Overvoltage at RPM

[rmcrom]

I am having a problem with my '96 Cherokee. When my RPMs rise to 3000 or higher, my Cherokee's electrical system rises to ~18-19V. When I drop RPM, the voltage drops back to normal operating voltage. I know it is not a bad gauge reading because when it goes into over-voltage condition, the A/C blower fans turn into jet engines and the light behind the gauge cluster turns into a spotlight. I have also noticed the charging system won't over-volt after a cold start until the engine has warmed up.

So far, I have cleaned both battery terminal posts. Cleaned the positive battery cable terminal and replaced the ground terminal. I also checked all ground and alternator cables to ensure they are snug. Where should I go from here for troubleshooting?

 

[old_man]

It sounds like the voltage regulation of the alternator is not working. Before we can start, we need the year of the vehicle. The Renix era systems used an alternator with the voltage regulator mounted inside the alternator. The later HO years used the ECU to control the alternator charging.

If it is a Renix, then you have an alternator problem. If it is the later model, you can have a lot of things going wrong. It could be a wiring back to the ECU and then from the ECU to the alternator, or it could be the ECU. It would have to be a fairly screwy failure within the alternator to cause it to charge too much. In order to charge too much, too much power is being fed to the field coils inside the alternator. The power to the field coils should be coming from the ECU. If that is the case then you either have a bad ECU or somehow the ECU is not sensing the battery voltage correctly.

 

[rmcrom]

It's a 96. Is there any way to test the wiring to/from the ecu and alternator?

 

[8Mud]

The dark green/orange wire is power into the alternator, comes from the ASD relay. The dark green wire is the ground (regulated) by the ECU. It regulates the power output by how much the alternator windings are grounded through the regulator in the ECU.

You can disconnect the orange/dark green wire and the dark green wire and ohm test from either pole separately to the case and it should be an open circuit. If it has ohms from either pole to the case the alternator has an internal short.

The dark green/orange and drk green are either end of the field winding. And will have continuity with some resistance when ohm tested.

Old man is likely right. But in many cases the regulator either works or it doesn't, unusual for it to screw up like you described. The usual solution is to replace the ECU, they are expensive and some years like the 96 can be hard to come by.

Worth the time to check the windings, just in case. I'd check them both hot and cold.

Disconnect the dark green wire from the alternator and the ECU and ohm test it, just to make sure it isn't chaffed and slightly grounded. Or the connectors are full of water or coolant. Water or coolant can slightly ground also. Test either end of the dark green wire to battery negative with both ends disconnected, look for moisture in the connectors.

There are other alternatives, like adding an internal (Toyota) regulator to a Denso alternator and rewiring. Or an early Dodge or Ford external regulator outside of the ECU and rewiring. Your check engine light will stay on.

That's all I got without special testers.

The key test using your check engine light should show codes 12 start of test, 41 alternator problem, 46 over charging, 55 end of test. I may have the 41 and 46 backwards, I'm old and my memory isn't what is used to be.

A DIN code reader will show more detailed results. On some 96 XJ's DIN readers can be iffy.

 

[rmcrom]

Thanks for the suggestions.

I took the field connections off the back of the alternator and didn't get continuity from either pole when tested against the case.

I also disconnected the wires from the back of the ECU and tested the field wires (alternator end) for continuity with ground (used alternator case for ground). The green/orange wire does show continuity. I noticed from a schematic that the green/orange wire also goes to the injectors so I don't know if that is why I get continuity with ground.

 

[rmcrom]

I peeled back the wrapping from the B+ on the back of the alternator and discovered this masterpiece.

I cut back to shiny copper and crimped on a new connector. A quick test drive shows that it may be fixed.

 

[Hypoid]

Good job!

 

[rmcrom]

I thought I had fixed it, but once warmed up on a longer drive the problem came back.

 

[8Mud]

Never tried it, but try disconnecting your battery temp. sensor. Your CEL will come on, but see what happens.

I've had issues with two already, the original and the replacement.

 

[8Mud]

Low voltage in or a poor ground and the regulator in the PCM (ECU) is going to try and adjust the voltage, garbage in garbage out.

The red wire is fused voltage in, the two black and tan wires are the grounds. Volt test the red with it plugged in and motor running. Ohm test the grounds at the PCM harness connector to battery negative.

I'm out of ideas at the moment.

 

[8Mud]

I really don't know where the regulator circuit gets it's voltage values to control the regulation.
Whether it is the red wire power feed to the PCM (ECU)or it is a feedback circuit from the alternator field winding or someplace else? Maybe the orange/dark green circuit)? All I know for sure is that the regulator is designed to compensate for low voltage, where it reads the low voltage is anybodies guess. I don't remember the book (FSM) being in anyway specific about this.

 

[rmcrom]

Never tried it, but try disconnecting your battery temp. sensor. Your CEL will come on, but see what happens.

I've had issues with two already, the original and the replacement.

I didn't think I had one of those sensors. There is nothing attached to my battery tray or any hole that looks like a sensor belongs there. However, I have noticed this thing dangling from above the horn and never knew what it was. Would this happen to be a battery temperature sensor?

http://i.imgur.com/1PNB27p.jpg

 

[8Mud]

Looks just like mine.

 

[rmcrom]

Didn't fix it, but at least now I know what that thing is. At this point, I'm just going to try replacing all the battery cables and if that doesn't fix it put in an external regulator.

 

[rmcrom]

Another tidbit, I get 13.6v running at idle with everything off except the radio. I get 12.6v with high beams on, a set of hella 500 on, AC blowing on max.

 

[rmcrom]

I wired in an external voltage regulator and I get a nice 14v while driving. However, instead of a voltage spike over ~3000 rpm, now I get a blown fuse on the +12v source wire feeding the voltage regulator. So stock voltage regulator = voltage spike, external voltage regulator = blown fuse. Would it be reasonable to think it's the alternator at this point?

 

[rmcrom]

I put in a re-man alternator and that seems to have fixed it. I switched back over to the original voltage regulator too.