alternator, bad gauge, or what?

Last week I noticed the voltmeter is reading barely above the red mark. I have checked all the electrical connections that I could find and they are tight. The battery is one year old with 4K miles on it. The braided ground wire from head bolt to firewall is replaced.

I measured the voltages across the battery terminals, with a cheapo multimeter and below are the readings:

12.7 - 13.3 volts = engine off
11.9 - 12.1 volts = engine cranking
14.3 volts = engine idling with all accessories off
13.3 - 14.2 = engine idling with all accessories on (A/C on, blower on high, blinkers on, headlights on, etc)

What's going on? I have driven like this for a few days and have had no starting issues. Could it be that the gauge went south all of a sudden. Should I take her to a shop for a more "professional" diagnosis?


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Sounds like a shonky gage to me - especially if it's RENIX. The IP gage is a notorious liar, and it is usually "calibrated" with a magnet stuck on the back to alter the reading somewhat. This magnet will slip over time, and with getting knackered about.

Your DMM readings are good, so do what I usually end up doing - take a grease pencil and make a mark where the needle sits when you know you're getting good charge. Use the IP voltmeter for "relative state of charge" (if the needle wanders off the new mark, check.) Else, ignore it unless it starts to wander more.

I find a red grease pencil works well. You want to use a grease pencil in case the magnet moves again - you can wipe the mark off and make a new one.

Mark the plexi face, not the gage face proper. Makes it easier to wipe the mark off if you need to.

Well your voltage seems good, but thats only doing a test on your battery really. Alternators are tested measuring current.

Seems to me your gauge has gone bad, or your getting an excessive draw somewhere, maybe via a short.

You can always get the battery and alternator tested for free.

Muad'Dib said:

Well your voltage seems good, but thats only doing a test on your battery really. Alternators are tested measuring current.

Seems to me your gauge has gone bad, or your getting an excessive draw somewhere, maybe via a short.

You can always get the battery and alternator tested for free.

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Yes, you can - but they don't usually load the alternator for that test.

Running a check on the alternator in situ is better than the common bench anyhow - if the current output isn't up to scratch, the voltage output won't be. If you want to work your alternator, check it with the engine idling then start turning things on (aircon, radio, electrical accessories, lighting, ...) If you don't have enough current capacity to power everything, voltage drops sharply - cf. Ohm's Law and Watt's Law (there is a direct mathematical relationship between voltage (EMF or V), current (I in equations, A in specs), resistance (expressed in Ohms), and power (watts or W.) If one changes, the others will swing as well.

Don't bother with the alternator tests at most big chains - I've said it before and I'll say it again, they only really check voltage, and the key element is current, as previously mentioned. Find a rewind shop if you want to wring out your alternator/starter/battery, since they're geared to really check it.

Thanks guys for the good feedback. I sure appreciate it.

I forgot to post a pic. When I was checking the cables, I noticed the negative battery cable attached to the oil dipstick seemed rather "worn out". I am not sure if that could be a casue but I want replace it. What gage wire is the factory cable, and what heavier gage should I buy?

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Pretty ugly...

Please see link in sig.

Buy 5-90's cables, or at the minimum replace yours with 4 gauge. That cable you posted is crap, and could certainly be why your having issues. I would also add a ground cable from the alternator to the chassis, and from the battery to the chassis if i were you.

5-90 said:

Pretty ugly...

Please see link in sig.

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Hell that ain't nothin, shouda seen the mess I had when I bought it. My diesel was even better, they ran green insulated house wiring on it, LOL.

my gauge on my 90 xj did the same thing a while back for about a month then my great 1k cold crankin amp batt finally quit pullin off a start, changed the alternator & fixed it

Ecomike said:

Hell that ain't nothin, shouda seen the mess I had when I bought it. My diesel was even better, they ran green insulated house wiring on it, LOL.

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Oh, I've got horror stories from some stuff I've had to clean up!

Still, "house wiring" isn't anywhere near good enough for underhood use - the only wire I'd use underhood is SJOOW - oil resistant, water resistant, and highly flexible. Nothing else is worth bothering, if you're going to the hardware store.

Took the Jeep to the shop for electrical testing and everything checked out OK. Went ahead and changed the main cables to 5-90's cables. The OEMs were in a very sad condition. 5-90's cables are substantial for sure. Easy swap. Thanks 5-90.

The gauge still reads low, so the verdict is bad gauge.

What are my options? Chage the guage only and where to get it from? I don't like it reading low like that. Are there some kind of upgrade kits for the whole instrument cluster?

By the way, changing out the main cables to 5-90's kit made my stock headlights noticeably brighter which I love, but the instrument clusterter and the interior lights are still rather dim. Anything else to check or any other cables to upgrade?

