Tracking down an electrical issue

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TimGee
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Joined: Wed Apr 28, 2021 3:01 pm

Tracking down an electrical issue

Post by TimGee »

Alright, at risk of looking like a total moron, I need to confess that I made a newbie mistake while installing a new stereo in my 1987 Grand Wagoneer. I reversed the leads on the battery and killed the car. I took it into a mechanic this week and they got it back up and running ($167) but when I brought it home yesterday I noticed that the gauges for fuel, temp and battery are non functional. I looked at the fuse block and I can't find a failed fuse although I think that some of the fuses are the wrong amperage. They were that way before the idiot mistake so I'm not sure that could be the issue. Is there another area I could check for a failed fuse? I searched the forum and the net and I wasn't able to find anything. Some posts referenced another fuse block in the engine compartment and one somewhere by the passenger dash but I haven't been able to find them. I'm wondering if wiring of the '87 is different from the '86. Here's a pict of the fuse block:
IMG_6236.jpg
It's embarrassing to admit this but the leads to the battery were both black and in my flawed logic at the time I assumed that the positive terminal was the one with the most leads coming off it. I should have traced the grounding wire. An expensive lesson.

Thanks for the help.

Tim
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1987 Jeep Grand Wagoneer. 360ci V8. Holly Fuel Injection
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tgreese
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Re: Tracking down an electrical issue

Post by tgreese »

Sorry to hear of your troubles.

Used to be that all the positive cables were always red. This seems to have gone away for our older cars, likely to cut costs for the suppliers. Sucks. I put a piece of red heat shrink tubing around the positive cable just for my own peace of mind, but that's not something that easily available to everyone.

I would suspect that you have killed the gauges. The fuses won't care about the polarity, but the gauges might. Use your multimeter to test the fuses one at a time. A fuse does not care about the direction of current, only the amount of current. Anything electronic-ish will be vulnerable to reversed polarity.

I don't know what the cutoff is, but some time in the '80s Jeep changed the dash design for these cars. Before the change will have an ammeter, after the change will have a voltmeter. There are wiring diagrams specific to the '87 here - https://oljeep.com/gw/elec/GW_wiring.html

Image

The first section shows a voltmeter, which indicates the later design. There is a complete '89 technical service manual (TSM) on that site, free to read and download, that you can use to evaluate the gauges. The '89 also has this later style dash, and the procedures and specs in the text will be the same.

You test the gauges in two parts. Disconnect the sender and measure the resistance of the sender to ground. Use a fixed resistor between the sender lead and ground to test the performance of the gauge.

Your catastrophe at the time was probably the alternator acting like a direct short when faced with reversed polarity. I have never seen someone reverse the cables like this, but this outcome seems likely. This probably melted one or more fusible links in the wiring harness. Looking at the scrap above of the wiring diagram, I would expect the clock to be dead (VFD type), the low fuel module, the gauges. Anything called "module" likely contains integrated circuits, which will go poof in response to reversed polarity.

I wrote this article about gauge testing on a CJ Jeep: http://www.earlycj5.com/xf_cj5/index.ph ... er.143458/ This will show you the concept in more detail, but the specifics will be very different for your GW.
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Last edited by tgreese on Thu May 27, 2021 10:00 am, edited 1 time in total.
Tim Reese
Maine beekeeper's truck: '77 J10 LWB, 258/T15/D20/3.54 bone stock, low options (delete radio), PS/PDB, hubcaps.
Browless and proud: '82 J20 360/T18/NP208/3.73, Destination A/Ts, 7600 GVWR
Copper Polly: '75 CJ-6, 304/T15, PS, BFG KM2s, soft top
GTI without the badges: '95 VW Golf Sport 2000cc 2D
Dual Everything: '15 Chryco Jeep Cherokee KL Trailhawk, ECO Green
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Topic author
TimGee
Posts: 28
Joined: Wed Apr 28, 2021 3:01 pm

Re: Tracking down an electrical issue

Post by TimGee »

tgreese wrote: Thu May 27, 2021 9:14 am Sorry to hear of your troubles.

Used to be that all the positive cables were always red. This seems to have gone away for our older cars, likely to cut costs for the suppliers. Sucks. I put a piece of red heat shrink tubing around the positive cable just for my own peace of mind, but that's not something that easily available to everyone.

Thanks for the info Tim. Very helpful. I put some red electrical tape on the positive cable. We'll see how long that stays on but shrink wrap is a good idea.

Thanks for the links and I'll post what I find out about the gauges.

