Fuel Gauge, Oil Gauge

[Elwiscon]

Hello, so I have a 1989 MJ, stock. I dropped the tank, cleaned it and raptorlined the tank exterior then replaced the fuel pump last year, i gutted the interior among other things to fix a rusty floor pan, replace the HVAC box, add AC etc. The tank sat dry for about 1 year. A couple months ago I put some gas on it to run the engine to make sure it and everything else worked fine after some extensive engine work and to keep things moving. A couple days ago I connected the gauge cluster (6-gauge with tach), the fuel gauge went up to full and stayed there, the tank has about 1/2 or less. I checked the wiring, added a ground jumper near the fuel tank (per cruiser54), replaced the battery cables and ground jumpers (with a kit from jeepcables.com). 

 

-Long story short, my fuel gauge stays past full, i tried another gauge cluster (same year, style etc) and did exactly the same thing so it isn’t a faulty gauge. I know the tank isn’t full, there’s less than 1/2, I traced the tan wire and it has continuity, there is no open/shorted out connections on it. At this point I think it could be a faulty fuel sender, I doubt the float switch got stuck in the full position but who knows. 

 

-Second issue is the oil gauge, i replaced the original oil pressure sender and the oil pump, I used a high-volume Melling oil pump (recommend in some XJ/MJ Forums) when the engine is running the pressure goes close to 80, I’m assuming the high volume pump increases the pressure, should I be concerned? Picture shows both gauges, with the key in the off position.

 

What do you guys think it could be? I want to make sure everything is in good working condition before taking my Comanche to the body shop for a paintjob and before I re-assemble  the interior. 

 

Thanks!

 

[eaglescout526]

For sanity sake, you got the right sending unit right? Renix?

 

All I can think of is the windings for the rheostat for the float is either damaged or has dirty contacts between the windings and float arm. I recently ran into that issue actually. 

[Elwiscon]

6 minutes ago, eaglescout526 said:

For sanity sake, you got the right sending unit right? Renix?

 

All I can think of is the windings for the rheostat for the float is either damaged or has dirty contacts between the windings and float arm. I recently ran into that issue actually. 

Correct, I have a 1989 4.0 Renix

I wanted to get the MTS unit but they were out of stock, so I ended up getting an “A-Premium” Amazon unit, it fit perfectly, it pumps. I just read some new comments on amazon and apparently others are having issues with it, I guess i’ll have to get the MTS unit if they have it, unless they’re out of stock. This is what I got:

 

A-Premium Fuel Tank Sending Unit Compatible with Jeep Comanche 1987 1988 1989 1990 4.0L, GAS https://a.co/d/1XO7p19

 

[ghetdjc320]

Can you check the resistance from the sending unit to chasis ground at the fuel tank connector?

[Ωhm]

Might be off base here but doesn't the ground wire for the sending unit on the pump, ground at the bottom of the resistor for RENIX and at the top of the resistor for SBEC. Can't tell from the photos. Gauge still should work with half tank of fuel.

 

Does Coolant Temp Gauge peg pass 260° when KEY is in the CRANK position? If NO, check GAUGES FUSE (7.5A). If GOOD, look for voltage on C203_6 & C203_18.

 

 

[Elwiscon]

On 5/24/2025 at 1:41 AM, ghetdjc320 said:

Can you check the resistance from the sending unit to chasis ground at the fuel tank connector?

I will look into that, I think it is the pump sending unit 

[Elwiscon]

On 5/24/2025 at 6:32 AM, Ωhm said:

Might be off base here but doesn't the ground wire for the sending unit on the pump, ground at the bottom of the resistor for RENIX and at the top of the resistor for SBEC. Can't tell from the photos. Gauge still should work with half tank of fuel.

 

Does Coolant Temp Gauge peg pass 260° when KEY is in the CRANK position? If NO, check GAUGES FUSE (7.5A). If GOOD, look for voltage on C203_6 & C203_18.

 

 

Gauges fuse works, temperature, voltage and tach work perfectly. I will look at the coolant temp while cranking it. I have voltage and continuity on C203-18, tan wire, I will check C203-6

 

[Elwiscon]

When I cleaned and raptor-lined the tank exterior, I installed an amazon “A-Prime” fuel pump/sending unit a couple years ago, but didn’t get to check the gauge until recently, as the interior is in pieces pending paint job and other repairs, so their 30-day warranty is long gone. According to some recent Amazon reviews some buyers are experiencing the same erratic gauge issue. The pump seems to be working fine except for the fuel gauge. I ended up with the Amazon pump because MTS didn’t have them in stock, I think I’ll get one from them, they’re supposed to be great quality. 

[ghetdjc320]

1 hour ago, Elwiscon said:

I will look into that, I think it is the pump sending unit 

I misspoke, test between the sending unit ground pin and the fuel level sensor wire. That should give you the correct ohm reading. 

[Elwiscon]

Just now, ghetdjc320 said:

I misspoke, test between the sending unit ground pin and the fuel level sensor wire. That should give you the correct ohm reading. 

Does the sending unit and pump use the same ground pin on the harness? I added an extra ground connection from the body near the pump (per cruiser54) for more reliability,

[ghetdjc320]

Yes it does. That one ground wire is the reference ground for both. 

[Elwiscon]

1 hour ago, ghetdjc320 said:

Yes ur does. That one ground wire is the reference ground for both. 

I think it’ll be the amazon pump sending unit,

the wiring looks good, checked for shorts or open wiring. I’ll replace it with a higher-quality MTS pump, hopefully that will be the solution.