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fuel gauge needle hits the oil needle

D

D2 Cat

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south of Kansas City
What's your experience, the gauge, grounds, or sending unit in tank problem?
I'm ready to drop the tank and start from there, but then if it's the gauge.... I don"t need the practice!!
 
Leo -

Before you get too ambitious, run a gauge check. To check your gauge, disconnect the lead at the fuel tank. Ground the lead and turn the key on. The needle should swing to the empty mark and stay there. Remove the lead from ground; the needle should swing past the full mark. If the needle moves as described, your gauge and wiring are OK. If the gauge reads empty all the time, check for a grounded lead; if it stays at full all the time, check for an open.

With an empty tank, the sending unit should read between 0 and 3 ohms. Full, you should see between 88 and 92 ohms. Getting to the pins on the FSU connector is a real PITA. It's a BIG help if you have a piece of tank harness to connect and test from.

I would check the gauge first. It's the easiest. And realistically, if the gauge checks good, then you know your harness is also good. That leaves only one thing - the FSU. Happy hunting!
 
Also a ground on the frame that your FSU grounds too. Check that by fuel tank.

I did everything possible besides replace my dash cluster due to this same thing, it would spin dance and go all over the place, sometimes going north, south, east... 360's.. Even get 'hung up' on the OIl pressure needle...

When I grounded it out it read 3/4 tank. I think the gauge lost its home position.

I ended up getting an aftermarket fuel gauge for 90 ohms....

And I permanately grounded the factory gauge so at least it read 3/4 and doesn't dance.
 
Matt, sounds like mine. The oil needle was "caught" on the fuel needle yesterday until I shut it off and started over!

Dave, do you have a schematic of the fuel system? It can't be too complicated so I'm going to start by finding the wire at the tank. Thanks guys.

len
 
Len -

I'll look for a schematic. What I have is the GM shop manual set for my 94. GM seemed to rely on drawings and written instructions. I'll keep looking.
 
D2 Cat said:
Matt, sounds like mine. The oil needle was "caught" on the fuel needle yesterday until I shut it off and started over!

Dave, do you have a schematic of the fuel system? It can't be too complicated so I'm going to start by finding the wire at the tank. Thanks guys.

len
Click to expand...

Just so you know the #1 reason this does this is that ground on the frame rail (topside) for the FSU needs to be removed and wirebrushed... didn't fix mine, but it may fix yours. I've researched this issue a lot in the past 6 months.

I have 2 tanks and an automated tank balance module... so it just makes things way way worse to fix.
 
The FSU ground on the frame rail, where is it? Do I need to remove the tank to get to it? I felt up on the top of the tank from the pass. side and I could feel two wires, but I could not pull them down far enough to see them.

I came from the driver side and there is a connection with two wires. The two wires come out of the harness taking all the wire to the rear of the truck. I disconnected the plug. One wire is black, one is purple. I didn't get any testing done, because another project required immediate attention!

The only ground I saw under there is from the fill tube to frame rail.
 
D2, it is on the top of crossmember above the axle. It is somewhat cumbersome to reach.

I replaced my FSU due to rusted lines and hoped it would fix the gauge reading problem. I cleaned that ground to within an inch of its life, too. Guess what? Still f'd up. Instead of 3/4 being full, it was worse- full to cap is shown as 1/2 tank now. I even played with the FSU float while grounded and out of the tank. It did not work on the gauge.

My oil pressure gauge also acts stupid. Goes straight down with key off, and goes up to 0 when driving (typically 40psi).

I have cleaned grounds over, and over, and over, and over. No difference.

GRRRRRRRRRRRRRRRRRRRRRR
 
I assume the gauge(s) are flaky or I have an under dash ground prob... or maybe some good ol crappy GM engineering. I love this truck but it has to be the worst wiring I have seen in 25 years of working on vehicles.
 
I got an instrument cluster from the re-cycler. Disassembled it. The gauges come right out with the remove of two screws.

It sure looks to me like I could remove the (gas) fuel gauge and just plug it in the diesel board. Then the "Diesel Fuel Only" logo will still be in place. I could fill up the tank and install the needle to the "F" and run it to see how it responds! The needle with fit on the pin in any position.

All four gauges look identical, fuel, battery, oil, water. Do you suppose the function off the same resistance? If they did you could install the gauges in any position for any function.

Matt, that's an interesting (and long) thread, thanks.
 
I do not have mine apart. I only have the gasser apart. From the resistance on the needle on the gasser compared to how the diesel needle spins and flops, I'd say the diesel gauge is wore out, kaput.

I will check and double check wires and grounds before removing the diesel inst. panel, but I've got a feeling it's needing replaced.
 
Thats normal if the FSU is bad. I still think its your FSU giving the problem. If you know your parts guy take your truck up and see if he will let you plug in a new FSU.

Just unplug the old one back at the frame then plug a new one in with it out of the tank obviously. I am able to see the gauge cluster from the rear, but you may need someone to watch the gauge as you slowly move the float around. See if it act normal then.
It would take 2-3 minutes to check that way out in the parking lot.
 
The FSU sends resistance values to the gauge to indicate levels in the tank. I remove the plug to the FSU and the gauge remains pointed to the 5 o'clock position---wouldn't this indicate the FSU has no impact on this needle moving? And, the ground being good or bad would not matter in this test, would it?

I've got a 90 ohm potentiometer I could wire to a plug for a portable FSU test unit!! But, I don't see how the needle can stay in the same position whether connected or disconnected
 
How good is the frame and body grounded to the battery? The crappy small flat strap that goes from the back of the passenger side head to the frame, where you cant see it because of the downpipe is marginal ,at best. I've started running multiple grounds from the engine to frame and engine to body with large battery cable to eliminate all the grounding problems these trucks have. I'd check all wires associated with the gauge with a ohmmeter before buying ANY parts. Check all connectors for pushed out pins and pins that aren't locked in. My 95 LD is doing the same thing,bouncing all the way around and hooking on the oil pressure needle, but only does it some times. I haven't tackled it yet, but this thread got me thinking about it again.
 
D2 Cat said:
The FSU sends resistance values to the gauge to indicate levels in the tank. I remove the plug to the FSU and the gauge remains pointed to the 5 o'clock position---wouldn't this indicate the FSU has no impact on this needle moving? And, the ground being good or bad would not matter in this test, would it?

I've got a 90 ohm potentiometer I could wire to a plug for a portable FSU test unit!! But, I don't see how the needle can stay in the same position whether connected or disconnected
Click to expand...
With the harness unplugged and dangling, the gauge circuit is seeing max resistance. So the gauge should be reading "full". Since it's reading "empty", then either the circuit is seeing no resistance to ground or the gauge itself is bad. If you're going to pull the instrument panel, disconnect the harness behind the panel and ohm the harness from the connector at the tank to the connector at the panel. Should read infinite resistance. Low resistance will indicate a short somewhere in the harness.
 
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