TIA

Depending on your skills set, I would pull the dash, and look into the gauge, gauge wiring and connections for any more poor contact problems, especially at the fuse box on the firewall, and the starter ignition switch down under the dash on the steering column. Might also consider just running some 16 gauge wires from the gauge to the battery. I suspect a fresh ground wire from the gauge to the battery negative will solve the display problem, unless you have some problems at the igntion switch wiring and / or the fuse box, both of which contribute to the gauge problem when the head lights and the E-fan and AC blower are all running and the contacts have gotten oxidized and overheated.

For one of the greatest contact cleaners (mechanical) I've ever found, see if you can't find a schoolboy's "ink eraser" - they're sort of a light grey, vice the normal "Pink Pearl" colour.

The "Pink Pearl" is second best, followed by a white draughtman's eraser.

If you have an "electric eraser" (used by draughtsmen) available to you, get the ink bungs for it (also a sort of steel grey colour) and use those. Clean up the contacts on the backside of your IP.

I think I've got a couple of RENIX IPs floating about, but I don't know what shape the voltmeter is in - and I haven't seen my variable DC voltage supply in a while, so I can't really test them for response and accuracy at the moment. Still, if you're willing to take a chance, we can work something out. But, I'd check voltages at the battery first (I've outlines the voltages you should be seeing elsewhere - I really should write that up one of these days) to make sure that the problem is actually with the voltmeter, and not with the system. Then, get out your grease pencil, make a new "nominal" mark, and use the IP voltmeter for "relative state of charge" instead of "absolute voltage reading." I've had five RENIX rigs - they're all liars.

Update;
The gauge is erratic now. It’s reading varies day by day. Some days it reads a full 14 volts, and some days it’s down to 9-10 volts. It’ll stay at whatever voltage it shows when the Jeep is started first in the morning, all day.

Ecomike said:

Depending on your skills set, I would pull the dash, and look into the gauge, gauge wiring and connections for any more poor contact problems, especially at the fuse box on the firewall, and the starter ignition switch down under the dash on the steering column. Might also consider just running some 16 gauge wires from the gauge to the battery. I suspect a fresh ground wire from the gauge to the battery negative will solve the display problem, unless you have some problems at the igntion switch wiring and / or the fuse box, both of which contribute to the gauge problem when the head lights and the E-fan and AC blower are all running and the contacts have gotten oxidized and overheated.

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Thanks for the tips. The dash has to come off sometime this summer since my OEM radio is not working and I need to find the problem. I’ll take a look then. It’s ironic how many XJ problems can be traced back to inadequate wiring and/or poor ground.

5-90 said:

I think I've got a couple of RENIX IPs floating about, but I don't know what shape the voltmeter is in. Still, if you're willing to take a chance, we can work something out.

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Thanks for the offer, but if I were to swap gauges I’d rather find a better quality set.

I don't think the gauges are the real problem, I think the location of the +13 to 14 volt tap (sample point) and bad, loose variable grounds (the body and frame and the old wires) are the real problem.

I have also noted that what looks like 9 to 10 volts on the gauge is actually about 12 to 12.8 volts. So 80% of the gauge span must run from 12 to 14 volts, inspite of the numerical labels on the gauge, in other words it is very non-linear!

Check the location of the needle with the power on, engine off, on mine it is just to the right of the red area so it must be 12.7 volts, but the gauge numbers (which are too sparse) makes it look like about 9 or 10 volts. If the running voltage of the gauge is at or above the non running voltage on the gauge, then you are OK. I have been working on the grounds so far all over my jeep, adding new grounds and replacing old onesand the voltage gauge with the engine running is responding much better now.

Maz said:

Update;
The gauge is erratic now. It’s reading varies day by day. Some days it reads a full 14 volts, and some days it’s down to 9-10 volts. It’ll stay at whatever voltage it shows when the Jeep is started first in the morning, all day.



Thanks for the tips. The dash has to come off sometime this summer since my OEM radio is not working and I need to find the problem. I’ll take a look then. It’s ironic how many XJ problems can be traced back to inadequate wiring and/or poor ground.



Thanks for the offer, but if I were to swap gauges I’d rather find a better quality set.

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Maz said:

Thanks for the offer, but if I were to swap gauges I’d rather find a better quality set.

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No worries - just thought I'd offer.

(Your attitude probably lines up with why I'm thinking of rebuilding my gage cluster using aircraft gages one of these days...)

Ecomike said:

I have been working on the grounds so far all over my jeep, adding new grounds and replacing old onesand the voltage gauge with the engine running is responding much better now.

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I have cleaned, and reattached with Ox-Guard, any ground that I can find with visual inspection, over time. The problem is not knowing all the ground points on the Jeep. I do have FSM and all the wiring diagrams, but that does not help much since a wiring diagram just shows schematic and not physical location of all the ground points. One has to trace wires to find the actual location of ground points and that is very time-consuming and a pain in the neck job.

BTW, thanks for all the explanations.