98 XJ - Alternator cutting out and other gremlins

Was out playing in the mud last night. Once it got dark we parked the Jeep and got the fire going. On the way home a few hours later, the CEL came on and the voltage gauge dropped to 9V and I got a code P1682 (charging system voltage low).

So I knew we're running on battery at this point and immediately suspect the alternator brushes were sticking. This has happened before a few times, 2-3 years ago. Fixed by giving the Jeep a bath.

This time however, in combination with the alternator dying, my idle RPM kept sticking around 1100-1500RPM until I kick the gas a few times, then it'd come down.
I've also noticed the fuel gauge is wildly innaccurate for the past few weeks. I fill up and it shows full, I drive 100km and it shows 50%, then drive another 50km and it shows a bit over 75%, then it seems to come down normally. I'm positive I'm getting about 20mpg so it's definitely the fuel gauge's fault and not my foot .

So while these issues all seem unrelated to me, perhaps there's something I'm missing? Something the PCM might do in a low voltage situation that may cause the gauge cluster to give me bogus readings?

We made it about 45km running on battery (on the freeway, no where to pull over, so just drove till the headlights failed and pulled over at the earliest convenience. We used my buddies truck to charge my battery, hoping to get me home on battery alone, and while charging I figured I'd bust out my speciality alternator hammer. Alt-hammer did the trick, voltage came back up, we disconnected my buddies truck and I ran for 10 minutes generating my own power, then alty died again so we just boost-charged the battery and had my buddy follow me home. Alty came back to life after another 35km city driving and I got home under my own power. This morning the alty worked for 5 minutes, then failed again on the way home.
Seems every time the alty fails, even temporarily, my idle throttle bumps up to around 1200. This only happens when the alty fails between restarts.
Can the fuel gauge, throttle, and alty all be one in the same problem?
(my guess is that the PCM goes a bit sideways when it's got low voltage and possibly enriches the A/F, but without going into closed loop).

i know for a fact the grands need to be "re taught" how to idle after dead batt situation. not sure of the technical term tho. or if a cherokee needs the same. maybe once you have all this sorted out (i just did grounds,alt and batt ) you can then track down the other issues.i had a semi cut and corroded plug wire on my alternator right by the frame rail. i assume the grounds on later cherokees are way better than the 89's. i have fixed a lot of issues by fixing my grounds, but i dont know about the later models, if they are as crazy with weak grounding. and for me, from here out i keep a spare alt onboard with me. ten bucks well spent.

SECRET:

Computer controlled vehicles use a "reference voltage" signal to the sensors. If the "reference voltage", usually about 5 volts, give or take a little, is out of whack, then the computer can't interpret the sensor signals and things go WONKY.

WHEN THE SYSTEM VOLTAGE AVAILABLE DROPS BELOW 9.0 VOLTS the available "reference voltage" is too low and the system goes to rubbish.

Get your battery and alternator tested, get the system voltage back up where it needs to be, and then let us know how your problems are doing.

joe_peters said:

SECRET:

Computer controlled vehicles use a "reference voltage" signal to the sensors. If the "reference voltage", usually about 5 volts, give or take a little, is out of whack, then the computer can't interpret the sensor signals and things go WONKY.

WHEN THE SYSTEM VOLTAGE AVAILABLE DROPS BELOW 9.0 VOLTS the available "reference voltage" is too low and the system goes to rubbish.

Get your battery and alternator tested, get the system voltage back up where it needs to be, and then let us know how your problems are doing.

Click to expand...

Thanks Joe. That's the answer I'm looking for. I'm always wary of "Jeep wisdom", as I've heard too much, but you've never steered me wrong in the past. I'm surprised that no amount of searching brought this up (though I'd suspected as much to begin with).

I unfortunately, live in Canada where no one is competent enough to accurately bench test either a battery or alternator. Shops here are awful. Will pull the Alt now and take a look at the brushes.

