Another fuel gauge issue

[Norwester]

My '86 has dual tanks. About 8-9 months ago, I installed the GM Sports combo tach/fuel gauge and everything has worked flawlessly...until last week.

I filled both tanks and selected the RH tank. The needle pegged to the right and stayed there. I switched to the left tank and same thing....needle at the 3:00 position.

I switched back and forth several times and finally the left tank pegged and then settled back to the full position. The right tank just stayed pegged.

I was just messing with it again and both tanks would peg and then settle back to what appears to be the correct full position.

When I switch the tanks back and forth, I can hear the valve clunking.

What can I be checking? Thanks, Folks!

 

[highdesertrange]

if the gauge pegs past full it means an open circuit in the sensing wire. highdesertranger

 

[Norwester]

My understanding of automotive electrical systems is right there with my understanding of voodooism so be patient.

Would the fact that the gauge is finding its way back to the proper position be consistent with your suggestion?

Does the sensing wire affect both tanks? I guess I thought of it as essentially two separate circuits controlled by the switch.

Is this something I can access with the truck sitting in my driveway? I have no way to raise it in the air.

Thanks for your reply

 

[Billy]

Check your grounds. If unsure, disconnect and clean them. Good luck!

 

[chengny]

My understanding of automotive electrical systems is right there with my understanding of voodooism so be patient.

Would the fact that the gauge is finding its way back to the proper position be consistent with your suggestion?

Does the sensing wire affect both tanks? I guess I thought of it as essentially two separate circuits controlled by the switch.

Is this something I can access with the truck sitting in my driveway? I have no way to raise it in the air.

Thanks for your reply

Hey Dave-O. Jerry here. I thought that (given all your delayed wiper experience) you'd be a certified automotive electrician by now.

You ask a valid question:

Does the sensing wire affect both tanks? I guess I thought of it as essentially two separate circuits controlled by the switch.

And what you suggest is exactly right - up to the very last word.

There is only one lead connected to the gas gauge and that lead (PNK 30) does split off into two branch circuits (to cause only the in-use tank to drive the gauge).

It is only where you are thinking the split occurs that is wrong.

In your truck, the common PNK 30 lead splits off (into the PNK/BLK 830 & PNK/WHT 930 at the Pollak valve. So by far, most (probably 90%) of the sensing circuit is common - on the PNK 30.

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An open - in that PNK 30 - would indeed cause the gauge to react exactly the same no matter which tank was selected.

To trace the PNK 30 is wicked easy. For most of the way, it runs along the outside of the LH frame rail (in the same loom as the rear lighting leads). Just under the rear wall of the cab, it pops out of the plastic conduit, and after that it is on its on - over to the transfer valve.

 

[Norwester]

Hey, Jerry! Thanks for responding. Assuming I can find the pink wire, what will I be checking for? Loose connection? Bare wire?

On the delayed wipers, an interesting thing happened. Bear in mind, my delays have never worked at all........I was going through the car wash the other day and the wipers were set on delay. Twice during the wash, my wipers made a sweep. When I left, I tried to duplicate it but no luck. Got me thinking.....why did they sweep during the wash? Only thing I can come up with is maybe the very high pressure of the water got the linkage moving. The blades were jumping around like crazy......maybe my linkage needs a good lube? Can you think of any other reason?

 

[chengny]

Hey, Jerry! Thanks for responding. Assuming I can find the pink wire, what will I be checking for? Loose connection? Bare wire?

On the delayed wipers, an interesting thing happened. Bear in mind, my delays have never worked at all........I was going through the car wash the other day and the wipers were set on delay. Twice during the wash, my wipers made a sweep. When I left, I tried to duplicate it but no luck. Got me thinking.....why did they sweep during the wash? Only thing I can come up with is maybe the very high pressure of the water got the linkage moving. The blades were jumping around like crazy......maybe my linkage needs a good lube? Can you think of any other reason?

Assuming I can find the pink wire, what will I be checking for? Loose connection? Bare wire?

Very loose connection or a break in the wire.

Also, I think it may be a misleading coincidence that your gas gauge appears to be indicating correct (i.e. full) tank levels.

I was just messing with it again and both tanks would peg and then settle back to what appears to be the correct full position.

For whatever reason, the behavior you describe is generally how the gauge reacts when there is very high - or infinite - resistance in the sensing leg. They shoot way past full when power is initially applied and then the needle slowly falls back. It eventually settles at a point past the F (like when you pump the tank really full - as in up into the neck). The only way to know for sure would be to drive around. And then, the tank level is lowered, observe whether the needle tracks downward and continues to provide an accurate reading.

Here are some images of how the PNK 30 is routed:


Penetrates the firewall connector here:

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Enters the loom along with the rear lighting cable and the two leads that provide power to shuttle the Pollak valve.

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Exits loom under back wall of cab and is spliced to an extension lead - that runs over to the valve:

You must be registered for see images attach

 

[Norwester]

Thanks for the pictures, Jerry. They will help me locate the wiring. I went out and laid in the driveway and checked it out. My truck is lowered so I couldn't get under it very far, just my fat head. I did see a bundle of wires going down both inside frame rails. Maybe I can jack it up and get under there.

I've been driving the truck and the gauge seems to be coming down in what appears to be the proper rate. Of course I won't know until I fill it up.

Thanks again for your help.