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  #1  
Old 01-12-2022, 06:30 PM
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Curly5759 Curly5759 is offline
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Location: Verde Valley, Arizona
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Help me figure out this fuel gauge problem

This is more of a puzzle than a repair question. I'd like to know why this is happening.
1981 Wagoneer/4.2/727


So, my gas gauge just quit on me. I have it diagnosed as a bad sender, but it doesn't make sense. Maybe you can figure out what is going on.



Project jeep, has new (BJ's) sending unit, about 6 months old.

Gauge reads empty, about 3/4 of a tank of gas.

Ground the power wire and the gauge reads.

Sending unit has continuity through the terminals, and continuity to ground.
Sender reads 58 ohms. Specs are 73-10 ohms

Using the old sending unit I took out and jumping it to the wires- the dash gauge reads properly.


Power wire has 4.8 volts on it, ground wire has 0 ohms of resistance

So............. I've got power and ground, a working dash when tested with another sender, continuity across the sender. Why doesn't the gauge work?
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  #2  
Old 01-13-2022, 10:15 AM
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tgreese tgreese is offline
 
Join Date: May 29, 2003
Location: Medford MA USA
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Quote:
Originally Posted by Curly5759
This is more of a puzzle than a repair question. I'd like to know why this is happening.
1981 Wagoneer/4.2/727

So, my gas gauge just quit on me. I have it diagnosed as a bad sender, but it doesn't make sense. Maybe you can figure out what is going on.

Project jeep, has new (BJ's) sending unit, about 6 months old.

Gauge reads empty, about 3/4 of a tank of gas.

Ground the power wire and the gauge reads.

Sending unit has continuity through the terminals, and continuity to ground.
Sender reads 58 ohms. Specs are 73-10 ohms

Using the old sending unit I took out and jumping it to the wires- the dash gauge reads properly.

Power wire has 4.8 volts on it, ground wire has 0 ohms of resistance

So............. I've got power and ground, a working dash when tested with another sender, continuity across the sender. Why doesn't the gauge work?
Grounding the sense wire is not the correct test for these. Jeep specified a decade resistance box, set to the resistances given in the TSM for empty, half and full. Ther is a picture of the tester in the TSM. The gauge should never see a full ground of the sense wire; there is always ca 10 ohms of resistance even when the tank is full (lowest resistance).

You can make your own tester or get some clip leads and fixed resistors and clip in the specified resitances. You can get the clip leads on Amazon and the resistors are just a few cents total on one of the electronics supply sites like Digikey or Mouser.

I built a gauge tester and wrote it up here - http://earlycj5.com/xf_cj5/index.php?threads/143458/ - you may have to join the site to see the pictures. This is really fancy, but only about $20 in parts.

The harness has either three or four lengths of wire between the gauge and the sender: gauge to the bulkhead, bulkhead to chassis harness, chassis harness to sender lead, sender lead to sender. You can unplug the harness at any of those connectors and patch in your tester or leads or whatever. The wire to the sender is usually pink. With the plastic tank, your sender will have a dedicated ground wire to the chassis. I would measure continuity between its connection on the sender to the body, and to the frame, and to the ground trace on the cluster. Divide and conquer.
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Maine beekeeper's truck: '77 J10 LWB, 258/T15/D20/3.54 bone stock, low options (delete radio), PS, hubcaps.
Browless and proud: '82 J20 360/T18/NP208/3.73, Destination ATs, 7600 GVWR
Copper Polly: '75 CJ-6, 304/T15, PS, BFG KM2s, soft top
GTI without the badges: '95 VW Golf Sport 2000cc 2D
ECO Green: '15 FCA Jeep Cherokee KL Trailhawk

Last edited by tgreese : 01-13-2022 at 10:30 AM.
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  #3  
Old 01-13-2022, 04:26 PM
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tgreese tgreese is offline
 
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Location: Medford MA USA
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A little more ...

Your problem: the gauge reads empty when the sender is conneced in the Jeep. That means the sender wires are open somewhere. The empty reading is the highest resistance, 73 ohms when empty. So an open circuit - infinite resistance - will read as empty, but actually be way way beyond empty.

You have an open circuit somewhere. When you bypass part of the wiring by unplugging the sender and adding the old sender by itself, you are fixing the open circuit somehow.

Some of the most common electrical failures are mechanical connectors. The connectors are there to make assembly easier at the factory, not to make the Jeep more reliable. They have the opposite effect.

