99 XJ Sport 4.0L.

My voltage is intermittently spiking high (19V+) when driving and after a minute recovering and zipping straight back to 14.25. A few times I've caught it drifting up to 15/16 when parked.

Multimeter straight between the battery terminals shows 12.6 when off, right around 14.25 when running, but when I've caught it running high it was spiking from 14 to 15 and 16. It only goes to 19V+ when driving, so I haven't been able to verify this at the battery but given that I've caught it running high at park it doesn't surprise me.

I splashed through a foot-deep puddle right before this started, so I thought the alternator got a bit of splash and would recover. But watching it today recover from a 19V+ spike to exactly 14.25 looks a lot like a failing voltage regulator. I know this is really bad for all of my electronics and sensors, but I want a second opinion before I make any buying decisions. My driving is all <5 miles at the moment, but I have a 2000+ mile driving coming up in a little under a month.

My current thought is to install an external voltage regulator until I can find a new PCM/ECU. Does anyone have a recommendation as to brand/PN etc?

(As a side note, about a month ago I rewired some of the harness on the left side of the block, distributor+pickup, ignition coil, and the ground at G101/G103 (the two side-by-side grounds next to the ignition coil on a 99). There were a couple of ring terminals on these grounds that I may have gotten mixed up when re-attaching them. They should still have continuity, but I'm thinking that this might have changed the resistance between the electronics, alternator, and battery somehow. Does anyone know the correct routing of the grounds at this location for a 99?)

 

I've been assuming that the only voltage regulator is in the PCM/ECU, but is there one internal to the alternator as well? 99 4.0L w/ AC if that matters

 

shameless bump

 

This is correct, voltage is regulated by the pcm. Theoretically, yes you could run an external regulator if you wanted. I do on mine, I used a regulator from a Dodge ramcharger (my reasons for this are different though). I would do that as a last resort, because it's just a band aid fix.

 

I just pulled my battery and checked the resistance on my battery temperature sensor (BTS). It's showing ~ 22K ohm resistance, and the normal range is 9.9 - 11.1. This is a $25-40 part rather than a new PCM or new alternator, so I'm really hoping this is the problem


<edit> My garage is colder than I thought ... 10-11k is at room temp (75-80 F) and I took the reading at 41F. False alarm

 

I'm getting continuity (1.9 ohms) between my + and - battery terminals ... think I've found my problem. Time to start hunting for shorts

 

I read on another forum that if the line from the alternator/generator to the PCM (solid green) is shorted to ground, the PCM will run the alternator in "full-charge mode". I was getting a connection between ground and the return line (B10 on PCM) somewhere between the connector next to the battery and the PCM plug itself. I cut it and splced in a fresh wire and my in-the-garage test shows my voltage sticking right at 14.25-14.30.

I'll need a test drive tomorrow to see if this actually fixed the problem.

PS. The continuity between my battery terminals was a red herring. This was caused by my hood light and courtesy light circuits.



<edit> Success!

 

Which color wire did you splice and run? What pin was it on the pcm connector? I'm chasing the same problem right now as we speak. I wired a separate regulator and it now quite over charging but won't rise from the battery voltage and charge.

 

That was over 2 years ago.....I'm no help except to mention you want to make sure your battery terminals are good and clean. Also old style regulators energize the alternator field circuit. It sounds like the OP found the control here is being grounded by the PCM.

 

Yeah I know it was awhile ago. I'm already past the point of checking terminals and all the easy no brainer stuff. Was just hoping he would chime in about that wire. I'm sure it was the b 10 pin in the white connector but just wanted some reiteration.

 

In my notes I've got that it is the return wire from the alternator, "usually green".

On my 99 it was an 18-gauge dark green wire. The other end goes to the PCM B10 pin. That should be the middle connector (with a white clip). Pin is second one from the right on the bottom row... I'm guessing looking into the harness connector, but in any case it should have a dark green wire out the back

Pinout: http://www.jeepforum.com/forum/f12/n...-here-1111789/

I just went out and checked, it looks like that's the only straight-dark-green wire I've got in that bundle. I spliced it just after the airbox, there should be a bit of room to work there without pulling things, at least to run a test wire and make sure this fixes your problem. On the alternator side, this hooks to a connector right behind your positive battery terminal and then dives down to the alt connection.

And since doing this, I have had 0 voltage regulation problems.

 

I've installed a external regulator kit in my 99 xj. From day one it would spike up to 15.50 volts at times. My gauge would peg all the way out. The only way I could see my real output is hook a voltage meter to battery. I have double grounded the regulator with two wires going from regulator to alternator instead of one. It still would do the same at times. I decided to install a real mopar regulator in place of the aftermarket unit I was using. It fixed my problem for a couple of months but i noticed the last time I drove it it spiked again while it was sitting in idle. I am tired of dealing with it. Anybody got a solution to my problem?