Drops when You put it in gear... First clue!

Yep. If I start it, and put it in reverse, the voltage drops immediately and when I hit the brake the lights dim and voltage drops



Isn't that a 6g? Whats the difference 4g-6g are they interchangeable?

so what should we check? voltage goes to normal off idle in gear just at idle in gear the voltage drops for mine.

Stock/Factory alt works fine in our out of gear.

Idk..its just been that high.

Does the new alt have a larger pulley??? Often they are just slightly larger which will cause charge problems at idle... Otherwise if the internal regulator is OK(likely)the new alt requires more RPMs to charge than does the stocker... Likely you need to raise idle speed and/or switch a undersize pulley...

6g has a bigger case than 4g. The 6g is deeper. Also 6g has better cooling and also better bearings compared against 4g. Of course over the lifetime it is said that the improved bearings and cooling really have no affect. 6g was the only alternator offered in 130 amps for Police although 4g and 6g were also wound for lower amps for civilian cars. It seems like Grand Marquiss always got 4g while Crown Victorias got 6g.

Ok... Your gauge and your measurements with a meter have concluded that the alternator is working okay except on idle in gear.

That is not unusual. There a two very common things that can cause that. One is the drive belt, the other battery connections including grounds.

Given the fact that it is not malfunctioning except for idling in gear you should make sure the tension on the belt is good when there is a load on the motor. You can do it by checking the tensioner. Given the age of the car that is a highly likely source.

You seem reluctant to check that part. It is not expensive or difficult to just replace. It should be hard to release the drive belt. If it is easy to release then the tensioner is too worn.

There is the possibility that since this is a higher output with therefore more windings, it may require a higher rpm. Ford set the idle higher in gear for police units for this very reason. They know what they are doing.

As suggested you could raise the idle with a tuner.

I did check the tensioner.. Couldn't find anything wrong with it. I really am not sure how to tell if the tensioner is bad or not though.

It seemed to take quite a bit of force pushing down to release the tension.

Looks like my next step is to compare the pulleys on both alternators and see if one is larger than the other

Battery connections should be all good. It's grounded to the factory green ground

Update.. Checked this pins on alternator harness, all good

I put my old alternator (stock, 12 years old) back on and my voltage is much better.

It no longer drops in gear. Slight drop when I turn on my ac (which is why I decided to get a new alt) but it's nothing bad.

I will be returning my alt and maybe just buy a new OEM one

I know my tensioner is good cause it is brand new when I put the new motor in. Also belt is new and tight. it is a gatorback belt 1" shorter than stock.

I may see about getting a smaller pulley.

okay well had you said that then I would have moved on earlier LOL

More than likely you have to step up the idle in gear. Bigger windings would do that if my fading knowledge of making electricity are correct. More windings but low speed making for an issue... seems to hold up in theory?

Anyway good luck...

My '99GM came with a 6G alternator.

Any update? What did DB Electric say about this issue?

P_Weston is running a 6G 130 amp P71 alternator in his Grand Marquis and he has not bumped his idle with a tune or P71 PCM swap and he is not having any voltage issues as far as I know.

Umm I thought he said he was running a 220amp?

I know he is, I was just saying that the 130 amp has more windings than the civilian alternator and yet it survives at the lower RPM of stock Grand Marquis programming...lower idle speed than the P71s.