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Engine won't rev past 2000 rpm's unless alternator is unplugged

With the alternator unplugged the engine rev’s fine . I tried a different alternator today & same problem . Any ideas ?

We need Car Talk’s call screener!! … lol …

Is this a newer car, like a GM version that has that computer-controlled charging system design?

Otherwise, I got nothing. What if the alternator is left plugged in, but the alternator belt is loosened so the alternator doesn’t turn?

Sorry , meant to put in that info . 2005 Jeep Liberty 3.7 V6 . Haven’t tried running it with the alternator connected & belt removed .

Is the check engine light still on?

Yes , has a code for the battery since the alternator is disconnected , a code for the ac , a code for injector # 4 & a new code for IAT . Didn’t have a battery code until battery started getting weak , The IAT code is new after this was already happening . At one time was down to just the injector code . Codes are P0204 , 0622 , 0533 & 0113 . I replaced # 4 injector & still have the code for it .

Correct the faults and the PCM will restore the rev limit to normal. Is there a reason to exceed 2000 RPMs in park? Does the vehicle have a PTO?

The way an alternator works might provide a clue. The power output (and therefore load on the engine) from an alternator is proportional to the product (multiplication) of the engine rpm and the alternator field strength. When everything is working correctly, the field strength is reduced by the voltage regulation circuitry as the engine rpm increases, to maintain a constant alternator power output (for that amount of electrical power needs). So the engine load is more or less constant. But if the field strength somehow was stuck to a certain level, then as the rpm increased the engine load produced by the alternator would increase, until the engine reached an rpm where it couldn’t overpower the alternator load, at which point the rpm’s couldn’t increase further.

@GeorgeSanJose , if I’m understanding you correctly , if power from the alternator is being called for , the faster the alternator spins the more power it puts out & the harder it is to spin it & at a certain point the engine can no longer produce enough power to spin it ?

Does car drive ok? What rpm is 1-2 shift? Above 2000 rpm? What happens when you accelerate hard from a stop? Leave shift in low, accelerate, engines dies at 2k rpm? Even a POS jeep 3.7 will rev past 2k without blowing. Maybe?

@Nevada_545 , There is no reason to exceed 2000 rpm’s in park or neutral . Are you saying you think the engine is supposed to do what it’s doing as far as rpm’s go ? I bought this vehicle knowing it had a bad engine & I replaced the engine . To this point I’ve never had it road legal , no insurance or license on it yet so I can’t road test it legally . Where I live it would be really risky { ticket wise } to road test it .
I did drive it home on the previous owners license & insurance when I bought it . Everything seemed fine at that point except the engine which had been tested to have low compression which I also verified after I bought it .
I already have 3 licensed & insured vehicles for a 2 person household so I have been in no hurry to legalize this one .

Engine rev’s with alternator plugged in & belt off so obviously as long as the alternator isn’t spinning engine rev’s aren’t affected . Have code for # 4 injector , noid light doesn’t flash when hooked to connector of # 4 injector . Have power to one of the connector wires so must not be getting pulsing ground from ecm . Anyone know if the injector wires stay the same color all the way from the injector to the ecm ?

Traced ground from injector to ecm , color stayed consistent . Using an ohm meter established continuity from injector to ecm so no broken wire . Went to rural property I own & retrieved ecm from wrecked jeep . Installed it & engine started & ran for 3 or 4 seconds & died , did this about 4 times & then wouldn’t do anything when I turned the key . Reinstalled original ecm & engine starts & runs just like before .
I’m beginning to get the feeling I’m talking to myself , I guess everyone reading this is just as lost as I am but I’ll get it by & by .

The immobilizer secret key data in your second PCM does not match the the data in your immobilizer module, the donor vehicle may also have had a factory alarm enabled.

If you disconnect a fuel injector on a vehicle with a NGC PCM while the engine is running the PCM will set a fault and disable that injector. You must install the noid light with the ignition off, then start the engine for the light to flash.

I figured out since trying the the other PCM that it wouldn’t work without being programmed for the vehicle that it’s being used on . I disconnected other injectors & got a flashing noid light for those . I have verified power to one wire on the injector connector & verified continuity on the other wire from the injector connector to the PCM . Could the camshaft position sensor cause one injector not to fire ?

If the wiring checks out it seems that the injector driver failed in the PCM. I wonder if this was the cause of the misfire on the original engine.

If you use the immobilizer module, ignition lock cylinder and key from the other vehicle that you removed the PCM from you may be able to use that PCM. It takes ten minutes to replace these parts, you may need a T-15 or T-20 tamper proof Torx to remove the screws.

The PCM may not work if the other vehicle has a factory alarm and this vehicle does not.

The original engine had high mileage & bad compression . A compression test had been done on it before I bought it & I did another one after I bought it just to satisfy myself that the compression was indeed bad . After I verified that it was I didn’t do any additional testing on that engine . Maybe I’ll try swapping the parts you mentioned & see what happens . Thanks for responding .

Have you measured the voltage output of the alternator as the rpm’s increase? Usually that voltage if fairly constant w/rpm due to what I explained above. But maybe the alternator output voltage is getting too high for some reason, and there’s a built-in safety mechanism to prevent damage to the sensitive computer circuits and sensors that limits the engine rpm to prevent an over-voltage condition from the alternator.

For comparison, on my Corolla the battery measures 12.5 volts prior to a cold start with the engine off. After the engine is started it measures about 14.5-14.9 volts, and stays about the same independent of rpm. Gradually as the engine warms up that voltage will drop as the battery is re-charged, to about 13 -13.5 volts.

If he wiring to injector 4 is okay you should see 12 volts at the connection to the ECU for the return side of the injector circuit. I suppose there could be something wrong with the CAM reluctor for cylinder 4 if that is how it is designed to operate the injectors, or something inside the ECU has failed.

Check the charging voltage as @GeorgeSanJose suggested to see if there is an overcharging issue when the engine RPMs get close to 2K RPM.

Has anyone figured out what the problem was? I’m having the exact same issue with my jeep

@Hodgepodge53 , have you measured your alternator output voltage vs rpm yet?