Gauge/Idle

[Shawn]

For whatever reason, my gage/idle fuse keeps blowing intermittently.

I have no idea what is causing it. I've started removing the PO's aftermarket alarm system to get rid of extra wiring (that was done terribly).

I can't find any bad looking grounds.

My neighbor grabbed a copy of the '87 GMC wiring book from his work & I copied it & printed my own. Tracing wires & pulling out what hair I have left.

Any ideas?

 

[Jims86]

For whatever reason, my gage/idle fuse keeps blowing intermittently.

I have no idea what is causing it. I've started removing the PO's aftermarket alarm system to get rid of extra wiring (that was done terribly).

I can't find any bad looking grounds.

My neighbor grabbed a copy of the '87 GMC wiring book from his work & I copied it & printed my own. Tracing wires & pulling out what hair I have left.

Any ideas?

1st, try replacing the fuse, and unplug the connector on the brake switch with the purple wires, and see if the fuse keeps blowing. your trans wont go into lockup during this time, but you need to get to over 45 mph....if it doesent blow, ther is possibly a short in the power wire to the The lockup solenoid.
also,
Check the fuel pump circut. pay special attention to the grey wire. also, check the plg to the fuel pump relay its self for charing.
that same fuse goes to the instrument panel, via a pink wire with black stripe. it is in the plug that goes to the printed circut board, pins 4 and 17.

 

[Shawn]

1st, try replacing the fuse, and unplug the connector on the brake switch with the purple wires, and see if the fuse keeps blowing. your trans wont go into lockup during this time, but you need to get to over 45 mph....if it doesent blow, ther is possibly a short in the power wire to the The lockup solenoid.
also,
Check the fuel pump circut. pay special attention to the grey wire. also, check the plg to the fuel pump relay its self for charing.
that same fuse goes to the instrument panel, via a pink wire with black stripe. it is in the plug that goes to the printed circut board, pins 4 and 17.

I'll have to try the brake switch when I get back from my trip. I'll be in Idaho until Thursday night.

The reason I replaced the fuel pump relay earlier was I was trying to find the reason my fuel pump would randomly turn on after shutoff for LONG periods of time, eventually draining my battery dead. The fuel pump relay was charred at 120 (the center connection to the tan/white wire). The connector was also charred a bit, but I couldn't find a replacement at AutoZone so I replaced the fuel pump relay only. I used electrical connection cleaner on the connector and plugged it back in.

Here's a picture of it I took:

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[Jims86]

I dont know why they have 2 FP systems, but i think you have the top one.
only the 1 tons have the hot fuel module.

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[foamypirate]

I dont know why they have 2 FP systems, but i think you have the top one.
only the 1 tons have the hot fuel module.

One is for C/K trucks and the other is R/V.

 

[Jims86]

One is for C/K trucks and the other is R/V.

Good catch...i have a C/K harness in mine, thats why I said grey wire for the fuel pump. So, The attention should be on the tan and white wire. thats the main wire to the fuel pump. Seeing that burned like that, as I have seen many times, I would go ahead and inspect the entire fuel pump power line, from the Fuel relay to the pump. You will have to drop the tank, or lift the bed...it will be worth the work. the best way to do it, is to unplug at the tank end, and probe between the charred terminal and a ground with a meter set for continuity. it should indicate open if there is nothing chaffed or chewed, if it shows closed, there is a short rubbing against the frame or body.

 

[Shawn]

Good catch...i have a C/K harness in mine, thats why I said grey wire for the fuel pump. So, The attention should be on the tan and white wire. thats the main wire to the fuel pump. Seeing that burned like that, as I have seen many times, I would go ahead and inspect the entire fuel pump power line, from the Fuel relay to the pump. You will have to drop the tank, or lift the bed...it will be worth the work. the best way to do it, is to unplug at the tank end, and probe between the charred terminal and a ground with a meter set for continuity. it should indicate open if there is nothing chaffed or chewed, if it shows closed, there is a short rubbing against the frame or body.

Thanks! Now I'm just going to spend until next weekend thinking all about it until I get back to the house...

 

[Shawn]

1st, try replacing the fuse, and unplug the connector on the brake switch with the purple wires, and see if the fuse keeps blowing. your trans wont go into lockup during this time, but you need to get to over 45 mph....if it doesent blow, ther is possibly a short in the power wire to the The lockup solenoid.
also,
Check the fuel pump circut. pay special attention to the grey wire. also, check the plg to the fuel pump relay its self for charing.
that same fuse goes to the instrument panel, via a pink wire with black stripe. it is in the plug that goes to the printed circut board, pins 4 and 17.

What problems will I run into if I unplug the brake switch? I'm finally home and want to try it out. What do you mean by the trans won't go into lockup?

 

[Jims86]

What problems will I run into if I unplug the brake switch? I'm finally home and want to try it out. What do you mean by the trans won't go into lockup?

That break switch that you are unplugging, interrupts the hot sidenof the torque converter lockup solenoid when you hit the brakes, one of the many circuts on that gauge idle fuse. wont hurt anything to drive it a few miles just to see if thats the problem.

 

[Shawn]

That break switch that you are unplugging, interrupts the hot sidenof the torque converter lockup solenoid when you hit the brakes, one of the many circuts on that gauge idle fuse. wont hurt anything to drive it a few miles just to see if thats the problem.

So I need to go over 45mph to test it though, right? Sorry, I'm learning everything as I go. I'm having fun learning for sure, but using Google a LOT.

Most work I ever did to any vehicle before this truck was change the oil and I helped my grandpa change the brakes on his van when I was 12.

I'll find the brake switch and test that sometime today after work.

 

[Jims86]

So I need to go over 45mph to test it though, right? Sorry, I'm learning everything as I go. I'm having fun learning for sure, but using Google a LOT.

Most work I ever did to any vehicle before this truck was change the oil and I helped my grandpa change the brakes on his van when I was 12.

I'll find the brake switch and test that sometime today after work.

Most tbi systems are programed to lock up at around 34 mph in third, and 38 mph in od.
I would do the electrical testing on that fuel pump line first, since there is evidence of a problem there. should never get hot enough to char a plug.

 

[Shawn]

I've almost got the fuel tank all the way out. Some hard to get to bolts. Just need to disconnect the connections all the way.

Only reason I had to stop was for a wedding, then right back to it!

 

[Shawn]

Found this:

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Gray wire is 921B and goes into fuel pump.

 

[Jims86]

Found this:

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Gray wire is 921B and goes into fuel pump.

Score!

 

[Shawn]

Also fixed this behind the dash (PO removed driver's side tank).

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