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kryptronic

HO: Voltage Guage Reading Off

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Vehicle specs: 91 High Output MJ 4.0L AX-15 4WD

 

A couple of weeks ago the truck started cranking slower, and my radio would dim with my flashers on, so I knew it was time for a new battery, and thought it might possibly be time to replace the alternator.  Voltage guage looked low too. I installed a new Bosch size 34 battery to replace the size 58 that was in there.

 

With the key turned to ACC with the engine off, this is what I see:

 

20180811_101142.jpg

 

20180811_101301.jpg

 

The guage looks like it's reading a tad low.  With the engine on and the alternator charging the battery:

 

20180811_100824.jpg

 

20180811_100930.jpg

 

So it looks like the alternator is just fine, but the guage reads low, by more of a margin.

 

So what's the fix?  Does this reuquire a new guage, or cluster, or is this an indication of a bad connection or ground somewhere?  Clean cluster contacts?  Any inights are welcome.

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My voltmeter has read exactly like that for the twelve years I've had the full gauge cluster in, a needle's width below 14V with the engine running. And I get the same charging voltage across the battery as you using my DVM, ~14.5V. It's nearly impossible to trace it the voltmeter + input wire back from the gauge tell using the FSM to see at what point electrically the voltmeter gets it sensing voltage, but it appears to be at the ignition switch. I've stopped worrying about it, never had a charging problem. But if you find a fix, let me know.  :laugh:

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44 minutes ago, HOrnbrod said:

My voltmeter has read exactly like that for the twelve years I've had the full gauge cluster in, a needle's width below 14V with the engine running. And I get the same charging voltage across the battery as you using my DVM, ~14.5V. It's nearly impossible to trace it the voltmeter + input wire back from the gauge tell using the FSM to see at what point electrically the voltmeter gets it sensing voltage, but it appears to be at the ignition switch. I've stopped worrying about it, never had a charging problem. But if you find a fix, let me know.  :laugh:

 

10-4.  Thank you.  I had a feeling your answer was what would come back in the replies.  I'm going to dig into it further.  Maybe run the same tests on the 92 at some point.  I was worried the alternator was going bad. 

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2 hours ago, kryptronic said:

So it looks like the alternator is just fine, but the guage reads low, by more of a margin.

 

So what's the fix?  Does this reuquire a new guage, or cluster, or is this an indication of a bad connection or ground somewhere?  Clean cluster contacts?  Any inights are welcome.

 

I wouldn't worry about it. Any stock automotive voltmeter is not a high-quality, precision instrument. If the system checks out, just get used to where the needle sits and pay attention if it ever points somewhere else.

 

On your gauge, if we assume a liner sweep (which I think the voltmeter is, there's a 5-volt step from 9 to 14, and another 5 volts from 14 to 19. So the white tick between 9 and 14 is 11.5 volts. Midway between that tick and the 14 mark would be 12.75. Your needle reads only slightly below that in the engine off photo (12.94 volts at battery), which can likely be attributed to voltage drop through the wiring.

 

Voltage with the engine running should be not less than 13.5 and not more than 15. You are reading 14.48 at the battery, which is within spec and shows that the alternator is working. The reading at your gauge is just a hair under 14, which again is probably due to voltage drop in the wiring. If you see the needle pointing at that tick between the 9 and the 14 marks when the engine is running, that's the time to sit up and pay attention.

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I agree with Eagle.  The instrument panel Voltmeter in most vehicles, particularly the older stuff like an MJ, are considered "trend indicators" at best.

 

In other words, they are NO WHERE near accurate enough to indicate anything more than if the system is charging or discharging the battery, and that is done by observation and experience of the guage reader (aka, you, the driver).

 

I suppose one could go crazy and attempt to "recalibrate" the guage on a workbench outside the vehicle.   However, there is no guarantee that process wouldn't break the gauge completely, so it's a risky proposition.

 

If it was me, I would learn to live with it.

 

PS....Eagle's assumption that the gauge has a linear sweep is up for contention.  Those older instruments tended to be very NON-linear.

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3 hours ago, AZJeff said:

PS....Eagle's assumption that the gauge has a linear sweep is up for contention.  Those older instruments tended to be very NON-linear.

 

The actual response may be flaky, but look at the numbers. 9 + 5 = 14, 14 + 5 = 19. Equal sweep for equal numeric values. Linear.

 

Then look at the temperature gauge. There's no way you can interpolate accurately on the temp gauge, but the oil pressure gauge is also calibrated on a linear scale.

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Thanks for the input.  I'm going to just leave it be for now.  It sounds like it's working as well as it was designed to. 

 

I could swear, though, that my 95 YJ read dead on when it ran last, but I've got to finish reassembling the vehicle before I can verify that.  

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Hey Don. do the HOs have the same instrument panel grounds as the Renix era?

 

Here's why I ask:

 

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, seat belt and key warnings, transmission power/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 share 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. Use a coating of OxGard at all ground contact surfaces when attaching the screw and nut.