Again, divide and conquer. It's unlikely that a wire has failed mid-way between the sender and the gauge. More likely is corrosion in a connector. Unplug, test, spritz with contact cleaner and scrub with a plastic brush. Put back together with a little dielectric grease on the contacts, to prevent future corrosion. Test every wire and connector between the sender and the gauge.
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Tim Reese
Maine beekeeper's truck: '77 J10 LWB, 258/T15/D20/3.54 bone stock, low options (delete radio), PS, hubcaps.
Browless and proud: '82 J20 360/T18/NP208/3.73, Destination ATs, 7600 GVWR
Copper Polly: '75 CJ-6, 304/T15, PS, BFG KM2s, soft top
GTI without the badges: '95 VW Golf Sport 2000cc 2D
ECO Green: '15 FCA Jeep Cherokee KL Trailhawk
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  #4  
Old 01-13-2022, 05:42 PM
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letank letank is offline
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Location: San Francisco
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usually bad ground to the tank, the ground is on the frame that support the rear bumper, by the driver's side rear wheel well.
I may have a pict somewhere

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74 wag, 349Kmiles on original ticker/trany, except for the rust. Will it make it to the next get together without a rebuilt? Status: needs a new body.
85 Gwag, 228 Kmiles. $250 FSJ test lab since 02, that refuses to give up but still leaks.

See Ouray 2013, Engine bits and Fuel and brake lines, and Body work
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  #5  
Old 01-19-2022, 05:03 PM
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Curly5759 Curly5759 is offline
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Location: Verde Valley, Arizona
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So, I think I found the problem. I switched out the new (non working) sender for the old one. It worked and gave the correct reading. It had worked when I layed it on the carpet and hooked it up. So, all is good now.



I inspected the newer sender that I had removed and found what I think was the problem. The sending unit wire was pushed up against the metal body of the sender hard enough to cut through the insulation and cause a short to ground. At least, thats my theory. I the 2 pics below, the wire has already been pushed away from the metal. Look closely and you can see a small hole in the insulator and discoloration on the metal.

https://photos.google.com/album/AF1Q...pcNaBqK-ZHaF9U


https://photos.google.com/album/AF1Q...ZSpwClnHqWhsI6
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  #6  
Old 01-20-2022, 09:47 AM
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tgreese tgreese is offline
 
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Location: Medford MA USA
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That seems backwards. A short to ground should read full, and will burn out the gauge eventually. Set the options on your images so that anyone can see them. They require a Google login now.
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Tim Reese
Maine beekeeper's truck: '77 J10 LWB, 258/T15/D20/3.54 bone stock, low options (delete radio), PS, hubcaps.
Browless and proud: '82 J20 360/T18/NP208/3.73, Destination ATs, 7600 GVWR
Copper Polly: '75 CJ-6, 304/T15, PS, BFG KM2s, soft top
GTI without the badges: '95 VW Golf Sport 2000cc 2D
ECO Green: '15 FCA Jeep Cherokee KL Trailhawk
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  #7  
Old 01-20-2022, 10:37 AM
Curly5759's Avatar
Curly5759 Curly5759 is offline
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Join Date: Mar 08, 2015
Location: Verde Valley, Arizona
Posts: 222
Quote:
Originally Posted by tgreese
That seems backwards. A short to ground should read full, and will burn out the gauge eventually. Set the options on your images so that anyone can see them. They require a Google login now.




I'm new to google photos, I don't see an option to make the photos public.
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  #8  
Old 01-23-2022, 11:58 AM
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letank letank is offline
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Join Date: Jun 03, 2002
Location: San Francisco
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Ironically, my gas gauge started to indicate lower than what was filled up... So yesterday it was time to find the high resistance... the sender gave me the right value of 38 ohms which is about 1/3... but read empty on the gauge... testing the ground wire gave me 110 ohms...

So I tried to clean the ground which had been moved from the rear frame to the rear taillight ground behind the speaker panel... still bad at 50 ohms, even after derusting the contact point or relocating the ground... the taillights are still operational.... after 1hr of messing around... I pulled a 6 foot wire to the dashground point that I use for other accessories ... et voila...


looking at my pict, the last time I messed with it was 2016


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Michel
74 wag, 349Kmiles on original ticker/trany, except for the rust. Will it make it to the next get together without a rebuilt? Status: needs a new body.
85 Gwag, 228 Kmiles. $250 FSJ test lab since 02, that refuses to give up but still leaks.

See Ouray 2013, Engine bits and Fuel and brake lines, and Body work
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  #9  
Old 01-23-2022, 12:49 PM
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Curly5759 Curly5759 is offline
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Join Date: Mar 08, 2015
Location: Verde Valley, Arizona
Posts: 222
Pretty much what I did. I had only 10 ohms on the ground wire, but a new temporary ground didn't help. I wound up reinstalling the original old sender using the ground with the 10 ohms and it reads correctly.



I thought I had it figured out, but as Tgreese pointed out, a shorted sender would read full all the time. I'm just glad it's working again.






Quote:
Originally Posted by letank
Ironically, my gas gauge started to indicate lower than what was filled up... So yesterday it was time to find the high resistance... the sender gave me the right value of 38 ohms which is about 1/3... but read empty on the gauge... testing the ground wire gave me 110 ohms...

So I tried to clean the ground which had been moved from the rear frame to the rear taillight ground behind the speaker panel... still bad at 50 ohms, even after derusting the contact point or relocating the ground... the taillights are still operational.... after 1hr of messing around... I pulled a 6 foot wire to the dashground point that I use for other accessories ... et voila...


looking at my pict, the last time I messed with it was 2016


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