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STRANGE fuel pump/rear window problem, out of ideas

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bowtieboy42

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Hi everyone. Hopefully someone can point me in the right direction here. I have an 87 k5, with a 350. Earlier today I put my rear window down to access the tailgate, and the window wouldn't go back up. This has happened once or twice, but it has always fixed itself quickly after driving a bit, so I assumed it was a loose wiring issue.

Anyway, I set out (in the rain, with the back window all the way down), and my truck stalled while driving, which it never does. When it did stall, the fuel gauge on the dash spiked to full, and past. When I restarted the truck, the gauge dropped back to normal.

I got a new fuel pump relay, and no help. The truck starts and idles fine, but when driving around it will intermittently loose power to the fuel pump, truck stalls, gauge spikes. I can actually start to see the gauge rise som etimes before it stalls, and giving it a lot of throttle seems to help.

My first thought was a bad ground, so I cleaned up the ground on the top of the frame rail. While messing with it in my friends garrage I started to notice some odd things (this is where it gets weird)

-When I cycle the key in the ighnition I don't hear the fuel pump prime (truck still starts fine, I'm aware of the oil pressure override)
-When I push the window switch in the cab, every 3-5 presses of the "window up" switch causes the fuel pump to prime
-Using the key in the tailgate does not produce the same result (no fuel pump prime).
-Either way the rear window won't go up, and the fuel pump is still intermittently having problems.

I tried finding wiring diagrams, but they were general k10 diagrams and none included the rear window switch. I'm guessing its some sort of electrical gremlin, but I'm not even sure where to begin, or if this is a common problem at all.

Anyway, thanks in advance for the help! Hopefully Ill get this old k5 back on the road soon
 
Its almost certainly a ground. When you press the window button........Wait a minute.

A ground is still possible, but it sounds like a main power feed issue. I'll check my schematics, but I would check the fusible links under the hood, and where they hook up a the starter.
 
Red wires coming off the starter solenoid going up to the firewall. They can melt inside the insulation, so you need to pull on them slightly to see if they stretch.

Also, if you want, just to eliminate a variable, you can get a piece of heavy wire, 6 gauge or so, and run it from the battery negative to the tank or its ground.
The gauge spiking, combined with the pump stopping, is a loss of power to the tank. And it has to have ground as well as the positive to work.

What throws me, is it working when you put power to the window switch, but its possible that doing that is supplying a ground to the rear area somehow.
Just because the ground connection is nice and clean, it does not mean that the wire its self is good.
A temporary known good ground is a quick and dirty way to test it.
 
Okay, I checked the main power feed off the battery to the starter and alternator, no fusable links. I'll go check from the starter back in a second.

The ground wire off the tank looked to be intact. Where would I run the temp ground to on the tank?

Also I noticed after letting it sit for a while it would prime on the key turn, only on the first turn though, repeated key turning did not produce multiple primes (not sure iof it should). However multiple window button pushes ALWAYS produces another prime.
 
Okay two of the 4 wires off the starter have little 2 inch black round foamy pieces, are these the fuasable links? Also after each of these things theres about an inch of bare wire. Not sure if these were bouncing around and shorting off things. I'll tape them up and see how it goes.
 
No change, still loosing the fuel pump intermittently. And the back window still won't go up.

If its still screwy tomorrow I guess I'll try to make that temporary ground.
 
From what I can tell, 87-88 was a change over year. I have schematics showing me two different things.
If the two wires off the starter are purple, then one of them seems to have an inline fuse and that wire changes to purple/white after the fuse.
You may have a single purple wire all by its self on a terminal. That is the wire that turns on the starter, and is not one of the ones I mean.

If you have the purple-purple/white wire, then the fuse should be a 3 amp and powers the engine control module. If you were not getting full power to it, then you might have these problems.

Most of the time when you have weird stuff like this, its a bad ground, and things are getting grounds from other places.
Sometime the bad ground can be a long way from the apparent problem.
I fixed a intermittent fuel pump in a Buick that turned out to be a bad master ground behind the driver's side kick panel inside the cab.

Before you run the temp ground, double check the grounds around the battery. Many trucks have a little short ground wire from the battery terminal to the body.
The reason I bring these up, is because they are easier to check than running a temp ground, and if you find one bad, you may not have to.
Check the easy stuff first.

The fuel pump in the tank and the fuel gauge sending unit share a ground. If you lose it, then the gauge will go flaky and the pump will quit.
Unfortunately I do a lot more work on Fords than GM, and I am not sure where that unit gets it's ground.
If it grounds to the tank, and there is not some corrosion where the unit bolts into the tank causing the problem, then if you can get a good ground to the tank it should stop the trouble.

