Two Blue Jeeps

Have 1987 2.5 Comanche with 61 AMP alternator (now new) and newer battery. Alternator dash light came initially. Tested for and needed new alternator. Both battery and alternator bench tested fine. Cleaned batt cables. Still putting out only 12 volts with ohm. Light still stays on. Any thoughts?

VTJeep

Sounds like the signal isn't grounded well enough, or maybe it's loosing current thru a short? Is the def. the right size alt?

Two Blue Jeeps

Yes. Replaced with identical size (61 amp) basic 4cyl no A/C etc alternator. Do both battery cables need replacing or which ground should I check. Thanks.

RTorrez1

You need to check all of your ground. That would be the battery and the one on the back of the head to the body. That is if the 2.5 has one back there. Anyway you should have at least one more ground other the battery.

Two Blue Jeeps

Yes the 2.5 has that ground strap to the firewall. Problem seems intermittent especially when you turn on headlights and/or turn signal. With daylight I will remove and clean the strap. Also saw ground on dipstick. Will clean all and maybe upgrade to new battery cables. Thanks.

ET JEEP

Not sure where you are checking for voltage. A few quick tests you can do. One, with engine off, test battery voltage. Then test for voltage at the large terminal on alternator. You should read the same battery voltage. If you have no voltage or lower voltage at the alternator, you have a break or high resistance in the feed cable/circuit.

Two, start engine and test for voltage at the alternator. You should read 13-15 volts. If no voltage or lower than 13 volts, you have an alternator or regulator problem. Use care doing this test with engine running.

Three, engine running, if alternator reads 13-15 volts. Test battery voltage. It should read the same. If not, there is a problem in the alternator feed to battery.

tytower

Hopefully you know whats what with these.

I have a 97 Cherokee 4L and I drive it for 15 mins and the alt light comes on and stays on until I switch off and start again when the same behaviour is exhibited.

RTorrez1

It can be a number of thing going on with you Jeep. I would first start by having your alternator check out. Along with that check the battery cables and all your ground wires. It would hurt to have your battery checked while you're at it.

Robert

Two Blue Jeeps

Follow up regarding alternator issue: Cleaned all three ground connections as suggested and problem appears to have gone away completely. The ground connection was probably the underlying condition to begin with. Comanche only has 89,000 miles so all connections were original and never have been touched in 25 years.

tytower

Can anyone point me to a diagram of the alternator usually fitted to a Jeep cherokee 97 ?

A How to would be good or a workshop manual but just a parts diagram will do

5-90

Don't trust the parts house benches - I see them pass bad parts all the time. It will pass it on voltage, but it only does so at 0.5-1.5A (which isn't even enough to run your ignition or your fuel pump, and it's barely working current on the ECU just to perk it up.)

I've posted a simple test procedure for checking your alternator, starter, and battery - take you half an hour, do it with a DMM, and while it's not a quantitative test, it's a rather thorough qualitative test.

And, never trust the RENIX IP voltmeter for an absolute reading - it's got a well-deserved reputation for being a liar. I've "recalibrated" the IP voltmeter in my 88 with a grease pencil, and use it as a "state-of-charge" indicator, more than anything else.

If you suspect a problem based on IP readings, the very next thing you should do is actually verify that there is a problem using an independent "known good" DMM.

Side note - if you have the CS121 alternator (used with some 4-150 engines) you're limited in upgrade options. If you have the CS130, you can get alternators rated for up to 100A max output as stock replacements - and the CS130 can easily be upwound to 140-150A by any competent shop in very little time, and for a reasonable cost (Rod does 140A for $140/A, and does rock solid work.)

I think the CS121 can be upwound, but I don't know how far. It can likely be replaced with the CS130, but you may have to change bracketry (and if you do have the CS121 and decide to replace it with a CS130, I'd like to talk the old CS121 out of you. I've been looking for one for a separate project, and finding a core around here has been difficult...)

5-90

I believe they were mounted to the lower passenger side of the engine all the way up - but your 1997 (all 1991-up Jeep) will have a Nippondenso alternator with the regulator circuit in the PCM - the Delco is not a direct swap for it.