IP ground location

**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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On 8/11/2018 at 3:48 PM, kryptronic said:

Vehicle specs: 91 High Output MJ 4.0L AX-15 4WD

 

A couple of weeks ago the truck started cranking slower, and my radio would dim with my flashers on, so I knew it was time for a new battery, and thought it might possibly be time to replace the alternator.  Voltage guage looked low too. I installed a new Bosch size 34 battery to replace the size 58 that was in there.

 

With the key turned to ACC with the engine off, this is what I see:

 

20180811_101142.jpg

 

20180811_101301.jpg

 

The guage looks like it's reading a tad low.  With the engine on and the alternator charging the battery:

 

20180811_100824.jpg

 

20180811_100930.jpg

 

So it looks like the alternator is just fine, but the guage reads low, by more of a margin.

 

So what's the fix?  Does this reuquire a new guage, or cluster, or is this an indication of a bad connection or ground somewhere?  Clean cluster contacts?  Any inights are welcome.

 

Stumbled on this post because I'm having the same issue. I found this fix: 

 

https://www.jeepforum.com/forum/f11/renix-voltage-gauge-fix-684375/

 

Haven't tried it yet. But I might. 

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10 minutes ago, ratty said:

 

Stumbled on this post because I'm having the same issue. I found this fix: 

 

https://www.jeepforum.com/forum/f11/renix-voltage-gauge-fix-684375/

 

Haven't tried it yet. But I might. 

 

Good fix, BUT ...

 

If you read the comments, it is noted (somewhat indirectly) that this fix eliminates the fuse for the gauges. The OP then comments that he needs to add an in-line fuse ... to which I would absolutely agree.

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5 minutes ago, Eagle said:

 

Good fix, BUT ...

 

If you read the comments, it is noted (somewhat indirectly) that this fix eliminates the fuse for the gauges. The OP then comments that he needs to add an in-line fuse ... to which I would absolutely agree.

 

Ah. Missed that part. Thanks for the heads up. 

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I got rid of my fused links and went to a battery mounted breaker. 

Upon ignition the truck is more responsive, fuel pump jumps to full pressure instantly and engine starts quicker. 

May be anecdotal or may be that 32 year old links have become resistors. 

 

Just a thought and I'm not suggesting that you do what I did. And of course consider that every connection in between including the grounds are corroded.

 

 

QCbRKHd.jpg

 

 

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On 2/17/2019 at 6:58 AM, Jeep Driver said:

I got rid of my fused links and went to a battery mounted breaker.  

 

QCbRKHd.jpg

 

 

So no more fusible links coming from starter relay to everything else, looks really clean. Did you only wire fused links to breaker or all wires that were attached to starter relay?

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7 hours ago, Me&my86MJ said:

So no more fusible links coming from starter relay to everything else, looks really clean. Did you only wire fused links to breaker or all wires that were attached to starter relay?

As viewed in the pic-

 

Left lug- 4g wire directly to alt. 

Right lug- 6g-4g lug, six 12g wires soldered to lug, each wire replaced the four fused links and connects directly to the six wires that were connected to the fuse links that were mounted to the starter relay. 

Center lug- 4g wire directly to the starter relay. 

 

From the starter relay- one 4g wire to the starter, non-fused. 

One 4g wire to after market fuse panel for accessories. 

 

Also, remaining but unused- one 175amp Megafuse that used to protect the alt, will be used in the future to protect 4g wire to two new amplifires, splitter will be fused before amps. 

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15 hours ago, Jeep Driver said:

As viewed in the pic-

 

Left lug- 4g wire directly to alt. 

Right lug- 6g-4g lug, six 12g wires soldered to lug, each wire replaced the four fused links and connects directly to the six wires that were connected to the fuse links that were mounted to the starter relay. 

Center lug- 4g wire directly to the starter relay. 

 

From the starter relay- one 4g wire to the starter, non-fused. 

One 4g wire to after market fuse panel for accessories. 

 

Also, remaining but unused- one 175amp Megafuse that used to protect the alt, will be used in the future to protect 4g wire to two new amplifires, splitter will be fused before amps. 

Oh very clean and thank you for detailed explanation. Looks like a very nice clean upgrade

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I got rid of my fused links and went to a battery mounted breaker. 
Upon ignition the truck is more responsive, fuel pump jumps to full pressure instantly and engine starts quicker. 
May be anecdotal or may be that 32 year old links have become resistors. 
 
Just a thought and I'm not suggesting that you do what I did. And of course consider that every connection in between including the grounds are corroded.
 
 
QCbRKHd.jpg
 
 
Can you post some links? I'm having trouble finding exactly what you have there.

Sent from my SM-G920P using Tapatalk

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4 hours ago, knever3 said:

Can you post some links? I'm having trouble finding exactly what you have there.

Sent from my SM-G920P using Tapatalk
 

Go here-  part number given.

 

 

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