That said, if the brushes are shot, I'll either replace them (which means ordering from the US cause no one sells a brush kit in Canada), or replace the alternator (because that may be my only option here) or possibly upgrade to something bigger.

That said, if I go the upgrade route, I've got two questions:
1. What alts drop in without serious modification, into a 98xj? I've heard the 9x ZJ's should drop in, with a bit of grinding, for 136A. Any wiring upgrades required for that?
2. I've been dreaming about a weldernator for years. I've read a tonne of writeups on the subject, but none really cover what I want. I'd much prefer a single alt that can be used for welding and running the Jeep, as opposed to a second alt dedicated to welding. There is a (enormously expensive) kit available for a single alt weldernator, but I'm sure someone must have done this themselves at some point yea?

Actually, while we're on this, does anyone know where to buy a replacement brush unit?
I got my alty out and it does look like the rear brush is worn real short. Seems like the obvious culprit. Cant' find a replacement unit though. I'm assuming someone must have one?

Rockauto dot com usually has alternators parts, brushes and diode sets.

Rockauto don't have it either . Found one on ebay that looks right, but says it's for another PN.

That said, as I mentioned perhaps this is time to upgrade. I can get a 200A alty (ZJ) for $130. Assuming I'm not adding any big draw electronics just yet, I shouldn't have to upgrade any wiring for the larger alt right? And when I do, I pretty much just need to upgrade the main leads from battery and ground yes?

You might want to upgrade the B+ wiring eventually to take full advantage of it, but the field wiring should be fine.

Your alternator brushes are definitely toast - happens when you feed em mud. Sometimes they come back, sometimes they don't, generally it's temporary... roll the dice!

Factory alternator B+ wiring for an XJ, at least on a 94/95, I'd have to check other years:
big heavy wire from B+ terminal through single-pin connector (near passenger side motor mount) to PDC. PDC has two 60A fuses in it in parallel, they are fed by the alternator and the other side is connected to the big input power lug on the end of the PDC, which is hooked to the battery wiring.

Depending on how you prefer to do it, you can either simply run a new cable to a 175A or 200A fuse (I like the MEGA/AMG style ones) directly to the battery, or upgrade the wiring into the PDC and swap out for a pair of 75-80A or 100A fuses. The first option is probably easier. You can find 175A MEGA/AMG fuses and holders for them under the hood of 90s F150s all day at the junkyard, they are on the passenger side of the firewall just below the hood.

Thanks Kastein. One further question I suppose. Will my stock voltage regulator (assuming it bolts up) be sufficient? I can't imagine why not, but better to ask now
Looking at this one right now: http://www.ebay.com/itm/Alternator-Jeep-Grand-Cherokee-Wrangler-Comanche-94-95-97-98-HIGH-AMP-Generator-/130872024025?pt=Motors_Car_Truck_Parts_Accessories&fits=Make%3AJeep%7CModel%3ACherokee%7CYear%3A1998&hash=item1e78949bd9&vxp=mtr

The stock regulator is actually built into the ECU - and it should be fine. In fact as long as you have the "funky 4-ring-terminal connector" style alternator rather than the one with a small two pin plastic connector for the field wiring, that should bolt right in, aside from possibly needing some grinding on the mounting bracket to clear the alternator housing.

What is the "funky 4-ring-terminal" connector anyhow? I always thought it was the voltage regulator, though I have heard the reg is in the ECU (which doesn't make sense to me).

*edit* I found a local source for the brush holder assembly for my stock 90A finally. $15 (plus BC's rape tax). I'm torn between upgrading to the 200A unit now. I do plan to run a 12k lbs winch in the future. Is this where a 200A might be required? Will the 200A put more strain on my engine than the stock, while generating stockish power (ie: under 90A) ?

It's basically just a piece of molded plastic around the wires - makes it way easier to get them all on at the same time, and in the right spots. The two large lugs are both grounds, the two small lugs are the field winding terminals.