Hey everyone! Thanks for taking the time to read this post.
I have a 2 Door 1993 Jeep Cherokee Sport. My battery gauge reads real low the last few days, around 9v and lowering into the red. I had the alternator checked and the shop said it’s fine. I replaced the battery and it worked great for a day. So with a fully charged battery and working alternator, randomly the jeep drops to 9volts again on the battery gauge and is stuck there now. When the gauge drops to 9v the jeep also takes a quick drop in performance as if it completely looses power, and then it bounces back. Afterward the gauge is still at 9v.
Anyone please throw something this way as to tips, tricks, or processes for diagnosing and fixing this issue. Perhaps fuses or power relays?
First thing, use a portable volt meter and see if it agrees with the dash gauge.
If you took the alternator to the shop and had it “bench tested” . . . that was a mistake, in my opinion
It would have been more accurate to test everything, while it was installed on the truck
I am leaning towards the alternator being bad, especially if it’s still the original.
Are your connections on the alternator and battery clean and tight?
Is the belt and tension good?
Alternators aren’t that expensive, and you’ve got a new battery. The next thing I’d do . . . if you don’t want to pay for diagnosis . . . is to just replace the alternator with a new/rebuilt unit
The fact that the charging system briefly worked correctly, after replacing the battery, indicates that your fuses and fusible links are okay
If the belt and tensioner are in marginal shape, now is the time to replace them
Replace the alternator and report back, please
yeah, if the alternator is failing intermittently , test may not catch it…
I have a 1992 jeep that had the same issue, seemed randomly that the volt guage woudl drop low, and I would notice that the jeep would also loose power level at same time, I to had taken the alt out and bench tested at store, tested good, but wehn put back in jeep would drop to low volts, removed again and had tested and once again tested good, I found that on thesse jeeps the voltage regulation is preformed in the ecu and not the alternator, The ecu tells the alternator how much to charge. THere is a 2 wire cable going to teh ecu, a reference from alternator telling ecu output voltage of alternator, and also a return from the ecu which tells teh alternator how much to charge. If i remember right (others with more know can correct me if i am wrong) but on the return from the ecu should be in range of 7-14 volts, the higher voltage the more alternator should charge, if you are getting batt voltage on the line coming from ecu then alternator is bad regardless if it bench tests good, once i replaced my alt ( i was getting 12 volts on lead from ecu) i had no further issues. I have heard that these type of alternators where they are controleed by ecu often test good on bench but are bad as bench only tests them at max charge but oftehn they won’t charge on lower charge levels.
if not getting the return from teh ecu then alternator will not charge and issue is elseware (ecu, wireing between alt and ecu)
I hope this helps,
Good informative post @Dekota . Modern cars – I can’t say for certain this applies for this particular Jeep – but it’s not at all uncommon for the charging system to rely on a computer algorithm running on the ECM to determine how much engine power should be allocated to the alternator at that particular moment. The more charging current from the alternator, the higher the engine load due to the alternator. So if the engine is already loaded up by going uphill with the AC compressor on, or the engine is at idle with the transmission in D with the AC compressor on and therefore near a stall condition, the ECU will defer battery charging – to eliminate the engine load caused by the alternator – to avoid stalling out the engine. When the AC compressor cycles off, the ECU will then allow the alternator some time to charge the battery.
The manufacturer likely provides a diagnostic method to test this function in their brand-specific scan tool. But if you don’t have access to that scan tool, or don’t want to pay for a diagnosis, replacing the alternator isn’t an unreasonable thing to do. And it might well work. Suggest however to replace with an OEM alternator, and definitely don’t return the existing alternator for your core fee until you are certain the new alternator is working correctly. The last thing you want is to replace what was actually good alternator with a bad one.
The info I saw shows that there are two fuses in the panel under the hood that tie to the output lead of the alternator. Make sure those fuses are good, along with any others in there. There is also a dark green or gray wire that ties between the alternator and the PCM. I suspect that is the field wire for the exciter inside the alternator. That wire should have close to battery voltage on it while the engine is running. If it doesn’t then that needs to be looked at. It could be a bad PCM or something related to it is causing the issue. Make sure all the fuses in the dash fuse panel are good and have voltage on them while the engine is running.
I had the same issue with my 1996 XJ (4.0 engine). What ultimately solved the problem? Cleaning (with a bit of fine-grit sandpaper) the contacts on the battery cables, then tightening them tight tight tight. No problems since. I hope your fix is that easy.