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Renix Gauge Question

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My gauges are off and I was wondering if anyone has any tips of what could be the problem. "87 Comanche 4.0L "well 4.7 actually" auto 4x4


My oil pressure gauge is about 20psi low

My volts is low, around 2V below what is measured at the battery

I don't know if the fuel is low because when the light comes on I fill it up and it goes from empty to full without problem

Tach works fine but I have no way to test against the gauge.

Spedometer seems accurate, but I haven't used the GPS to verify "probably should"


What could cause the first two to be much lower than normal?  The oil pressure sender is original but I can't see why it would go bad.  I don't know what picks up the voltage.



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18. Improving the instrument panel ground


The ground point for the complete instrument cluster on your XJ or MJ is located up under the driver’s side dash. If you lay on your back and look up under there with a flashlight, without wearing a hat, you will see a black wire attached to a shiny piece of metal almost directly above the hood release knob. The screw will have either a ¼” or 5/16” head on it.


This ground point is responsible for handling the ground circuit for the following items: Dome lamps, Seatbelt and key warning, trans comfort switch, wiper switch, headlamp switch and delay module, fog lamp switch, cargo lamp switch, all instrument panel grounds and illumination, power windows and door locks, cruise control dump valve, and a few more things.


The problem is that where the ground point is located does not have a good contact with the chassis where the ground should be. The solution is simple.


Make up a jumper wire with #10 gauge wire about 10” long. On one end, crimp on a ¼” round wire terminal. On the other end, crimp on a 3/8” round wire terminal.


Remove the screw from the existing ground wire and attach the small terminal of your jumper so that the original wire and your new jumper share the same attaching point, one over the other.


Look above the driver’s side plastic kick panel just forward of the top of the hood release knob. You will see an 8mm stud there. Attach the large terminal end there with a washer and nut over it tightened securely.


**Special note for Comanche owners: Make your jumper wire 12” long and attach it on the driver’s side kick panel close to the fusebox on the 8mm stud.**



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Revised 11-29-2011

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Temperature and oil pressure gauges in my '88 XJ read low, and have been that way for the past ten years or so. I've replaced the oil pressure sender three times, and it made no difference.


Factory gauges aren't the most accurate in the universe. I regard them as more of a "GO / NO GO" indicator. As long as the needle points where it usually runs, I figure everything's okay. If it moves away from whatever it considers "normal," then I start checking things.


Still better than idiot lights -- although IMHO an ideal setup would be both.

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The oil pressure gauge is directly dependent on the variable resistance generated by the oil pressure sensor. Since the low ohm change generated by the sensor dependent on oil pressure is low (less the 300 ohms or so throughout it's range), corrosion at electrical junction points between the sensor and the gauge can cause a false gauge reading. Since you have added a good ground to the cluster, yes, change out the sensor. I've seen variations of 20-40 psi differences between various pressure sensors on the same gauge. Some are more accurate than others.


The voltmeter is easy to check; it reads voltage directly from it's positive sensing location. Throughout the years the sensing points of the VM were changed. To check the accuracy of the gauge itself, remove it from the cluster and directly apply voltage to the gauge terminals observing polarity from a voltage source like a 9VDC battery, comparing it's reading to a known standard, like a calibrated multimeter. If both voltage readings agree, the dash VM is good, and the problem is either the dash VM sensing point, or again, corrosion in the wiring between sensing point and the gauge.


And as Eagle said above, if you get a ± 20% accuracy tolerance on the factory gauges, UB doing well.

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