New alternator, slightly rough idle

I just had a lot of work done on my '88 XJ-S V12 convertible with 39,500 original miles. After test drive, I filled it with gas and the alternator died, so mechanics came and rescued me. I picked it up yesterday and noticed when I’m stopped, the car idles a little rough. RPM’s barely fluctuate, but it is still noticeable. It did not do this before alternator was replaced. Does it just need a little fine tuning or will it sort itself out as I drive more? Thanks for any and all advice.

Can’t help with your diagnostics but I remember back in the 1980s a Series III XJ6 came in running ROUGH!!!

I noticed the battery/alt light was glowing dim. The voltage was good but the readings were strange.

I disconnected the battery wire on the bulkhead to the alt. I started the engine and it ran SMOOTH.

I reattached the alt and the engine ran like ‘crap’ again.
I hooked-up the Allen Diagnostic analyzer oscilloscope and the alt was putting out 3 phase AC (the diodes had completely failed).

I called the customer to tell him the news about the alt. He said he did not want the alt repaired/replaced because it charged the battery OK and it had the glowing light for a few weeks now.

I told him that the alt repair would also fix the poor running engine but he would not believe me.

I think he came and picked up the car and took it somewhere else for a second opinion?

Some electronics do NOT like alternating current in the circuit. Maybe too much ‘ripple’ in the replacement alt?


Oh, me!!!

  1. Yeah I can see the “question” WTH does the alternator have to do with a smooth or not so smooth idle??

  2. Humour. “Ripple”. Reminded me of an ancient TV show. “Sanford and son”. They ran a junk business,.
    The elder liked “Ripple”. A non descript red wine!!


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I watched the show when it was running on TV ‘back-in-the-day’. It is still on in reruns on one of our local TV channels in the afternoon.


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I can see that, though. The whole EFI system runs on system power, and cycling between 12V and 14V might impact how the injectors are moving. At idle the alt, even functioning properly, might barely be keeping up with loads.

The language thing again. No, I did not know that. My response was to motorcarman and his customer’s differences??

  1. Would it matter to a carb’d car, I doubt it. But, the battery and light bulbs!!!

  2. Or would an EFI car care, it the AC e power were adequate in voltage and amps.

  3. Back a bit, I saw an “off the frid” TV show on You tube. A fellow “gutted” an alternator of diodes and stuff and ran it “straight”. Coupled it to a stream turned wheel. it lit up his cabin.

  4. Many decadesa go, Pal roy bought a semi derelict 28 A Ford. We planned to reconstruct it to resemble the one from high school days. . Rough and ragged, but it ran well… 60 bucks!! We cut the top off and removed the bumpers and fenders. On the original 28" tires and wheels, off to the desert to hunt rabbits!! It got dark. On with the seal beam converted lights. Good illumination!! Got stuck in sand. Gunned the 4 banger. Lights got really bright then dark!!

Got it free and went home. Aha, the PO tapped the cut
out atp the generator for juice for the head light switch!! Before the battery to “calm” the volts…


Hi there, thanks for this, it makes sense. The car did idled without issue before the new alternator went in. It is a noticeable stutter when I stop and start.

Thanks, motorcarman. This seems to hit on my problem, the RPMs barely flutter, but it is noticeable at stop/start time, and it ran much more smoothly before the alternator was replaced.

I forgot to mention, I also had the AC overhauled with a new compressor and fan. Could that be the problem or be contributing to it? It idles a little rough with or without air on.

I think the only way the A/C compressor could be contributing is if the clutch is slipping. When the clutch is slipping, either the gap in the clutch is misadjusted or the clutch coil isn’t getting full 12V when engaged. You might wanna check that voltage. Of course, it could also be a failing clutch coil…

Is the new fan out of balance?

Thanks guys. It is in the shop again for this problem, with notes from you to accompany. It may just need a tune up…but it seems like every time something is fixed, something else is broken, and these guys work on classic Jags all day long. I really appreciate being able to pass your insight on, and I hope that’s okay since I’m new to the forum.

Way out thought. The old alternator was “dead”. Imposed no load on the engine. Best seen at idle.
New one works, but at a price. Absorbs torque.
Again, best seen at idle.


Hi everyone, thanks for all of the comments and suggestions. It was the spark plugs. Had them all changed out, and it runs like a champ now. The mechanic sent me a photo, and I’ve never seen a spark plug look so sad. Problem solved!


Thank you for posting back with the good news. Sometimes you get lucky and it turns out to be the simple things.


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Aye on the simple stuff. So many times the answer when we skip it and go to complex!!


I am a strong believer of the Pareto Principle and suspect that my experiences over the years match the 80:20 ratio. I sometimes have to throttle myself back when troubleshooting and remind myself to check the easy, inexpensive, and likely things first before looking for a more complicated answer.


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You all had great insight into the potential problem, and I greatly appreciate it. I was very lucky it was an easy fix. I have an excellent mechanic who treats my car right and doesn’t kill me on labor costs. Thanks again everyone!

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