Since this is meant to be just a temp ground to see if it stops the problem, you can be fairly creative about hooking it up.
If you can get to the edge of the tank, you could scrape a clean spot, and clamp the wire to it with vice grips.
Kind of a chintzy way to do it, but we are just doing a test, not a permanent modification.
 
Okay, I went out today to check it out and now the gauge is pegged to full all the time (even with the truck off), and the truck won't start (dead fuel pump).

I wasn't sure if I could just ground off the tank or if it had to be off the sending unit/assembly so I went ahead and dropped the tank. I tried running a 0 gauge wire from the top of the pump to the frame, gauge is still pegged, no change.

The battery grounds to the frame and the body, both of these look good, but I'll go ahead and clean them up anyway. I'll also check out the power supply again, although I searched it as far up the firewall as my arms would fit and couldn't find a fuse.

Wish me luck! I'll keep you posted. Thanks so much for all the help
 
OKay, a few updates:

Got the window up today. Although now the pass side doesn;t work. As odd as this sounds, its normal for one to work and not the other. The pass side window actually doesn't work a lot.

The temp ground from the tank to the frame didn't change anything. My buddy had his multimeter and the resistance from the battery to the frame/truck was good, meaning the frame should be grounded correctly? (all while the gauge was still pegged).

We jumped the power wire in the relay harness, and the fuel pump primed as it should. After we did this, the gauge dropped back to normal and the truck started and idled. Around the same time I also noticed the ground from the relay itself to the truck was broken. I ran a piece of thin wire straight to the battery.

However after running it a little and turning it off, the fuel gauge eventually spiked again and the truck won't start. I'm thinking the window was unrelated after all. However even with the relay grounded its still spiked. And now its staying spiked, and even jumping the power in the relay harness won't make the pump prime. Is it possible its just a dead pump that I'm over-diagnosing? Or do you think the random spiking of the gauge definitely points at an electrical issue?

I'm gonna get back out there and keep messing with stuff and I'll keep you posted. Thank god I have a motorcycle to ride while the truck is down.

Thanks again!
 
Okay, well I think this update will be most definitive,

At first I didn't run a temp ground straight to the battery as I didn't think I had any long enough thick wire, but then I split my set of jumper cables, and I had it haha. Anyway, after grounding the tank, the gas gauge DID drop back to its normal position. However the truck still wouldn't start, and when I tried to jump the primer I realized it still wouldn't prime. I took a hammer to the top of the tank and after 2 taps it started to prime.

So I guess what I'm thinking is the fuel pump is just dead? Maybe the pump itself has an electrical problem? The fact the the pump would only prime intermittently under direct current (and only after some taps) has me thinking the pump itself is just dead. But if you have any other thoughts I'd love to hear them as I have no money for a fuel pump right now (and I refuse to buy another airtex POS pump)

Also looking on rockauto, I see a lot of separate pumps/sending units. When I did mine before, I got the whole assembly as one. To be honest I'm a little confused of the actual difference between the two... Is it one of those things that while I'm there I might as well change the whole thing? Or is it likely only the sending unit (like a solenoid on a starter?) that needs replacing?
 
OK, just got home. Believe it or not, I think we are about to find the problem, or at least isolate it.
First, I want to apoligize, I had never typed at you before, and had no idea of your abilities.
So, I was trying to err on the side of caution, and was doing everything I could to avoid suggesting dropping the tank.
Now that it is down, we can find it once and for all.

First, lets find out what is wrong with the gauge. Hopefully it is still pegged. If so, unplug the connector at the tank.
You should see a pink wire, and a black/white wire. I'm going off a 1990 book and schematic at this point, but it seems to agree with my '87.
With the key turned on, and the gauge pegged, hook a short jumper from the pink wire to a good ground.
There should not be enough current there to even notice, but if you have a wire with a 2 or so amp fuse and want to use that just in case, go ahead.
The gauge should go to empty.
If not, then the pink wire is probably bad somewhere.
If it goes to empty, take the jumper loose and it will go back to full.
Then use the same jumper to jump from the pink to the black/white. It should go to empty again.
If not, then the black/white ground wire is bad.

Every time you get a good reading, give the harness a good shaking and pulling in case there is a broken wire in there somewhere.


I kinda suspect that you are going to find that the black/white wire is not supplying a good ground.
That would cause problems with the gauge and pump.

If not, then you could check the pins those two wires hook to on the tank with an ohm meter.
It should read between 0 and 90 ohms.
My other mistake, was having you ground the tank. Older units got their grounds from the tank body, but yours has a separate ground.

As for the pump, hopefully that ground will turn out to be bad. If not, then it could be bad.
I have a place marked on my two car's tanks to hit with a 2X4 if they fail to crank. They are Mercedeses, and the head mechanic at the dealership showed me where to hit.
When the pump gets worn, or gets a piece of trash in just the right place, it can fail to start.
If you turn the key to crank and whack the tank where it touches, you will often jar it into running.
He said if it cranked when I did that, do not turn it off until I get it to the shop or home, because it probably will not crank again.

