F355 Alternator Problems - Latest Update

Voltage regulators on alternators rapidly burn up if the ground or positive connection to the battery is/are bad (hat tip: Ken and RoloCR).

 

YEAP!! check the cut off swicth, people normally take it for granted, a loose kill swicht is like running the alternator with the battery disconected and it burns the ALT in MINUTES!

notice that everytime they change the ALT it works even less than the previous one, so it shows the problem is getting worse

 

What was the output voltage when the reg failed?

 

Are you thinking dual failure: alternator *and* VR??

 

VR's die if the positive OR ground connection is poor. I'm not an expert, but you can't short them out otherwise (without a deliberate effort).

Your fault is somewhere in the charging system; as I am unfamilliar with your car I can't help you more than that. ROLOcr has it right; a negative battery cutoff switch that is bad can absolutely fry a VR, just as much as a bad + connection. You are frying some connection worse each new alternator; one would think there will be scortch marks by now somewhere! *S* Short of seeing that, you need to test each connection in the charging circuit with a meter; both ohms and voltage. I once had a bad ground that *looked* fine; you need to test the connections; not just look at them. It's a PITA but what else can you do? When you find the problem, it will be obvious.

Ken

 

Very interesting problem. I have a 355 in the shop right now that is doing almost the samething. Driving me up the wall.
When the car came in the light on the dash would flash on and off at higher RPMs. Took the alternator out and found 3 broken wires in the stator. Had the alternator rebuilt, installed and checked..14.3 at idle amperage would fluctuate as normal when accesories were turned on., battery recived a full charge and I gave it a load test before it left.

Car dies at a gas satation 3 days after delivery. Run out to the station..dead battery. Hummm...must be a bad rebuild. Boost car remove battery pack..car still runs..hummmm I put my mutlimeter at the boost point in the engne compartment..shows 13.2 at idle. Alternator is working. Drive the car back to the shop..load test the battery again..FAILED!!

Install new battery..check system again..everything is fine. Car arrives on a flat bed Monday, dead battery. No lights on the dash came on at all during the drive home. Charge the battery fully..start the car..retest..all is well charging a 14.3 WTF!! I say.Contact the customer to tell him everything seems ok at the momment, need more time. Car is still at the shop trying to replicate the issue.

Please keep us updated with this issue, as I may have the same problem here! if I find the cause I will post it.

 

Im thinking Tbakowsky and this guy both have nasty electrical gremlins on their hands. The complete charging system wiring harness needs to be checked IMO. As Ken just said, a major PITA.

 

Once when I was young and foolish, I unplugged the leads from an alternator while it was running. It must of been a really good v-belt, as it never slipped a bit as the alternator ball bearings turned blue and MELTED right before my eyes! I was in Chugwater Wyoming (honest, look at the map) at the time. Fortunately the battery had enough charge in it to start the engine the next morning and drive the twenty five miles to Wheatland, sans alternator, to the nearest NAPA store. Moral of the story? If the alternator doesn't sense system voltage, it will try to raise system voltage until it destroys itself.

 

Wouldn't you want to do the divide by two test here (rule out half of all possibilities at a time, progressively)?

I'd suggest temporarily disconnecting half of the wiring harness (e.g. all lights, gauges, stereo, A/C, etc.) as well as momentarily running a *temporary* ground cable from the battery ground to the alternator circuit (i.e. bypass the battery disconnect switch at the very least).

If the problem repeats, then disconnect half of the remaining wiring harness (continue until only the alternator, battery, and spark coils are on the circuit if necessarry).

This *should* isolate the problem to one half of the wiring harness/circuits.

 

Maybe so, but id be more inclined to run a 2nd earth strap actually.

 

OK, I can live with that. Add a 2nd ground strap from the alternator to the frame, plus add a new (but temporary) ground strap that bypasses the battery disconnect switch.


Might as well measure the resistance on the B+ alternator output cable, too. If it is exceptionally high then that could be a (perhaps "the") problem.

 

I asked the auto-elec at work the other day about this issue. He actually mentioned the 2nd earth strap. If that wasnt any help, then he had a strong feeling that there is a short there somewhere, in the charging circuit, that is frying the VR. I agreed with him, thats why he should bust out the mulitmeter out and start probing into it. Major time consumer, unless its an easy, quick find. Thats why the dealership mentioned the wiring harness replacement, gets them out of pulling their hair out trying to find the short circuit problem.

 

The more that I think about it, the more that I'd focus solely on the charging circuit (cables to battery, disconnect switch, cables from alternator, ground from alternator) rather than on the entire wiring harness.

I'd be looking for high resistance in the B+ cable or an open/broken circuit in the grounds...actual short circuits would be low on my list (wouldn't hurt to glance around for large scale burn marks, though).

 

add just the bypass one of the kill switch, and test it

what brand is your alternator,delco?
ROLO

 

the denso units are the good ones!

 

There is usually a smaller guage wire from the alternator output that connects to the starter solenoid and then a larger cable that runs back to the battery. So the smaller cable should be removed and tested while flexing the heck out of it. Also, since the failures happen while driving, the battery disconnect switch should be bypassed (both cables on one terminal). Lastly, Fluke makes a clamp on current probe and some data logging digital volt meters could log voltage and current until the next one dies. This is a fun problem.

Jeff Pintler
86 tr, 89 348tb(on the mend)

 

The 355 is still charging at this time. i left it running in the shop for almost an hour and then went for a drive with my mutlimeter hooked up directly to the battery with extentions leading inside the car to the meter. No drop out in voltage at all. Hummm...

 

Can I take a wild stab at this? Could a secondary circuit, like the A/C or aftermarket stereo, be causing a short that drains the battery? Perhaps the customer runs an accessory that you are not during the test, and that's the culprit?

Ken

 

If they are right, then you had two problems: 1 problem with the kill switch that they bypassed, and a 2nd problem with the bad alternator.

Hope that they didn't charge you for a bad part or for labor for a bad part/alternator!

...of course, with their track record so far, whether they are right about the bad alternator is open for debate.

You'll also want to insure that they leave that kill-switch bypass on. Might as well rule out the kill switch if the next alternator goes bad.

 

I ran that puppy with every option and accessory "on". A/c at max, radio on, wipers running, highbeams on, interior lights on..switched to the heater half way through the drive. Showed no real change in the output of the alternator other then the amp draw which is expected...Thanks tho'

 

could it be that while driving the bumps tend to "flicker "the kill swicth, killing the alternator just when driving around???