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post #1 of (permalink) Old 05-29-2017, 06:29 PM Thread Starter
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CJ7 Speedometer Help

I'm trying to rid the gremlins from my temp/ fuel gauges in my speedometer.
The shop manual says the constant voltage regulator is inside the fuel gauge.
A worksheet to diagnose these problems says it's the regulating strap that runs between the the fuel and temp gauge that steps it down to 5 volts.
Which is it?
And do the replacement gauges feed the voltage the same way?

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post #2 of (permalink) Old 05-30-2017, 08:09 AM
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John Strenk's CJ7 Gauge Troubleshooting

Newer gauges are normally not fed the same, nor read the same unless you get the Stewart Warner gauges.
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post #3 of (permalink) Old 05-30-2017, 08:18 AM Thread Starter
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Thanks.
If I need to replace these is there a "better" replacement?
Looks like my choices would be Omix, Crown or Stewart Warner.
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post #4 of (permalink) Old 05-30-2017, 09:16 AM
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1) Those gauges are generally pretty robust and reliable.

2) Almost all problems with them can be traced to faulty grounds.

3) The voltage regulation takes place within the gauge cluster, probably within the fuel gauge. Power comes in through a red 18 gauge wire that is spliced to a wire that also supplies power to the oil pressure gauge, so if it's working the problem probably lies in the cluster itself or its grounding.

It gets is ground through the housing to the mounting screws, through the instrument panel, the body tub and eventually the ground straps back to the battery. Before doing anything drastic, scrape a clean spot on the back of the cluster housing and run a good ground wire directly from the battery post. If you see a change in the reading when you scrape the wire to the clean spot, you know it's a ground problem. The most likely place is between the instrument cluster and the panel - rust and corrosion on the mounting studs and the cluster housing, which you might be able to correct simply by loosening and retightening the nuts.

This information comes from experience and my '78 shop manuals, but I doubt there's any significant change between then and '84.
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post #5 of (permalink) Old 05-30-2017, 04:40 PM Thread Starter
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Removed the speedo and completely disassembled. It has the aftermarket Stewart Warner upgrade gauges.
The Stewart Warner fuel and temp gauges that are in this speedo are two post and use 12 volts.
I can't remember if I changed these out 15 years ago or not or just added an additional ground to the case.
I'm guessing the specs for checking these is different than for the factory gauges.
Any ideas what my testing numbers would be, or if where I could find a Stewart Warner spec/install sheet for these?

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post #6 of (permalink) Old 06-01-2017, 12:42 PM
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Stewart Warner's website would be my only guess. There's probably nothing on line there, so send an inquiry to them asking for information.

But if they look like factory gauges with the SW logo stamped on them somewhere they may be original. The original voltmeter in my '78 died under warranty and the dealer replaced it with one that has the SW logo on the front. I have no idea if the original voltmeter had the SW logo, and it's the only factory gauge left, but SW was making gauges for Jeep in the late '70s so there's a good chance they made the instrument cluster gauges.

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post #7 of (permalink) Old 06-01-2017, 01:03 PM Thread Starter
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Looked to SW first with no luck. I may try a direct contact.
These gauges appear to be the "upgrade kit" factory gauges from Chrysler Mopar #4740791 using Stewart Warner 835878 - fuel and Stewart Warner 835879 - temp.
I did learn from reading that both gauges use 12v.


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post #8 of (permalink) Old 06-02-2017, 02:02 PM
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OK, so have you eliminated the possibility of ground problems as I suggested? What are the symptoms of the gremlin's work?

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post #9 of (permalink) Old 06-04-2017, 07:25 PM Thread Starter
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Here's where I am.
Verified fuel and temp gauges work by shorting to ground - both pegged out.
I have redundant grounds. I have one from the back of the speedo to the battery.
I have a ground from the fuel tank ground wire mount to the battery.
Grounds are good.
Voltage in on the fuel gauge is dependent on either battery power (12+ volts) or alternator (14.4 volts).
Voltage out to fuel sending unit and temp gauge is 3 volts less than the voltage in.
Wire connector (pink) at sending unit unit has the same voltage as the output of the gauge.
Still no working fuel gauge - suspect sending unit.
Still no working temp gauge - failed ohm test on sending unit - will replace.
Still hoping to hear back from Stewart Warner on my gauges to verify voltage needs. Without that I'm just guessing.
Does my process sound about right?
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post #10 of (permalink) Old 06-06-2017, 06:07 PM Thread Starter
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The temp sending unit was indeed bad. The new sending unit fixed the temp gauge problem.
Still no luck with the fuel gauge.
Next move is to drop the tank to verify the sending unit is bad.
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