Tim
1987 Jeep Grand Wagoneer. 360ci V8. Holly Fuel Injection

candymancan
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Re: Tracking down an electrical issue

Post by candymancan »

If none the gauges dont work. Its either the fuse under the dash by your feet. Or you have melted wiring somewhere related to that fuse.

I believe the fuse is the 3 amp clear purple accesory fuse youre looking for. Mine blew one time and my entire dash cluster didnt work.

Replace all the fuses with new ones. Theyre cheap enough.. mine were all replaced. I would deff find the fuse diagram for your 87 and make sure you have the correct amoerage ones too. I had a 30amp fuse in place of a 10 when i checked mine. I guess a previous owner thought thatd be cool.

And because of that when my visor wire got shorted.. it melted the wire all the way to the fuse box.. which is like 10 feet of wiring. And melted it into other wires.. if it was fused properly the fuse would have blown
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Topic author
TimGee
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Re: Tracking down an electrical issue

Post by TimGee »

candymancan wrote: Thu May 27, 2021 10:51 pm
And because of that when my visor wire got shorted.. it melted the wire all the way to the fuse box.. which is like 10 feet of wiring. And melted it into other wires.. if it was fused properly the fuse would have blown
Thanks Candyman. I just replaced all of the fuses and got it up and running. It was the 15 amp IGN/LPS fuse. Now all of the gauges are working but strangely, the rear defrost is on and if I try to turn it off it blows the same fuse. I created a new post that details this with picts.

Tim
1987 Jeep Grand Wagoneer. 360ci V8. Holly Fuel Injection

hanger
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Re: Tracking down an electrical issue

Post by hanger »

I know this is a rather old post but I am having a fuel gauge circuit issue and starting to dive into it. What size of resistor do you recommend using between the sender lead to ground to test the fuel gauge? I put a test light on this lead and it didn’t light up the light so I’m thinking there wasn’t enough voltage to operate the 12v test light perhaps or is my logic flawed?

I should add, the problem I’m experiencing with the fuel gauge is when I turn the key on it slowly goes from empty to full. Once in a great while (every 6 months or so) it will operate correctly for about 10 minutes and then goes back to reading full. Measuring the voltage at the lead removed from the sending unit I record 8.2 volts. The ohm reading across the leads of the sending unit briefly shows a measurement and then goes to open. I’m thinking perhaps it’s the sending unit. Testing the ground wire to ground (running a test wire to the negative battery post) shows continuity to ground.
Last edited by hanger on Sat Aug 06, 2022 6:13 am, edited 1 time in total.
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tgreese
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Re: Tracking down an electrical issue

Post by tgreese »

The sender is a variable resistor. it has standard values at full, half, empty. Those values are listed in your TSM.

Use a multimeter on the resistance setting. A test light may show you something, but it's hard to predict without knowing more about your test light. An old-fashioned test light is a bulb in series with the probe, and probably won't light from the sender current.

I presume the gauge forms a voltage divider with the sender, and displays the voltage across its half of the divider. The gauge circuit puts a very small current on the sender wire, which goes to ground at the sender. The variable resistor is connected to chassis ground at the fuel tank; that connection must be good.

The TSM has flow charts for testing the gauges. That's where I would start.
Tim Reese
Maine beekeeper's truck: '77 J10 LWB, 258/T15/D20/3.54 bone stock, low options (delete radio), PS/PDB, hubcaps.
Browless and proud: '82 J20 360/T18/NP208/3.73, Destination A/Ts, 7600 GVWR
Copper Polly: '75 CJ-6, 304/T15, PS, BFG KM2s, soft top
GTI without the badges: '95 VW Golf Sport 2000cc 2D
Dual Everything: '15 Chryco Jeep Cherokee KL Trailhawk, ECO Green
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hanger
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Re: Tracking down an electrical issue

Post by hanger »

Update

I should add, the problem I’m experiencing with the fuel gauge is when I turn the key on it slowly goes from empty to full. Once in a great while (every 6 months or so) it will operate correctly for about 10 minutes and then goes back to reading full. Measuring the voltage at the lead removed from the sending unit I record 8.2 volts. The ohm reading across the leads of the sending unit briefly shows a measurement and then goes to open. I’m thinking perhaps it’s the sending unit. Testing the ground wire to ground (running a test wire to the negative battery post) shows continuity to ground.
[/quote]
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tgreese
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Re: Tracking down an electrical issue

Post by tgreese »

How do you interpret the voltage you read at the sending wire? Relaying your observations likely won't help with electrical issues. You need the car in front of you and you need to devise tests that will tell you something.

