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Is it my Alt or Battery?

RadioShackSpecial

NAXJA Forum User
Location
Michigan
Tonight after work I noticed an odd electrical problem. While the heep was warming up before I take off I noticed that if I tried to roll down two windows (or all four) the engine rpm would drop about 100 rpm from idle and the volt gauge would drop to what I would say is the 10 volt mark.

Does this mean my Alt is weak and not charging 100% at idle or is the battery weak and in need of replacement?

This is a preliminary question before I have the Alt and battery bench tested, just wanted to know what direction I should be going in.

On a side note when coming to a stop, once I get below 15mph, the high idle drops and the engine feels like it wants to stall. If I release the brakes slightly while stopping it does not seem to do it as harsh. Is this a brake booster issue or more of a vacuum issue?

Thanks for reading this far. :smoker:
 
It runs like crap and wants to stall because there isn't enough voltage to run the engine management electronics.

Disconnect the battery. Charge the battery. Overnight (Ok), 24 hours (best).

While the battery is charging completely remove the main cables from the XJ. Clean both ends of both cables. Measure the resistance of the cables with an ohm meter, should be near ZERO.

While the battery is out, you might as well renew all of the grounds. Pay particular attention the self-destructing braided strap from the rear of the head to the firewall. It is the main unibody ground. Replace it with a cable. Make sure you clean the mounting points down to bare metal, Dremel works great. Use a corrosion inhibitor on the bare metal so it won't rust and ruin the connection. Install the new unibody ground cable and make sure it is tight.

Well, with a battery that is fully charged you really need to have it load tested.

Install the good passed battery or a fully charged new battery. Reconnect the battery with cables that passed or are new.

Start the engine. Measure the voltage across the battery terminals. Should be 13.6 ~ 14.5, if not have the alternator tested.

Note: reports by several members/users that on XJs equipped with the Nippondenso alternators, 91+, that when tested by the chain parts stores that pass the test, but are actually bad. May be related to the PCM controlling charging.

If you haven't found the problem by now, post back and we will keep trying.
 
I had a starter do that on me. The store tested it and said it was good but did nothing but try and chew my flexplate.

Thank you for the quick reply. That sounds like a great plan of attack. I will have to get to it first thing in the morning, its far to cold to try tonight.
 
I had a starter do that on me. The store tested it and said it was good but did nothing but try and chew my flexplate.

Thank you for the quick reply. That sounds like a great plan of attack. I will have to get to it first thing in the morning, its far to cold to try tonight.

It gets cold in Michigan? At night? What will they think of next!

My Dad was from St Paul, MN. He mentioned something about cool evenings! :D
 
.

On a side note when coming to a stop, once I get below 15mph, the high idle drops and the engine feels like it wants to stall. If I release the brakes slightly while stopping it does not seem to do it as harsh. Is this a brake booster issue or more of a vacuum issue?
I was wondering about this to, sounds like we have similar issue's.
 
Well, replaced the Alt and the belt and all is better now. Tomorrow I will pick up a new battery, the one in there is a few years old.

DakineJ, It does not do it as bad now. I would tighten the belt and see if it helps. The Alt on mine was not spinning freely and thus not producing enough of a charge at idle. New Alt and belt (just to be safe) helped considerably.
 
On a side note when coming to a stop, once I get below 15mph, the high idle drops and the engine feels like it wants to stall. If I release the brakes slightly while stopping it does not seem to do it as harsh. Is this a brake booster issue or more of a vacuum issue?

Thanks for reading this far. :smoker:

High idle--what transmission?
 
AW4.

While coasting the engine seems to hang around 1100rpm, but when coming to a stop it would drop to around 600rpm, it hung there for a second then 'something' kicked in and brought the rpm back up to normal speed (750).

The IAC might be lazy but the severity after the Alt and belt swap has gone down. More like a slight hesitation at this point, no major drop in rpm while slowing to a stop. So it may have been related to the charging issue.

The wires were in good shape, no frays and connections corrosion free for once (stupid salt). I would like to rewire it but dont see any need to fix whats not broken. I think I might grab a new Battery and IAC tomorrow just to be safe.
 
Yeah, 750 rpm in DRIVE, engine and transmission both fully warmed up, after about 20 minutes of operation.

I would try and clean the IAC and throttle body, if you haven't done that yet.

The electrical issue would certainly impact the IAC as it is a stepper motor.
 
Carb cleaner? Or something else for cleaning those items?

If it is marked as "throttle body safe" then that's Ok. You will probably want an old toothbrush, some cotton swabs, and take your time. The IAC can be broken. Keep after the passageway in the TB until it is good and clean.
 
Will do. Thanks for all the help.

The Alt sure was some fun getting off with the Factory skid in the way. Makes me want to ditch the AC comp and run the Alt in its place.
 
Great!

:party:
 
So I lied. After the new Alt and battery it is charging great but the slight rpm drop is still there.

The problem is when coming to a stop once the rpm gets under 1000 it wants to drop to about 500rpm till I lift up on the brake pedal. I only have to lift a slight bit and the rpm will go back to 750rpm.

If I roll to a stop, no brakes, it will not drop in rpm. Is this a vac leak? Booster issue? Or IAC issue?
 
It would have to be vacuum related, which could include the booster.

The IAC is ECU/PCM controlled, shouldn't be effected by the brake pedal.

Do you have cruise control? Have a vacuum operated cruise disconnect switch on the brake pedal?
 
I do have CC. What does the switch look like?

I do not use the CC becasue it will not turn off sometimes. I think the on/off button on the steering wheel is not working.
 
Look for a vacuum line attached to something attached to the brake pedal--best way to describe it.
 
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