Alternator warning light came on

I had the same problem two years ago so I took it to our local alternator shop and found that I had bad diodes, they rebuilt the whole Alt, with new diodes, bearings and brushes for $100 No more lights coming on since

 

It stays on until the rpm reaches about 2500rpm then goes off. The problem was I can drive my car all day and all night long but once you turn it off, it will not start back up, I always have to jump start the car everytime.

 

What is happening is the diodes in your alternator are going, there are six in the alternator that turn the AC output of the alternator to DC and charge the battery. When one or more go then the charge voltage is low and the system can't keep up. A voltage difference is created between the Alternator and the battery and the G lamp comes on, telling you your battery is failing or the alternator is not turning i.e. broken belt. There are several things that can cause diode failure, If there is a bad connection between the battery and the alternator, charging current will find an alternate route to flow to the battery, this over heats the diodes. A poor connection may be caused by corrosion or a broken battery cable. Diode overheating can happen when the alternator tries bring an undercharged battery up charge. This can cause excessive current that can overheat the diodes.

Diodes can also be damaged when you try to jump start, as an large surge of power can toast some diodes, it doesn't happen all the time but if they are on the edge they may be damaged by continuous jump starts

When you revved the engine and generated more voltage you charged the battery again and it lasted for a while but it will likely happen again and will continue to to get worse, as more diodes crash and burn. But its easy to repair. Just make sure when you put it back that all the connections are good the the battery is good and will hold a charge at about 12.5 volts. It also a good time to put in a new voltage regulator even if yours is good now and maybe some new bearings.

 

First thing is to rule out the easy stuff & make sure its not just a loose belt (though the behaviour seems a bit odd for that).

It should just be a Bosch alternator so any Bosch specialist should be able to rebuild it - just don't tell 'em what its from ;-)

 

I just had the alternator from my '86 328 rebuilt at a local Bosch alternator shop. The whole thing, including new bearings and shaft, complete rebuild, came back looking like new, cost $140. These are plain vanilla Bosch alternators, nothing "Ferrari" about them.

 

You can definitely remove the alternator on the '78 with the gas tank in place. I did it on my former '78 GTS, no problem. Yes, it's a bit tight getting at things, and you might have to remove the stainless heat shield, but you can get it out. It is much easier if you can get the car up in the air, and if you have someone help you so you don't have to twist yourself like a pretzel to get the wrenches where you need them. IIRC, I might have had to move the coolant pipe that runs through there as well to get it out and back in. Can't remember if we were able to drop it out the bottom or not.

Note of caution: If you are jacking the car up to do this, please, please, please be sure to use really good jack stands and wheel chocks. We don't want to have to nominate you for a Darwin Award.

 

the red alternator light on my 78 308 just came on when driving. when parked and with the engine running; at idle the light is very dim. when i rev it up, the light gets much brighter. i guess it is time to yank the alternator unless someone can think of something else..
all cables to and from the battery have been cleaned, tightened and i replaced the battery ground cable to the frame and cleaned teh corrosion there. the battery is relatively new and i have never had a problem storing and running the car without the use of a trickle charger....
thanks in advance

 

1st thing to do is measure the voltage across the battery with the engine off and then with it running. It should be around 12V & around 14V. If its not then you may have an issue.

The symptoms you have suggest the alternator is not charging the battery. That can only be a bad belt, a bad connection or a bad regulator/alternator.

 

thanks Ian, i will measure the battery voltage with car running and not. i suspect the diodes in the rectifier circuit of the alternator. the belt is on nice and snug, and cables are new.

 

i measured battery voltage at the battery terminals
12.00 volts when car is off
13.65 volts with car idling

so, any thoughts as to why my generator light stays lit all the time while driving?. incidentally, i drove the car 30 minutes with the light on, you would think if it was charging a deadish battery, the light would eventually go off - or is that not how it works.

 

Well, clearly those numbers indicate its charging - odd!

Needs someone with better understanding of the charging system than me I'm afraid.

 

bad cell in the battery. Replace the battery ASAP or it will damage your alternator and can damage the charging circuit.

 

dead cell in battery, thats intersting. thanks for the tip.
i could get the battery checked for that - correct?
and i should add alternator rebuild to the list - couldnt hurt..

 

i hear ya on pulling the alternator. it sure does look like a PITA.
thanks for alll the help, i wil keep you all posted.

 

FWIW I just had the alt on my 328 rebuilt (because the engine was out so it would have been daft not to).

There was a little play in the bearings (but you could check that without removing it) and the brushes were less than 25% worn after 37K miles. Again I replaced the reg pack (which includes the brushes) because it was cheap & it would have been silly not to - but it wasn't strictly necessary.

Unless you need to pull it because its not working or the bearings feel really bad, I wouldn't bother.

FWIW when tested my Alternator is kicking out 14.6V & well over 100 amps (spec 85) - it pegged the gauge on the test bench.

 

battery checked out fine, 799 amps from a 800 amp battery.

looks like i get to R/R the alternator.

 

It's a piece of cake..the rear heat shield bolts are a little hard to get to.....

Take the mounting bracket and the water coolant tube out of your way, disconnect the charging wire off the stud and out it comes!

If it doesn't clear the frame, your motor mounts have 'gone soft"....unbolt them and jack the motor to make enough room to get it out....

 

Pay attention to the rubber insulator on that stud, and the tubing protection of the wire going back to the starter soleniod (and from there back to the battery in front).

IF you accidentally ground out that wire against something on re-installation, the full current of the battery cable goes thru that #8 wire, and it doesn't LIKE it!!

In my case, that tube had filled with leaking oil and it made quite the smoke cloud!!
Until the wire vaporized.....

 

thanks Bubba, i will do that. i probably would have left that heat shield in place had you not chimed in. oh, and i yanked the ac compressor last year - so i hope it is easier with that out of the way.

now, where did i put that judas priest CD; since i will be bangin my knuckles, i may as well be bangin my head!!

i will keep youse posted on the progress.