And I have seen my mechanic friend drive three other cars and trucks off the roll back wrecker by doing that.
Sometimes I do the cranking and him the whack, and sometimes its reversed.
Either way, its an easy way to get it in the shop, and he goes ahead and replaces the pump, because it will do it again.
So, it it turns out to not be the ground for the pump, you will either have to save up for a new one, or rig a rope with a pivoting weight that you can pull to slam the tank.............
 
That idea reminded me of one customer who came into the parts store I worked at,saying he thought his starter might be bad--but had little cash,so he wanted to ensure it wasn't the solenoid on his Ford pickup that was not letting it crank sometimes....so I went out to the truck,he showed me how it failed to crank by turning the key,and I ensured it wasn't the neutral safety switch by jumping the solenoid,it still refused to crank...until I had him tap on the starter with a hammer...he seemed delighted to learn this new "trick",and said "Cool!--I'll see if I can get the money up for a starter by friday--I'll be back to get one from you if I can"...........................................................................................................................................................................................................A good 3 months later,he pulled into the parking lot...asks me if I sell new batteries,and I said "yeah,but what about your starter"??..I figured since he was short on cash,he had just got a used one from a junkyard..."Oh,its still going strong", he replies...when I went out to see what size battery his tray would accept, (he wanted the biggest one he could stuff into it",I saw a claw hammer hanging next to the starter,suspended by a screw eye in the end of the handle to the firewall with some wire,and a length of twine was wound around the handle,that led back thru the firewalll into the cab........................................................................................................................................................................................................I just rolled my eyes,as he showed me how "great it worked"...he turns the ket to crank it,then tugs on the twine,the hammer strikes the starter,and VROOOM!....I think it lasted a good year,before he finally bought a new starter!...I told him he was braver than I,to trust driving a truck like that in some not so good neighborhoods--he said "ahhh,if it dont crank someday,its a standard,I'll just get a push and pop the clutch!...........................As for the fuel pump problems,lots of new fuel pumps come with a new plug you must install on the harness,as GM's had a lot of issues with them not making good contact and having intermittent power & ground issues....personally--I hate electric "in tank" fuel pumps...nothing but a pain in the butt ,when one dies and your forced to drop the tank,and spend up to 200 bucks to buy one...I miss the days when you could use anything for an emergency gas tank,when the mechanical pump on the engine was easily replaced in 30 minutes for less than 20 bucks!...nowadays your boned if your fuel pump decides to quit,you need a tow truck and 300 bucks to get it to start again...is that "progress"...not in my opinion!....theres no good reason why an external pump couldn't have been used....
 
My 89 tank that I have sitting on my bench has 0 ohms from the ground strap to the tank body. Also when using the tank as a ground and reading the sending unit with my ohm meter it functions. And the pump also operates when I supply power to it with the tank as the ground.

Edit: I don't know why you're using such a large wire to operate the fuel pump. I can make mine operate using a 9v battery (hint hint).
And for those who don't know, the reason for the ground strap is because the gas tank is electrically isolated from the rest of the truck due to the anti-squeak pads on the tank straps and on top of the tank.
 
Last edited:
Fordum said:
OK, just got home. Believe it or not, I think we are about to find the problem, or at least isolate it.
First, I want to apoligize, I had never typed at you before, and had no idea of your abilities.
So, I was trying to err on the side of caution, and was doing everything I could to avoid suggesting dropping the tank.
Now that it is down, we can find it once and for all.

First, lets find out what is wrong with the gauge. Hopefully it is still pegged. If so, unplug the connector at the tank.
You should see a pink wire, and a black/white wire. I'm going off a 1990 book and schematic at this point, but it seems to agree with my '87.
With the key turned on, and the gauge pegged, hook a short jumper from the pink wire to a good ground.
There should not be enough current there to even notice, but if you have a wire with a 2 or so amp fuse and want to use that just in case, go ahead.
The gauge should go to empty.
If not, then the pink wire is probably bad somewhere.
If it goes to empty, take the jumper loose and it will go back to full.
Then use the same jumper to jump from the pink to the black/white. It should go to empty again.
If not, then the black/white ground wire is bad.

Every time you get a good reading, give the harness a good shaking and pulling in case there is a broken wire in there somewhere.


I kinda suspect that you are going to find that the black/white wire is not supplying a good ground.
That would cause problems with the gauge and pump.

If not, then you could check the pins those two wires hook to on the tank with an ohm meter.
It should read between 0 and 90 ohms.
My other mistake, was having you ground the tank. Older units got their grounds from the tank body, but yours has a separate ground.

