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pegged fuel gauge and no worky dash lights?

T

tuner-automotive

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1990 GMC Jimmy k1500
TBI 5.7L 350 700r4
I just bought this truck a month ago and have been ironing outr some issues.
Before I start I have read thru alot of threads I found oing searches , now I have to ask for myself.

when I first bought the truck , the speedo and fuel gauge lights did not work.
The oil/amp/etc lights do work.
Now all the dash lights do not work.
The fuel gauge is pegged about 1" past the full mark and goes down as far as the half full mark when its almost empty, so basically it fluctuats.

Any ideas?
I am going to try and remove the gauges and change all the bulbs aswell as clean the copper tewrminals and restretch them.
 
What I've learned to do is once I've got the gauge pulled, don't even bother hooking the speedo cable up or bolting it in place until I've connected it to the harness to verify what I've done has worked.

In the case of the fuel gauge, sounds like the pink wire is shorted to ground somewhere.

I'd be surprised if that is a gauge/cluster issue, but since you have to pull it to get the bulbs changed anyway, make sure all the printed circuit board to IP connnectors are clean and not screwed up. You should be able to test resistance on the fuel gauge wire (mines pink) since it should be between 0-90ohms. Fuel gauge (all gauges actually) are easy to diagnose, one wire is +, one is -, and one is the sender wire. The + and - are usually pretty easy and shared between multiple gauges.
 
So I followed my fuel sending units electrical wires from the unit on the tank to halfway up the frame rail .
Coreect me if Im wrong but it looks like the ground for the sending unit is bolted down on the frame rail right behind the driver side rear shock mount(on top) and then there are 2 other wires that go from the sending unit towards the engine.
Where do these other 2 wires come out at ?
I need to check there continuity or at least check to make sure they are intact.
 
I messed up on the first post, you don't have a speedo cable lol. :)

Other thing I left out is that the (newer) clusters (90-91 at least) use all common "grounds" and 12V where they need to. In other words, all gauges that work off of ignition 12V will share the same + connection, and all the same ground. (older clusters had the board divided into "sections" and had multiple 12V feeds and grounds)

There is a connector near the tank that will save you some work of finding a place to check their continuity. One wire is 12V for the fuel pump, the other is the fuel gauge sending wire. You can find the wire for the fuel pump by finding the one at the connector that matches one of the ones on the fuel pump relay, at least they SHOULD be the same color. Don't believe there were any splices in that wire that changed it's color, but there can be. You could also just as easily connect the voltmeter to ground and one of the pins in the connector, and turn the key on, see which one gets 12V for the pump.

Good idea to clean/inspect that tank ground too, as far as common problems, broken or poor connections on that wire are one of them.
 
excellent info.
I was actually replacing the rear diff fluid this weekend and realized I could basically sit upright under the truck facing the gas tank.
Thought I would inspect the connections and thats when i saw the elec. connector you are speaking of.
The one that cannects the fuel pump and sending unit to the forward cab.
I also then found the ground wire and followed it to its location on the frame rail above the dr. side shock mount.
I have afeeling my pegged issues are coming from a issue in the circiutry as it enters the cab .
I just lose the wiring along the frame rail once it gets about around the dr door.
 
I don't recall if that wire goes through the fuse panel or not...I *believe* it does. I don't recall running that wire seperately through my firewall, so I likely spliced into the existing fuel gauge wire on the fuse panel connector (engine side) when I wired it all up.

Disconnecting the connector near the tank with the key in "run" (or turn it to run after disconnecting) SHOULD force the gauge to go full empty. Grounding that wire should force it to go back to full. If it doesn't, then you've at least eliminated the sender and it's wiring.

Once you figure out which wire is the gauge wire, I'd look at the fuse panel connector and see if the same color wire is visible. At worst pull the connector to get a better look. Then you can check continuity on that wire, and/or see if its shorted to ground along the frame somewhere. GM was good about protecting that wiring along the frame, unlikely it's damaged, BUT as with everything, somebody could have messed with it, mice, etc.
 
