'87 R20 Fuel Gauge Not Reading Right

[Raffles]

I've made a couple of other posts on here re: my wife's truck, and my tribulations while fixing it. Today I was working on getting the gas gauge functional. Currently, it reads top dead center, regardless of actual fuel level. I just filled up the passenger tank completely today, but no change. While driving, you'll see it drift upward of the halfway mark, but it doesn't change otherwise.

Switching to the empty driver's tank also doesn't have an effect. I pulled the gauge and made sure the needle wasn't seized. Once I got under the dash, I started seeing some potential problems.

https://imgur.com/a/SZo4YYt

This album has what I found so far, including descriptions. Any ideas/suggestions?

 

[chengny]

I've made a couple of other posts on here re: my wife's truck, and my tribulations while fixing it. Today I was working on getting the gas gauge functional. Currently, it reads top dead center, regardless of actual fuel level. I just filled up the passenger tank completely today, but no change. While driving, you'll see it drift upward of the halfway mark, but it doesn't change otherwise.

Switching to the empty driver's tank also doesn't have an effect. I pulled the gauge and made sure the needle wasn't seized. Once I got under the dash, I started seeing some potential problems.

https://imgur.com/a/SZo4YYt

This album has what I found so far, including descriptions. Any ideas/suggestions?

Making any progress on sorting that mess out? I can help a little.

The orange lead is hot into the brake light switch (shown below) and the white is switched power out. After passing through the MFS in the steering column, it becomes part of the rear lighting harness - carried on the same leads as the directionals (YEL & DK GRN). They must have required a clean brake light signal - one which was segregated from the turn signals. And that is why they hacked into the harness between the brake light switch and the MFS - that section of the circuit is the only place to get that pure brake signal.

This is normally only done when a heavy class trailer is going to be pulled. But it has nothing to do with activating the brake lights, rather it is to activate an electric braking system - which is required on heavy trailers.

As I am sure you can imagine, the leads in the rear wiring harness would not be acceptable (because every time the directionals were operated, the trailer brakes would be applied at the same frequency - so nothing after the MFS could be used to apply the trailer brakes).

As for all that other "wiring" - what that is for is anyone's guess. I notice you have an after market radio installed, maybe its for that - IDK. But, I can tell you for certain that, GM did not use in-line fuse holders in their under dash wiring harnesses.

As for the gas gauge issue: have you eliminated both the tank transfer valve - and dash switch - as possibly being the problem?

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[RetroC10Sport]

I need to dig in to Bertha's fuel gauge not reading at all. I'm really really good with wiring if you need assistance.