Your 1997 is OBD-II, so here's an easy way to track it down:
- Go to your PDC (box with the fuses and relays underhood.)
- It should have two cables attached to studs on the front. One goes to the battery, ignore it.
- The other runs directly to your alternator (it will look smaller at first - the first 4" or so if it is 10AWG fusible link wire. After that, it's 6AWG red wire.)

Easy!

tytower

I have done your suggested voltage checks and nothing is coming out of the alternator . Both the static voltage checks are fine and there appears to be resistance in the field winding with it all disconnected but feeding some current through it with a 5 ohm resistor in series , regulator disconnected, did not produce an output so I am suspecting a fauilty alternator field winding. , The brushes needed cleaning but were good for another 30,000 miles

I think you are saying that the regulation is done by the PCM . What does PCM stand for P? Control Module? Can I access it to find out why my alternator is not working ? Can I replace the alternator with one with a built in regulator ?
Do you have any diagrams or how to's on this area?

You seem to sell replacements -what do I replace and how much is it to Fl

tytower

So if yo u cant sell something you dont bother 5-90
Or maybe you only trawl new posts and didn't see my edit
Can anyone else help here with an explanation?

EDIT
I found these pictures of the circuit and a description also on the net. Seems to confirm the above but I havn't found all the bits yet

5-90

No, it's more like I didn't catch everything, since I try to get in and help and might not catch all posts.

PCM = Powertrain Control Module.

As far as testing, I know things changed. The alternator used to be fed low DC voltage to the field coils to make current (0.5-4.0VDC,) but they've since swapped to PWM modulation - using a pulsed 12VDC instead (PWM = Pulse Width Modulated, the average voltage is set by the on/off times.) You'd probably need an oscilloscope to properly diagnose the regulator circuit, and I don't know what the waveform is supposed to look like offhand. It can probably be found somewhere, but I'm not sure where offhand.

You can replace the alternator with one that has an inbuilt regulator, but you'll have a problem with a persistent CEL/MIL. The PCM will see the regulator circuit not having an effect on system voltage, and will throw something like "Alternator Field Not Switching" - which you won't be able to bypass (at least, I haven't figured out how yet.) The engine will still run normally, but you'll have the CEL illuminated all the time - which can mask another issue that might kick off the lamp. And, if you're subject to regular vehicle inspections, you'll fail (due to having the CEL illuminated, despite it not being an emissions or driveability issue.)

If you decide to go with an internally-regulated unit anyhow, go with a Delco CS-130 that's setup up as a "one-wire" or "self-exciting" unit. Or, you can have the conventional regulator and adapt the wiring (I haven't figured out how to do that yet, either...) but you'll still be bypassing the regulator circuit in the PCM and still get a CEL/MIL.

Either way, if you get a persistent CEL/MIL, I'd strongly suggest you get a code reader and run it on your rig at least quarterly to make sure you're not masking another issue with the alternator lamp!

I don't do replacement alternator, I don't have a shop or a bench to do it the way I'd want to. However, if you can't find a shop locally (check your Yellow Pages under Auto Parts - Remanufactured, usually,) I've done a couple of pages for the shop I use and I talked Rod into doing mail-order a couple of years ago - I don't make anything on it, I just want to send some business his way because he does damned good work. Scroll down on the main page, and click the "San Jose Generator" link. Rod keeps banker's hours out here in CA, but I've long found his work to be well worth it. But, check locally first.

I don't suggest going with a chain store that offers a "lifetime warranty" - you end up needing it far too often, and the best warranty is the one you don't need.

Look at it this way - go with a chain store, and you've got 1:100 chance that the unit you are buying has been actually tested - maybe (that bench in the store isn't worth calling a "tester.") Go with a local shop, and it's a general rule that every single piece leaving there has been wrung out on the bench, and on a bench that actually will test it properly.

I used to work in a chain store (Kragen's, before they sold out to O'Rielly's) and that was years ago - and the parts weren't worth a damn then. They've only gone downhill since.