Don't know if this helps or not.....

fuel-dia.gif
 
I think I may have misrepresented the problem Im having?

The gas gauge will work in the sense that when I fill the tank , the needle will move to like the 2 oclock position or about a inch past the full mark.
After driving for awhile itll drop down some , maybe to like 3/4 tank.
Itll go as far as 1/2 , sometimes but never past it .
It is a 30 Gallon tank , right?
So Im thinking now , maybe its the in tank sender ?
 
Gauges rarely go bad, however, on a 15 year old vehicle, it is possible.
But....from what you're talking about now - the sending unit is the most likely cause.
 
It can be a large job depending on how and where you do it.
Yes, you have to drop the tank to get to the pump/sending unit which is usually one whole unit.

I do at least one or two fuel pumps/sending units at work per week (Dodge dealership) and the biggest thing that I do that helps is to make sure the tank is empty. It makes a lot of difference when raising & lowering the tank considering a gallon of gasoline weighs roughly 8 lbs. If you have even half a tank that's 120 lbs. you're lugging around.

It shouldn't be too difficult if you take your time and do it right the first time...which means being careful every step of the way.
I'd advise getting the truck as high as you can comforably work on it, a floor jack (or a small transmission jack) and a large piece of wood to go on top of it, and making sure you disconnect all the electrical/vapor lines/filler neck(s) BEFORE you drop the tank. The floor jack/wood combo works well as a base for you to lower the tank with and makes it easier to put back into place as well. Also, try to get as much stuff out of the way as possible.
One other thing, MAKE SURE you relieve the fuel pressure before you disconnect any of the fuel lines...otherwise you'll be taking a bath.

BTW, if you need any diagrams or tech. info, let me know and I'll see what I can do to get it to you. We have an AllData subscription at work so I can get you pretty much whatever you need.
 
Last edited:
dyeager535 said:
Disconnecting the connector near the tank with the key in "run" (or turn it to run after disconnecting) SHOULD force the gauge to go full empty. Grounding that wire should force it to go back to full. If it does, then you've at least eliminated the sender and it's wiring.
Click to expand...
Have you done this? Don't go dropping the tank and $200 on a new sender unless it's failed this test. :deal:
 
spearchucker said:
Have you done this? Don't go dropping the tank and $200 on a new sender unless it's failed this test. :deal:
Click to expand...

No , Im going to do it this weekend.
Yeah , I aint dropping the tank until i eliminate ALL my options.

If you can get me a wiring diagram for the electricals on a .............
1990 GMC Jimmy SLE 350 5.7l 700r4 pwr windows and locks that would be great. :D
 
I couldnt find any diagrams for the doors, but I did get the windows.

Front window wiring.
jimwin.gif


Tailgate wiring.
jimtail.gif
 
spearchucker said:
Have you done this? Don't go dropping the tank and $200 on a new sender unless it's failed this test. :deal:
Click to expand...

Actually now that I confuse myself more, I got that backwards. If the gauge responds correctly to grounding/ungrounding the sending unit wire at the connector, then the gauge is GOOD, and the sender or sender wiring is the culprit. If the gauge didn't respond, then at least part of the problem would be the wiring from that connector to the gauge, or the gauge itself.

I went through a bouncy gauge issue, then an issue where the gauge didn't work accurately. Bouncy gauge is a gauge issue. Accurate reading in my case was the sender.

I pulled the sender, cleaned it (instructions were posted on here for that by someone else) reinstalled, and got a much more accurate reading, although still won't go quite to empty and reads just a bit over the full mark. I have a brand new sender, and tested it against the used one, and the gauge read correctly then, so I know the sender is really just shot, but until it starts reading way out of spec again, I'll live with it, since it does go through the whole sweep of full to empty, just not according to the gauge letters. :)

A gauge that is operating correctly will respond to float level changes slowly, so make sure to wait for a change to take place for a couple of minutes. I think the factory service manual says something like 10 minutes!
 
Yeah I was thinking about what you had said earlier and realized you had it backwards.
If the gauge works when grounded then the gauge is good, no response when grounding = bad gauge
 
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