More Gas Gauge

[kdml]

I know, nobody needs another gas gauge thread. I have searched and still can't figure out my gauge problem. For background truck is hybrid '83/87. Drivetrain is all '87 with TBI and 700R with dual tanks. I think the only part '83 is the cab and vin. Gauge is combo tach and gas gauge.

Problem is the gas gauge only stays around a quarter tank no matter which tank or how much gas. This morning drivers tank was showing about quarter tank and passenger tank was showing slightly below quarter tank. When I changed the selector switch in the cab the gauge needle moved between these two measurements. Took the truck to the gas station and filled the drivers tank (took about 11 gallons). Gauge now read slightly more than quarter tank. Filled the passenger tank (only took 1 gallon) and gauge went to about a quarter tank, when switched.

What I have done so far:
1) Selector valve on the frame is about 1 year old. It was leaking so I replaced it. Everything worked after replacing.
2) Checked and cleaned the 2 grounds for the sending units on the frame
3) Inspected all wiring on the frame.
4) Removed switch on dash and inspected wiring. The switch is not original (see pics), but did work when I first bought the truck. I planned on replacing the switch with something more original looking (bought Standard DS455, but the new switch has 5 tabs for wires and my current switch has 4 (see pics), so I am not sure how to wire it)

Anyone care to chime in with next steps? Is there some way to check continuity at different points to try to isolate where the problem is? Also, what's the best method to check to make sure the tanks are actually switching, regardless of what the gas gauge says?

Thanks
Doug

[hatzie]

The gauge is switched by the valve after 1981.
That home brew polarity reversing toggle switch looks like one of my creations. It's perfectly kosher for this system and it'll probably outlast one of those cheap plastic AC Delco rocker switches.

You probably have a loose bias resistor on the gauge can or the gauge itself is bad.
This is a ceramic bias resistor but not necessarily the right one...


Pull the fuel gauge out of the pod without pulling the pod itself. Remove the nuts holding on the resistor and measure with a meter. If it shows a resistance between 30Ω and 100Ω then put a couple #10 external tooth star washers under the retaining nuts so you can get a bite in a different location on the ceramic bias resistor than the nuts have been riding for the last 30 years. Carefully snug the nuts back down. Re-insert the gauge in the pod and turn on the ignition switch. If the gauge and resistor works it should read normally now.
GM used several different bias resistors with different colored centers corresponding to the value. The 70-72 Chevelles used a yellow 90Ω resistor on the fuel gauge. These will probably be different.
I have a couple different year clusters at home. If I get a moment I'll record the bias resistances for the gauges.

[hatzie]

You have a new 73-80 rocker switch there.

The OEM switch for 87-91 RV series is AC Delco D7089C I recommend using this switch for 81-91. It only has 4 terminals and the crossover wiring is inside.

However... I'd keep the home brew toggle switch it should outlive both you and the truck.

[kdml]

Thanks
Will try to find some time next weekend to test and will report back results

[Skunksmash]

My advice,

toss that factory junker and get an autometer gas gauge. Then at least you'll know for sure its not the gauge itself. Doing this solved 2 of my problems with gauges

[kdml]

Pulled apart dash today. Resistance was approx. 88Ω. Added star washers to the 2 studs holding the resistor. No change. Both tanks are full, but still registers the same as before. Second pic is drivers tank, third pic is passenger tank. Any thoughts on next step?

[hatzie]

The readings aren't exactly the same... You'll need a decent Digital Volt Ohm Meter with fresh batteries to chase this. I keep four or five sacrificial male and female Weatherpack etc terminals in my meter bag to keep from damaging harness and sensor connections as I probe them. Part #s (12089040 or 12124582 or 12124587) Male and (12089188 or 12124580 or 12124581) female. Alligator clip test leads are a real nice thing to have too

Unplug the tank valve and see if the fuel gauge needle pins itself at the top of its' travel. If it does then the gauge is likely OK.
You can double check with a roughly 90Ω 1/2Watt resistor between a good ground and terminal B of the tank valve plug. The gauge should read Full.
If the gauge passes those it's probably OK.
Just to be sure... ground terminal B of the tank valve plug. The gauge should drop to E. Again if it does the gauge is likely OK.
You can test the 1/4, 1/2, and 3/4 tank reading with 1/2 watt resistors to simulate the sender as well...
Testing 90Ω GM fuel gauges with resistors - YouTube

Check both sender resistances by hooking the meter between the valve plug terminal A and a good ground then between terminal C and a good ground. If the tank senders are good you should get a reading of 90Ω ±4Ω with full tanks.

If the senders and gauge test OK then your problem most likely lies in the wire running from valve plug terminal B to the bulkhead plug or less likely inside the cab between the instrument cluster and the bulkhead plug. Check for anywhere the wire might've gotten crunched against a good ground but not dead shorted.

This is the detail of the valve electrical internals and the Weatherpack plug.

[kdml]

Quick update….

Unplugged the tank valve and the gauge moved to full for both tanks. Grounded terminal B and the gauge moved to empty for both tanks. So, I am feeling good about the gauge.

Attempted to test the senders. Not sure I was doing this right. Placed one lead of the meter in terminal C of the selector valve plug and grounded the other on the frame. Meter read 100Ω for both tanks. Moved the lead from terminal C to terminal A and the meter read approx. 52Ω for both tanks. Is this right? Was I supposed to be getting 90 for both readings or do the different terminals relate to different tanks?

Assuming I have a issue some where in a wire, is there a way to probe the wires along the way to try to isolate the area?

Thanks
Doug

[hatzie]

Gauge should be OK. But 100Ω should move the needle to full too. You can bench test these fuel gauges away from suspect vehicle wiring if it comes to that.

