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Gremlins in Jeep CJ Fuel and Temperature Gauges


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#1 Kevin's 83-CJ8

Kevin's 83-CJ8

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Posted 31 July 2014 - 12:43 PM

Please help,

 

I need some help on a nagging issue. I have been messing with my fuel & temp gauges for a few years and cannot get them to work right. I have replaced the fuel-sending unit twice. I have replaced the temperature-sending unit 3 times. I have replaced the gauges several times. The last time I worked with Omix (the manufacturer of the gauges) directly to help solve the problem. They said it must be a ground issue and sent me a whole new speedometer, odometer and new gauges. This did not work either.

 

One set of gauges caused the ignition fuse to blow (that was a whole chain of repairs and investigation to learn that it was a fuse that would not let the Jeep start. I thought I had really done something bad). The second set worked well for a day and a half then the temperature gauge started to surge to hot. Then it would just jump straight and fast to hot. I started the motor the next morning for the first time and it snapped right to hot. It was off the gauge and looked like it was pressing against the needle stop. The fuel gauge continued to work correctly. Once the motor was stopped, the temperature gauge only returned to the middle of the gauge and is now stuck in that position. When I restarted the motor, neither gauge worked at all. I could get them to work intermittently by messing with them, but I have never been able to get them to work right or faithfully.

 

The wires are good coming from the fuel-sending unit. That ground is good. The symptoms do not correct when I run a new wire to the sending units. It has to be a ground or short somewhere or the company keeps sending me defective gauges. I have bought or replaced them about 5 times. Is there another company that sells these that work? Is there a magic trick to getting them to work? I want to bang my head on the dash to get them to work.

 

I really need help on this one. I would love to know how much gas I have during trips.



#2 Moses Ludel

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Posted 31 July 2014 - 03:12 PM

Kevin, it helps to understand the basic premise of how these gauges work.  Simply put, with the key "On", the gauge is "hot" all the time.  The supplied voltage does not vary.  What varies is the ground side in ohms-resistance. 

 

My tool of choice for troubleshooting is a digital volt-ohmmeter.  There are several troubleshooting options.  With the key "On" check the hot side of each gauge.  You should get a constant 12VDC here (unless the gauge system is ballast resisted to a lower voltage on the hot side).  Once you confirm correct voltage on the hot side at each gauge, move to an ohms-resistance test of the senders.  Or perform this preliminary, simple test:  ground each wire leading to a sender, just momentarily, and the gauge should swing to the high side.  Do not continue to hold the wire to ground!

 

Electric gauges have senders that serve like a rheostat, varying the ground completion, if you will.  The best example is the fuel tank sender, which you should know intimately by now!  That float arm moving the sender mechanism up and down is actually varying the ground resistance.  This variable (ohms-resistance) ground signal goes to the gauge as a variable ground signal.

 

Using the fuel gauge and sender as an example, you should be able to take a fuel gauge sender (outside the fuel tank) and make the gauge go up and down by simply raising and lowering the float arm.  You will need to ground the top flange of the sender to the chassis while performing this test.

 

For the temp gauge, you can simply disconnect the wire from the engine temp sender and hold it to ground momentarily.  The gauge should swing to the "H" or hot side.  The sender provides a variable ohms-resistance ground. 

 

The oil sender works the same way.  You should be able to momentarily ground the sender wire at the oil pressure sender and watch the gauge rapidly move to the high side.  This is also a good way to test whether the sender is any good.  (There are ohms-resistance tests in the factory workshop manual, if you need those details for testing a sender, let me know.)  Be aware that the tiny oil feed hole in the 258/4.2L OE oil pressure sender is notorious for plugging, which prevents the gauge from reading accurately—or at all.

 

So, if all of the gauges are swinging high, there is a short to ground or a direct ground to chassis/body on the ground side of the gauge wiring.  If the problem is isolated to one gauge, that one sender lead may be shorting to ground if the gauge, by itself, swings to the high side. 

 

Using the ohmmeter is very valuable here.  You can check a sender wire from the sender all the way to the gauge if necessary.  Disconnect the sender wire at the sender.  Take your ohmmeter and a jumper lead and check the continuity from one end of this wire to the other.  Then check for a short by holding the ohmmeter probes between the detached sender wire and a solid ground.  (The meter should be set for continuity.)  If the wire is not attached to the sender, there should be no continuity to ground—unless there is a short to ground somewhere between the sender end of the wire and the gauge.  A short can mean either a single wire lead or a common short/ground that shorts several gauge wires at one time.

 

We can discuss this further after you digest this post.  Pleased to walk through electrical troubleshooting as needed.

 

Moses



#3 Kevin's 83-CJ8

Kevin's 83-CJ8

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Posted 02 August 2014 - 03:57 PM

Moses,

Thanks for this reply. It is way too much to sort through quickly. It has been a long nagging problem so I guess the fix will be complex too. I have read your reply a few times and plan to print it off so I can read it in detail. It is very detailed. Thanks for that.

 

I have done most of the checks at one point or another but am a bit like a blind man stumbling around. I have never used a voltmeter. I have a cheep one but do not understand it or the settings. I am getting it. I just have to find a day and read up on it. Once I have the volt meter figured out, I can find the short…that is the plan anyway.

 

I think it is in the box that relays the wire from the engine area to under the dash. I have traced everywhere else. I have the manufacturers manual, your book and a smaller shop manual. I have been reading up on the issue lately. In the past I just replaced parts and figured it should work. This is more of a technical problem than most. I have used jumper wires from the sending units and grounds to make sure it is not the wires and it is hit and miss. So, I think it is a combination of a periodic short and bad gauges. I really want to fine a good gauge manufacturer.

 

Thanks again for the post. I will pick it apart and put it to good use.

 

Kevin



#4 Moses Ludel

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Posted 02 August 2014 - 05:37 PM

Good, Kevin...You really need a volt-ohmmeter, digital variety for accuracy at the lowest, most sensitive readings.  We'll move the discussion to the tools forum when you're primed and have the right volt-ohmmeter.  (I can even make suggestions here if you're shopping for the tool.)

 

You will be amazed what a volt-ohmmeter can share compared to either a voltmeter (only) or a continuity tester.  We can even discuss the lamp load test, a simple, homespun tool that can unearth the deepest gremlins, like an open or weak wire or circuit that seems to appear intermittently.

 

Happy to pick this up when you're ready!

 

Moses




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