Alternator issue

[paintdudeluke]

A few times in the last couple weeks I have some weird link between the tach, brake, and apparently voltage but not sure. When its warm I'll be driving along without issue and out of the blue it will want to die when I hit the brake. I noticed that when it happens the tach also drops down to zero almost instantly. Oddly it only happens under 3k-RPM and if I downshift quick enough to bring the Rs up it will continue to operate fine, but once I slow down and it drops under 3k as soon as I touch the brake the tach drops and it dies. Usually is starts right back up but the short persists. I noticed today that it was wanting to die rolling into my parking space so I used the ebrake, popped the hood (suspecting a weak connection at the MAF box and while I was looking, as soon as the fan kicked on it died.
Ring any bells? I live on a pretty choppy road so a bad connection is suspect, but Where is the likely junction based on the relationship between the symptoms?

[TX82FIAT]

There was a post last week about a voltage drop when the brakes were applied because of an increase in resistance within a circuit. If you have a voltage drop in a brake circuit is could effect other systems as well. I Suspect you would take your foot off the brake to downshift and prevent the stall.

We are always talking about grounds, I'd check all the grounds to make sure they are clean. Ohms law does not take a lot of resistace to drastically change the voltage requirments. You could also have the beginnings of a failing voltage regulator. Those little cigarette lighter voltage meters are very useful to see how the alternator is working and the change in voltage when you turn on various circuits as you drive down the road. A bad connection or corroded connection could increase resistance. When the fan kicked on the system needed more power, did the alternator (voltage regulator) instantly adjust for the increased need?

[paintdudeluke]

No , it instantly died. I don't suspect low voltage because it cranks like a new battery when I crank it back up. I will check the voltage however with a meter in a few minutes. Not a bad spec to check out at this point.

[paintdudeluke]

Well that solves that, tested at the alt.terminal, Idling at 12.05 when I rev it up it drops to 11.something. Sounds like an alternator to me. My 80 has a reg in the alt unit right, no external regulator? Just a buy and swap if that's it. Surprising how sensitive the FI and E. Ignition is to a slight voltage drop.

[paintdudeluke]

Question:

Can the regulator on the back of the alt be swapped out or just replace the unit? Where is the "special deal" on a new alt other than ebay? Are some models better than others?

[81SPIDERMATT]

yes you can swap the regulator .... there is a chance that only the regulator is bad and you would not need a whole unit .... not sure how to isolate that myself but auto parts store can hook you up and tell you if it the regulator or more

[paintdudeluke]

I renamed the thread alternator issue to be useful for the next guy doing a search. BTW< I remember the car dying when I had the lights on the other night. This is why it dies under 3k, Above that it must generate enough power to keep the voltage up. Nice assessment tip.

[v6spider]

Replace that alternator with a one wire gm alternator.. Problem solved..

Rob

[baltobernie]

Diagnosis by replacing parts can be frustrating and expensive. Test the alternator installed in your car. There are many You Tube videos and Internet links to this procedure. If the alternator fails the test, remove it and take to a local auto parts store, where they will bench-test it. Then you can evaluate your options.

[TX82FIAT]

Agreed that you should specifically test the alternator. A false positive is likely with voltage drops. The regulator is not hard to replace and i have seen a skilled mechanic with small hands change one out on the side of the road in 20 minutes. If you have not already done so, I would still clean all the grounds points and check connections. At least once every two years i clean all the grounds and check connections I know are prone to corrosion as normal preventitive maintenance.

The fact that the voltage dropped when you increased RPM's with no additional circuits to add resistance does lead me to the alternator/regulator as you suggested.

[kilodrew]

I have a alt for a 1978 spider for sale if needed.

I can post details if interested

[majicwrench]

I'm really leary of your alternator volt test. If indeed voltage was that low, lights would dim, radio would go off, would not crank when you went to restart it, and you would be constantly having to charge battery.

[paintdudeluke]

It was effecting everything too. When voltage is at 12 when running, alternator is not providing adequate voltage, period. A load test is another factor too, but low voltage is a dead give away. 12 was enough I guess to operate and crank up, but the ignition or computer system didn't like it and failed when voltage dropped below X, whatever that happened to be. Brake lights, wipers, headlights, stereo, whatever pulled enough current to drop voltage made the tach quit then the engine would die.

[paintdudeluke]

I put the new Alt in a few days after this post started. Totally solved the problem. Everything is operating better with 14v. I think the voltage drop was effecting the computer. BTW, That bottom bolt sucked, it was pretty corroded in place. I had to pull the radiator and use a long extension with the impact to spin the crap off of it so it would pull out of the lower bracket. This is why every time you work on something, squirt everything in sight with PB or WD or something. You never know when you might have to pull that rusty bolt.

[nalle]

Would you please tell the type of the old and new altenator.
Bosch.....something
I'm going to change the regulator in the future (and the brushes) but my alternator "type stamp" is missing /not readable.
And on the same time I can check the sliprings / insulating resistance (like with bigger generators)