C should be the LH tank sender. 100Ω should move the needle just past the full mark... so that one should work properly.
A should be the RH tank sender. 52Ω should put the needle between the 1/2 & 3/4 tank marks.

If you're sure the RH tank is full then the RH sender or tank to valve wiring is suspect. But that doesn't fix the LH reading.

Try jumping B to C and look at the gauge. If B-C reads Full... try jumping A to B. If A-B reads over 1/2 then you have a switch contact problem inside the valve.
If it stays the same start chasing wire B to the firewall and from there to the gauge.

Don't forget to check the ground and power wire splices in the cluster wiring. Six wires become two somewhere not too far down the wrapped harness from the cluster plug. Motornut has posted some pics of these splices in several gas gauge and instrument cluster threads.

[kdml]

Little more testing today…

Unplugged both senders from the wiring harness under the truck and tested resistance at sender wiring connector. Still getting readings around 100 and 52. Pretty sure passenger tank is full, so maybe I have issue with passenger sender. Stripped some insulation off passenger sender wiring and tested resistance ahead of connector - still at 52, so it's not the connector. I assume the sender wiring is permanently attached to the sender so I will need to drop the tank to get the sender out for further testing.

Decided I would like to at least get one tank reading right so I am focusing on the drivers side as it reads 100 at the sender connector and at terminal C of the tank selector valve plug. I plugged in the tank selector plug and followed the wire from terminal B (which I assume makes it way to the gauge) and found a connector under the truck. Removed connector and tested resistance - still at 100, when switch in cab is set to drivers tank. Replugged in connector and continued to follow wire and did not find any issues.

I assume if the sender is reading 100, that resistance should make its way all the way to the gauge, so I unplugged the gauge from the pod and tested the resistance from the connection I have circled in the attached picture. It read approx. 100 when the drivers tank was selected. If I am getting 100 at the gauge pod and the gauge isn't reading full, doesn't that mean I have an issue with the gauge or am I misunderstanding how this all works?

I ran a similar test at the gauge for the passenger tank and got readings in the 50s. So it appears I am getting resistance readings at the gauge, but the gauge isn't reading right

Any thoughts?

[hatzie]

It may still be worth checking the harness splices but it sounds like the gauge died.

[kdml]

Thanks

I'll keep inspecting the harness, with the hope that I find a different answer, as finding a tach/fuel gauge combo like mine won't be easy. I've searched different vendors and only found a tach or a fuel gauge, not a combined unit.

[kdml]

Thanks

I'll keep inspecting the harness, with the hope that I find a different answer, as finding a tach/fuel gauge combo like mine won't be easy. I've searched different vendors and only found a tach or a fuel gauge, not a combined unit.

[hatzie]

That's a medium duty 40-60 series tach setup. Those seem to be harder to come by.

You may be able to use a working meter can from one of the large light duty 10-30 series fuel gauges.

You'll have to remove the indicator needles and the rivets and then source fasteners... Maybe #4-40 machine screws? You can get 4-40 nylok nuts.
I'd be very careful to mark the shaft so that you can get the indicator back on in the same orientation.

There are a couple gauge repair outfits. I've dealt with a guy in SC that seems like a straight shooter. He is pricy but you get what you pay for.

[rich weyand]

If that resistor is cracked, it may read right when you have it out and measure it, and be broken when fastened on the gauge. Mine was temperature sensitive: all summer, when it was nice to work on the truck, it was fine, in the winter, it did the quarter-tank thing on me. That is characteristic of a bad resistor. The gauge is expensive, the resistor is cheap. Change out the resistor.

Not sure which one you need: my parts book is only through 1978. Can somebody with the right book find the part number for the OP?

[hatzie]

That's a 1970's MD tach.
Here's the page from the 1970's Medium Duty Parts and Accessories Catalog 78TM I scanned earlier this year.
http://gmpartswiki.com/getpage?pageid=176797

http://www.corvetteforum.com/forums/...post1566948802
Lt blue-123Ω
green -89Ω
yellow-88Ω
orange-86Ω
blue black-81Ω

[rich weyand]

Some additional reading. Note what it says about 1/4 full readings.
http://www.chevyclassicsclub.com/the...ry-1965-newer/

Here's the one I bought for my 78.
https://www.classicmuscle.com/p-6237...-resistor.aspx

[Jeepwm69]

Here's the resistor I used. Worked fine and was $11.

http://67-72chevytrucks.com/vboard/s...6&postcount=21

[kdml]

Thanks

I will try a new resistor

[kdml]

Installed the new resistor today…problem solved.

I also discovered the problem with the passenger side tank…it was not really full. When I fill it quickly the gas pumps shuts off, but when I pull the trigger half way and fill more slowly the tank fills. So, I have a different problem, but it's not a gauge/sending unit/wiring problem.

Thanks again

[rich weyand]

Excellent. Thanks for following up.

[hatzie]

Originally Posted by kdml

Installed the new resistor today…problem solved.

I also discovered the problem with the passenger side tank…it was not really full. When I fill it quickly the gas pumps shuts off, but when I pull the trigger half way and fill more slowly the tank fills. So, I have a different problem, but it's not a gauge/sending unit/wiring problem.

Thanks again

Filler neck vent line restriction of some kind.

[rich weyand]

I have brand new 86 tanks on mine, with new filler necks, hoses, and vent lines. Some gas pumps will shut off on surging in the tank if the pump is on the second position. I always fill the tanks on the first click on the pump to get them full. Even so, some pumps will stop at 15 or 15.5 gallons, when I know I have sucked the tank completely dry. Other pumps will run to 16.7 gallons on a dry tank.