After reconnecting battery, engine has low RPM or could die in D

Hello everyone.
Has this happened to anyone? It usually happens when the car is in D and I’m at a stop sign or a red light. I stop then the RPM gets very low. Sometimes so low that the engine shuts off. I have to put the transmission in N and keep the revs around 1000. This problem fixes itself after the 2nd startup.
Is it a sign of a weakening alternator?

You might have to reset the idle control valve, theres a sequence that you have to follow, its in the online XJ40 book on the old site.

Thanks Robin. I’ll do the test to find out if the idling is between 550-600.

Could be alternator or slipping slack belt

Larry, the belts are in good condition. This rpm drop happens a few times after reconnecting the battery then it fixes itself. Especially after the 2nd start up.

Clean intake butterfly.

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Thanks Gordon, I’ll check it out. I did the test Robin suggested. The idle speed fluctuated between 1100-1500 rpm.

Hi Joe, my car was doing the same thing a while back and it was the alternator that was at fault. When the RPM drops does the charging rate drop to around 11 volts then rise up if you increase the revs ? After I rebuilt the alternator and replaced the brushes it was fine. How old is your battery ? A borderline battery puts an extra load on to the alternator too so I would check this out as well.

Thanks Casso for the reply. Last time they checked the alternator at a local auto part store, it was charging fine although it’s not 100%. I bought my car in 3/2007 and haven’t replaced the alternator yet. I can have them check it again and see what they say.
I’ve already checked Denso alternators on ebay but having a hard time to find a brand new one. Or is Bosch the OEM in these cars?

Hi Joe I’m not certain which is the oem alternator but they are not to difficult to dismantle and overhaul yourself, especially if you are confident using a soldering iron. You can buy complete overhaul kits for almost any make of alternator that include the rectifier pack and all the other bits if you search E.bay. It’s usually the brushes that wear out first and if the alternator is still charging and hasn’t failed completely just changing the brushes will usually solve most problems. Obviously if it’s done a high mileage then its also worth changing both bearings too. The company I bought the overhaul kit from also did a ‘tutorial’ on facebook describing the procedure and taking you through all the steps.

Casso, I was thinking about rebuilding my alternator when it comes to that point. The OEM is Denso.
Thanks for the advice.