the same resistance checks to the tan wire. If they don't check, you have a dash wiring problem. If they check OK, your gauge is bad. Gauges can be bench-checked but this is best left to a specialist.


Rocky
 

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Discussion Starter · #11 ·
the sender checks ok. 60 ohms (i have about 3/4 tank now).

just wanted to make sure the shunt resistor is connected ok. The stripe is facing out..

Should the resistor be under or over the flat plastic connections? I forgot how it was

thanks

tom
 

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Discussion Starter · #12 ·
ok... it might have been a bad connection. I not have it on like this..

Flat plastic wireing contacts on the guage.
resistor on top of the flat contacts with stripe facing IN. (between the contact and resistor I placed a brass washer.)

the tank now reads just under full.

Tomorrow I'll take the car for a ride (with a gas can
) and see if the guage moves.

Ill keep you all posted.

thanks

tom
 

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Resistor needs a good connection. I mentioned that the one I send was worn some. The stripe in or out doesn't matter as long as it gets a good bite.
 

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Discussion Starter · #14 ·
The fuel guage is now fixed. Apparantly I didnt get a good enough connection when I installed the resistor John gave me. I originally used the washers like he said but I guess I either didnt tighten the nuts enough or the resistor wasnt seated where a good connection was made. Pulling out the resistor and putting it back in resolved the problem. Thanks so much for you help..


Tom
 

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Ya, it was a good part but I could see that it was worn. Sorry for the trouble. Best one I had in the junk box.
Nice to hear when a plan comes together.
 

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Sorry to reply on an old post... I've had the exact problem with my gauge in a 71 ss clone. The dash has the big fuel gauge instead of the tachometer. I put a new sending unit in today with same results. Traced all wiring and resistance readings back to gauge and everything checked out. I did notice that the plastic circuit board is bolted on the outside of the resistor. I'll pull the dash and put the resistor on the outside and see if that works. Thanks
 

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Alright just pulled resistor off and looks like I'm missing some metal on there. (Posted pic) now I'm led to understand that all the resistor does is slow the current down. I hooked up fuel gauge directly to circuit board and it will only read quarter full. I have 32 ohms in back and 32 ohms to the circuit board. 12.6 volts to the gauge and 2.6 volts on the sender side. Wiring has been confirmed again that it's good. Will a new resistor fix my problem or does it sound like the gauge is junk? Thanks
 

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I put in a 40 ohm resistor off of amazon. That fixed my problem. I'm trying to figure out exactly how many ohms those ceramic shunt resistors were on the back of the fuel gauges(stock). I know the temp gauges are 84 ohms. I will test this next time I get gas and fill her up all the way and compare ohm reading at the tank and where he fuel gauge reads.
 

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I wanted to post this earlier but here it is anyway:

I have a 1966 or 1967 Chevelle Fuel Gauge that I did some testing with the other day and here are the results with and without the shunt resistor across the back of the gauge:

33 ohm WITH Gauge Shunt


75 ohm WITH Gauge Shunt


33 ohm WITHOUT Gauge Shunt


75 ohm WITHOUT Gauge Shunt


220 ohm WITHOUT Gauge Shunt


JIm
 
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