As for the pump, hopefully that ground will turn out to be bad. If not, then it could be bad.
I have a place marked on my two car's tanks to hit with a 2X4 if they fail to crank. They are Mercedeses, and the head mechanic at the dealership showed me where to hit.
When the pump gets worn, or gets a piece of trash in just the right place, it can fail to start.
If you turn the key to crank and whack the tank where it touches, you will often jar it into running.
He said if it cranked when I did that, do not turn it off until I get it to the shop or home, because it probably will not crank again.

And I have seen my mechanic friend drive three other cars and trucks off the roll back wrecker by doing that.
Sometimes I do the cranking and him the whack, and sometimes its reversed.
Either way, its an easy way to get it in the shop, and he goes ahead and replaces the pump, because it will do it again.
So, it it turns out to not be the ground for the pump, you will either have to save up for a new one, or rig a rope with a pivoting weight that you can pull to slam the tank.............
Click to expand...

No apologies needed my friend. I appreciate all the help you've given me so far.

Anyway, the gauge today decided not to be pegged full, but the truck still won't start.

I went ahead and jumped the wires anyway, pink to a good ground and the gauge dropped to E. Pink to black also dropped it to E.

The Pump also has a green wire which is the ground to the frame. Do I need to get this involved? I can't find my multimeter, and my friend whose I borrowed before isn't here, so I can't check the resistance right now. Although now that I'm back inside I realize I forgot to give the wires a good shaking, I'm gonna run out and do that right now. Thanks again! Although I'm leaning towards a dead pump.

I'm also very familiar with the tapping the fuel pump/starter trick. I've used it quite a few times to get various vehicles home. I even had a POS dakota 5 speed that I drove around with a beat starter for 4 months haha. Parking on hills is your friend!

EDIT:
Okay, even after extensive jiggling of the wires, I couldn't get the gauge to spike again. My only thought on this is it all began on a very rainy day. Perhaps this had something to do with it?

I also tried jumping the pump again, which got it priming (and the gauge still at its true level). I got the truck to run like this, but it was stumbling and ran VERY poorly. It wouldn't idle for more then a few seconds if I wasn't on the gas.
 
At this point, I'm not sure. It sound like a bad pump, but the gauge part puzzles me. Its possible that the two are related, and the most common point is the ground.
My problem at this point, is that all the schematics I have for your year truck just say there is a ground, with no location.
I have a great book on a 1990 pickup that shows everything and where all the grounds and splices pertaining to your pump and gauge are.
But, I don't know if they are the same on your truck. The color codes seem to be the same, so just in case........
The master ground for those is either mounted on one of the bolts holding the top radiator hose fitting on the engine, where the thermostat is, or one of the valve cover bolts right next to that on the driver's side.
Different models have it in different places.
Look for a heavy wire hooked up in that general area.

But, that connection is ground for a lot of stuff, and if it was bad I would expect you to be having other problems...

However, all those grounds are spliced together inside the cab right under the fan motor.
Look for a big bundle of wires probably going through the firewall there, or maybe just mounted there.
The book is not clear on that.

If its there, it probably wrapped in tape. Examine it closely. If it feels funny when you squeeze it, or you see any signs of leakage or corrosion, you might want to unwrap it and look for trouble.

Of course, none of this might be on your truck, its just a best guess right now.
 
Well I know the pump grounds right to the frame rail, it's a small green wire grounds on the top of the driver side frame rail. I'm not sure about the master at the front, I'm out camping for the weekend now, I'll check on sunday when I get home.
 
OKay, long time update, Finally got the money/time to get the fuel pump together. Bought a new sender and pump, hooked everything up, got the tank up and in, and the truck still doesn't start. I can't hear the fuel pump prime with the key on or with the relay jumped.

So my question now is what essential wiring do I need to make everything work? I'm convinced theres a wiring problem somewhere, I just don't feel like chasing it all over. What would need to be done to just rewire the whole pump/sender? I know that some people bypassed the relay with a little toggle switch. Just wondering what else would need to be done (or if anyone has a wiring diagram)

Thanks!
 
Everything that is needed, is there. Nothing extra, unless you consider the oil pressure switch extra. Then again, that will prevent you from being stranded if the relay fails, and it's not really adding complexity. There is no chasing to be done, kind of need to start over from scratch.

Test for 12V at the relay from the battery. If 12V, then test continuity to the tank on the pump wire from the relay. No point in going any further past that until you come back and say both work. Bite it off in small chunks. The problem is either power or ground to the pump, that's it.

You may have mentioned doing it, but if you've got a good length of wire (or spare battery, don't forget to ground it), put power to the pump wire at the tank connector directly from a battery. If the pump still doesn't work, you've significantly reduced the potential problem area.
 
Okay, I'm making some progress I think. Ecm b fuse was blown. I can get a little driving in before it blows again. I haven't had a while lot of time to troubleshoot cause I'm just starting a new job, but I'm hoping this will help me pimpoint the short.
 
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