91 Fuel Gauge And Sending Unit Questions

[Keyav8r]

Dropped the tank on my 91 to replace the tank vents which were leaking, solved that problem.

The fuel gauge hasn't worked correctly since I've owned it. The gauge reading was floating from empty to about 1/3 tank regardless of the actual amount of fuel in the tank, but the low fuel light would come on when it actually got close to empty.

While I had the tank out I checked the sending unit output and had 113-114 ohms with float down (empty) and 7-8 ohms with the float up (full) at the tank side connector. This is close to spec and the error may be from the Harbor Freight meter.

I poured in about 10 gallons of gas and re-checked the reading at the tank side connector - 42-43 ohms. Cleaned the connectors as well as I could, plugged them back together, checked the gauge and it read about 1/3 tank. I'm thinking with a range of 110 empty to 3 full I should be seeing at or over 1/2 tank on the gauge. Tried to back probe the harness side of the connector but couldn't get a reading so I shaved off a little insulation and checked the ohm reading - 31-32 ohms. Losing about 10 ohms across the connector. At that ohms I should be indicating an even fuller tank.

Insulated the wires I had shaved, buttoned everything back up, drove to the gas station and filled the tank. The gauge floated up to a little over 1/2 tank and floats between that and near empty as I drive around.

I know I will need to check the wiring continuity and clean any additional connectors, but I'm wondering what else I may be missing.

AFAIK, this is the original cluster.

Should there be a ground strap/wire between the tank and the unibody?

Should the 91 have a ground wire from the dash to the unibody like the Renix MJs? I can't spot one.

I'm wondering if this could be a grounding issue as the voltmeter also doesn't work. Everything else on the truck works correctly.

Any suggestions?

[HOrnbrod]

I know I will need to check the wiring continuity and clean any additional connectors, but I'm wondering what else I may be missing.

AFAIK, this is the original cluster.

Should there be a ground strap/wire between the tank and the unibody?

Should the 91 have a ground wire from the dash to the unibody like the Renix MJs? I can't spot one.

I'm wondering if this could be a grounding issue as the voltmeter also doesn't work. Everything else on the truck works correctly.

Any suggestions?

 

Your fuel sender pot readings sound okay. My 91 fuel gauge reads fine, but I'll try to answer.

 

"Should there be a ground strap/wire between the tank and the unibody?" The fuel sender ground is behind the left tail lamp. Make sure it's clean, tight, and shiny. Also check it from the sender connector. AFAIK that's it for sender ground.

 

"Should the 91 have a ground wire from the dash to the unibody like the Renix MJs? I can't spot one."  There isn't one from the factory. I did find the black dash ground wire (about 12AWG) tied to to the dash lower left brace, using a dental mirror is the easiest way. Since I had no cluster problems, all I did was verify that the sheet metal screw connection was clean and tight. Pic below (not mine).

 

"I'm wondering if this could be a grounding issue as the voltmeter also doesn't work. Everything else on the truck works correctly.

Any suggestions?"  Pull the cluster and check the gauge mounting screws for tight and clean, and go over the circuit foil carefully checking for or partial PC breaks and/or cold solder joints. I have a few foils left for the HO clusters when I was calibrating them if you need one. Also make sure the two cluster cables are clean and fully seated.

 

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[dinghyboy]

I'll be real interested to hear how you finally resolve this problem. My 92 gas gauge never reads above 3/4 and I never really trust it below 1/4. I do not have the full cluster.

 

Thanks

[Keyav8r]

Thanks, Don! I knew Incould count on you for solid 91 information.

So the ground that's soldered to the outside of the senging unit/pump cover travels through the wiring harness t o the left reae taillight. I'll have to refresh that one and see if it has any effect. If not, I may jumper scross the connector to seme if that helps. Msy repest my continuos ground from the left taillight all the way to the to the negstive post on the bsttery with numerous body grounds along the way., semmed to clear up some problems unitntionally,,'

Could not see a dash ground wire, maybe one's there, i'll search more diligently.

 

TTFN - just got through installing a waterbheater and the old neburons aren' firing scross the synapses correctly.

[92tanMJ]

I'll be real interested to hear how you finally resolve this problem. My 92 gas gauge never reads above 3/4 and I never really trust it below 1/4. I do not have the full cluster.

 

Thanks

Same here

[Keyav8r]

Dang! Just re-read my Monday night post. Was way more tired than I thought. Probably will be a couple of weeks before I can get back on this project. May be even longer with Christmas and New Year holidays rapidly approaching.

But, to confirm, does the ground wire soldered to the outside of the sending unit go through the harness to the ground behind the driver's side taillight?

[HOrnbrod]

Dang! Just re-read my Monday night post. Was way more tired than I thought. Probably will be a couple of weeks before I can get back on this project. May be even longer with Christmas and New Year holidays rapidly approaching.

But, to confirm, does the ground wire soldered to the outside of the sending unit go through the harness to the ground behind the driver's side taillight?

 

Yes, the ground wire off the sender is tied to pin A of the 3-pin connector and goes to the tail lamp ground point. Test pin A of the connector to ground for continuity.  Pin assignment:  C  B  A - C is 12V, B is output to fuel gauge, C is ground.

[Keyav8r]

Thanks, Don. Now for the grubby part - tracing wires and cleaning connectors and grounds.