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What can cause low voltage other than bad alternator?

Lobster70

NAXJA Forum User
Location
Spokane, WA
I'm here on behalf of my son, who is the proud owner of a 1991 Jeep Cherokee Sport, 4.0L, AT. We have a problem with the electrical system that looked like a bad alternator, but may be something else.

Details:
  • Engine will start and run, but electrical system is low voltage (dim headlamps, turn signal won't blink, wipers very slow, etc.)
  • Gauge on dash shows low voltage (red zone, left side of scale, 8V or something)
  • Revving the engine makes no difference in headlight brightness or dash gauge.

Tested the alternator while on the car using a load test tool with an alternator test setting. It showed that the alternator was bad. It also showed that the battery is good. It is a "24 month" battery that is 4 years old.

Last night we fully charged the battery and replaced the alternator. To my dismay, there is almost no change in behavior. I tested with a volt meter on the battery terminals just now:
- engine off: 12.59 V
- engine on: 12.0 V
- engine on plus headlights, heater, radio on: 11.5 V

...so, it is not charging like it should. I would expect 13V or so.

One stupid thing I did was fail to have the old alternator tested, but I just called the auto parts store and they still have it. They will test the old one AND the new one when I bring it in. I will update this post after that. However, I have been told that the test off the car is less reliable than when in a running engine.

I know it is possible that the new alternator (a remanufactured unit) is also bad. Assuming both alternators are actually OK, what other component could cause these symptoms?

Other factors that might be important:
My son left his headlights on a few weeks ago. His friends, also 16-year olds, gave him a jump start. Could they have done something wrong that would cause this problem?
Also, the cheap aftermarket stereo that it came with acts up (he will be replacing it) and he is convinced that the stereo receiver is causing this somehow. I'm not.

Any advice will be greatly appreciated. This is my first post here, coming from another forum that is apparently losing members like crazy. I joined there unaware, but have been told NAXJA is the place to be.

Eric
 
Clean the battery terminals

Clean the battery-to-chassis ground, and clean the engine-to-chassis ground (strap at the rear of the engine). The ground path between alternator and battery is alternator-to-block (the mounting bolts), engine to chassis (ground strap), and chassis to battery (ground strap). If any of those are dirty it will raise resistance.

Check the hot wire between battery positive and alternator output, its fused and sometimes it pops unevenly.

Take the battery to Autozone or wherever, they can do a load test on it by drawing amperage, which will tell you more than you can get from a voltage test.
 
A slipping belt will usually squeal at least occasionally but but I would check it.

A bit of background might help. Have you had the Jeep awhile and this just started recently or what. Do you know history of drive belt replacement for example?

Not the head unit. Perhaps wishful thinking to accelerate purchase of the replacement?

And as mentioned, you need to take every cable connection apart, clean and tighten. This is just good maintenance even if it is not the immediate problem and a visual inspection often will not reveal a corroded connection.

Edit: ehall said it better...
 
Thanks for the quick responses! I forgot to mention that we did clean the "auxiliary" ground that connects to the body near the battery. The main ground looks good but I can clean it for good measure. We also cleaned the battery terminals and clamps before we reassembled. All the alternator connectors were also cleaned upon reassembly.

The battery test was via a load tester (the kind with a heating element inside) so I'm confident it is still good.

HOWEVER, it just occurred to me after posting my original question to check resistance between the output terminal on the alternator and the positive battery cable. It should be low. [There's a great video on YouTube from someone called Eric the Car Guy that shows how to test this. He also shows how to check whether there is a ground problem by going from the alternator case to the positive terminal, that checked out fine.] The value should be 0.1 or 0.2 at the most. Mine was reading 18!

That seemed so far off that I watched the video again and retested, same result. That revealed a problem in the connection between the alternator and the battery. Where might there be a disconnect in that electrical route...? Why didn't I think to check the two 60A fuses?!? BOTH were blown. No wonder the alternator couldn't charge the battery. (Good call ehall.)

So...off to the parts store for two fuses. $9 later they're now in and the reading on the battery while running with all accessories on is 14.1 V. Problem solved!

There's a fair chance that we didn't need to replace the alternator, but it's possible that it was bad and blew the fuses. Or that whatever caused the blown fuses (16-year olds doing a jump start maybe?) also affected the diodes as well as those two fuses.

Lessons learned:
1. Check fuses before replacing alternator.
2. Test alternator at parts store before buying replacement.

Thanks again for the help.
 
Any advice will be greatly appreciated. This is my first post here, coming from another forum that is apparently losing members like crazy. I joined there unaware, but have been told NAXJA is the place to be.

Eric

Oh i need a new high end stereo because the cheap one is killing my alternator, funny that never worked on my dad.

Never, Never, Never, Ever jump start a Jeep, the regulator is in the ECU (OBD1 and up), a voltage spike can take out the ECU. if your battery is dead hook jumpers leave the key off on the Jeep and just the running vehicle as a charger, make sure you have good quality cables, cheap ones will get hot.

in this case you had the alternator full field after the jump and that is most likely what killed the fuses.

BTW nice to see you on the better site and how is the headlights?
 
Never, Never, Never, Ever jump start a Jeep, the regulator is in the ECU (OBD1 and up), a voltage spike can take out the ECU.

I went through about 12 Dodge alternators in the 1980s on my 76 Charger and 78 wagon before I made the connection. And they did not have an ECU. Alternator usually lost half the diodes, one side of the bridge and drained the battery at night with the head lights on. Within 1-4 days it was a no start. I learned to watch the head lights dimming for an early clue, then finally made the jump starting delayed signs of damage connection.

:laugh3:I lost my jumper cables and have not seen them since then, LMAO!!!!
 
yup, 40/90 and 50/110 Chrysler alternators were the reason they went to GM for the CS, but then Chrysler had never built a good alternator. if i had a $ for every Chrysler alternator i replaced with a GM i.......o wait i do.
 
For knowlege that'll save me some trouble - which fuses?
I believe the two (2) that say "generator output". #1 and #9
2f6ad72a1e5498f13bbcdf0d3e050e64.jpg


Sent from my ONEPLUS A6003 using Tapatalk
 
As for jumper cables, I'd get the type with a fuse or such. Basically, prevents a surge that can take out the EMC. I do use and carry one of the jump start packs. I did have an issue with a '96 Ford Contour after I jump started a neighbors Saturn. Took out the brake light switch. Then had other issues. I think the squirrel under the hood did not help.
 
How did this thread go from low voltage causes, to a fuse topic? A blow fuse kills all voltage / power on the outbound side of the fused line. That image does not look like Renix late 80's XJ set up, mot like late 90s model. The early 85-91 models were known for undersized wiring, corroded and over heated connections that led to hot/voltage drop loctions at connections, and switches, Like the ignition switch and Blower fan, etc... Connections at the Battery as well is where one looks for corroded, loose contacts that are prro conductors that lead to a voltage drop.
For knowlege that'll save me some trouble - which fuses?
 
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