Fuel pump wiring

[mitten400]

Sorry if this is in the wrong thread.

I have exhausted the search button and google.

1998 k3500 dump truck. Had 2 fuel tanks with the balancer module. Previous owner removed the back tank and put a new pump/sending unit In the front tank. Gas gauge doesn’t work.

I pulled the pump and the sending unit is ohming from 40-250 as designed.

My gauge was acting funny so I put a different cluster in and the gauge is not all over the place anymore, but it does not appear to listens to the sending unit.

I have the black and black/white wire grounded together(straight to battery for testing reasons). Purple to purple and grey to grey. I removed the balanced module and relay completely.
Any thoughts?

 

[mitten400]

Anyone have a guess?

 

[Frank Enstein]

Bad ground? Because I always blame the ground first.
The sending unit to ground is the most common culprit in my experience.

Continuity test on the wires end to end?

Bad connection? Maybe you will find a break in one of the wires.
The pins in the dash plug (vehicle side) are often compromised. Gently clean them and bend the connectors (where the dot is) a TINY amount. Like a 16th of an inch.
Maybe the gauge guts are bad?
Sorry not much help tonight.

 

[mitten400]

Bad ground? Because I always blame the ground first.
The sending unit to ground is the most common culprit in my experience.

Continuity test on the wires end to end?

Bad connection? Maybe you will find a break in one of the wires.
The pins in the dash plug (vehicle side) are often compromised. Gently clean them and bend the connectors (where the dot is) a TINY amount. Like a 16th of an inch.
Maybe the gauge guts are bad?
Sorry not much help tonight.

So I found something interesting. The top peg of the fuel gauge was not matched up with the correct pin on the gauge harness. When I was checking for continuity from the sending unit to the gauge harness I noticed this. I pulled the pin and swapped it to the correct location to where it would contact the fuel gauge. However, it is still not working. I checked for ohm's from the sending unit to the cluster plug and it checks out fine. I put 4 different clusters in and I get different characteristics on each of them. I can't imagine I have four bad gauges in the different clusters. My only other thought buy an aftermarket gauge and test it that way. Is there a chance my gauges are set for 0-90 and my 40-250 sending unit is messing it up?

 

[Frank Enstein]

That is easy enough to check. Remove the sending unit wire from the sender and hold it in the air. It will read pegged past full because it is a bazillion ohms to ground. Now ground the wire and it will read empty. if it reads way below empty or bangs the needle down hard it is likely the 40 to 240. The 40 to 250 started in 98 so if you have an earlier gauge it will certainly be weird.

 

[mitten400]

That is easy enough to check. Remove the sending unit wire from the sender and hold it in the air. It will read pegged past full because it is a bazillion ohms to ground. Now ground the wire and it will read empty. if it reads way below empty or bangs the needle down hard it is likely the 40 to 240. The 40 to 250 started in 98 so if you have an earlier gauge it will certainly be weird.

Thanks for the advice. I will try that tomorrow. I thought I read that they switched over in 1998 like you mentioned, but when you google search for a cluster, they say they are for 95-99 so i'm confused.

 

[Dropped88]

98+ fuel gauge gets its reading from pcm.

So sending unit to pcm to cluster

And its a PWM signal

 

[AK49BWL]

Thanks for the advice. I will try that tomorrow. I thought I read that they switched over in 1998 like you mentioned, but when you google search for a cluster, they say they are for 95-99 so i'm confused.

The reason for that is, 98+ wires the sending unit to the PCM, which then sends a PWM signal to the gauge. The gauges are all the same.

Ah, ninja'd.

 

[mitten400]

98+ fuel gauge gets its reading from pcm.

So sending unit to pcm to cluster

I was under the impression that the PCM doesn't effect the reading in anyway though. It simply "reads" it for other functions. Is there a chance the truck is confused since their used to be two tanks with the balancer module which is now all gone?

 

[AK49BWL]

I was under the impression that the PCM doesn't effect the reading in anyway though. It simply "reads" it for other functions. Is there a chance the truck is confused since their used to be two tanks with the balancer module which is now all gone?

Possibly. The PCM contains the programming for fuel tank size and sender voltage tables that are probably synced to the old two-tank system still. It absolutely has ALL control over the gauge reading lol