The TSM has instructions for testing. I would substitute an equivalent resistance for the sender and see what the gauge reads. I'd also test the ground of the sending unit.
Tim Reese
Maine beekeeper's truck: '77 J10 LWB, 258/T15/D20/3.54 bone stock, low options (delete radio), PS/PDB, hubcaps.
Browless and proud: '82 J20 360/T18/NP208/3.73, Destination A/Ts, 7600 GVWR
Copper Polly: '75 CJ-6, 304/T15, PS, BFG KM2s, soft top
GTI without the badges: '95 VW Golf Sport 2000cc 2D
Dual Everything: '15 Chryco Jeep Cherokee KL Trailhawk, ECO Green
Blockchain the vote.

hanger
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Re: Tracking down an electrical issue

Post by hanger »

Figured it out. The sending unit is bad. Got one ordered. Thanks.
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devildog80
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Re: Tracking down an electrical issue

Post by devildog80 »

When you drop the tank to replace, look at your old sender, as when I pulled mine out of the tank it was crudded up with old dried fuel residue across the coils.

Initially had 0 ohm reading, so cleaned up real good, and sender worked perfect.

Not always salvageable, but if it is, it's worth saving.

And honestly I think as good as, or better, than the new one.

On the pic, gently bend back the 3 tabs just enough so the cover comes off, and coil is inside.
Some will have a broken coil, but others this is where they oxidize fuel residue across the surface, and the float tab that moves across the coil does not contact and read correctly, or at all.

I used Super Clean to soak for 2-3 hrs, then a soft brush & carb cleaner to remove the gunk.
As you do not want to submerge the whole sender in cleaner, soak a small rag with it, and lay on the coil to soak.

Image20220515_144435 by Scott Weckerly, on Flickr

Image20220515_144447 by Scott Weckerly, on Flickr

Image20220515_160447 by Scott Weckerly, on Flickr
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tgreese
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Re: Tracking down an electrical issue

Post by tgreese »

devildog80 wrote: Sat Aug 06, 2022 10:48 am When you drop the tank to replace, look at your old sender, as when I pulled mine out of the tank it was crudded up with old dried fuel residue across the coils.

Initially had 0 ohm reading, so cleaned up real good, and sender worked perfect.

Not always salvageable, but if it is, it's worth saving.

And honestly I think as good as, or better, than the new one.
...
Excellent suggestion. I'd expect the OEM part is of much higher quality than any aftermarket replacements - as long as the original is not worn out. And replacement senders often come defective from the aftermarket supplier; suggest you test any replacement well before you install. An analog multimeter would be ideal, but a digital multimeter will work ok.

Good, cheap analog multimeter: https://www.amazon.com/dp/B00MJKS0EM/
Tim Reese
Maine beekeeper's truck: '77 J10 LWB, 258/T15/D20/3.54 bone stock, low options (delete radio), PS/PDB, hubcaps.
Browless and proud: '82 J20 360/T18/NP208/3.73, Destination A/Ts, 7600 GVWR
Copper Polly: '75 CJ-6, 304/T15, PS, BFG KM2s, soft top
GTI without the badges: '95 VW Golf Sport 2000cc 2D
Dual Everything: '15 Chryco Jeep Cherokee KL Trailhawk, ECO Green
Blockchain the vote.
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tgreese
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Re: Tracking down an electrical issue

Post by tgreese »

Also, if you have a sender with a sunk float, a brass replacement like the original is available from Ford restoration places.
https://www.cjponyparts.com/fuel-tank-s ... 973/p/FSUF
Tim Reese
Maine beekeeper's truck: '77 J10 LWB, 258/T15/D20/3.54 bone stock, low options (delete radio), PS/PDB, hubcaps.
Browless and proud: '82 J20 360/T18/NP208/3.73, Destination A/Ts, 7600 GVWR
Copper Polly: '75 CJ-6, 304/T15, PS, BFG KM2s, soft top
GTI without the badges: '95 VW Golf Sport 2000cc 2D
Dual Everything: '15 Chryco Jeep Cherokee KL Trailhawk, ECO Green
Blockchain the vote.
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dodgerammit
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Re: Tracking down an electrical issue

Post by dodgerammit »

I'll add, if yours is later than 1980, you don't have to drop the tank to replace sender. Mush easier to flip rear seat forward and remove access cover ;)
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