Overheating Alternator???

After running fine for 15-20 minutes my alternator will begin to heat up until the point where it is too hot to even touch. As the alternator heats up the charge of the battery begins to drop until the point where the battery drains and the car dies. After sitting for enough time for the alternator to completely cool off the car will start up fine, but the process soon repeats itself.

I removed the alternator and had it tested professionally and was told that there were no indicators that pointed to it being a bad alternator. (They did not run it in the test environment for more than 2 minutes however). The leads all appear to be fine and everything is emitting the proper amps until the overheating begins.

Has anyone had a problem like this or have any suggestions as to what next steps I should take?

Thanks.

Bring it back and have them test it again (or bring it somewhere else), this time running it for however long it takes for this problem to re-occur.
I don't know how much heat an alternator should generate, but anytime the output starts to drop off, you've got a problem.
Are you hearing any noise that may be the bearings going bad?

I just bought and put in a brand new alternator. Within 20 minutes it's again too hot to touch and the battery charge begins to drop. Haven't taken it out for a long drive yet as I don't love getting stranded and waiting for a tow truck.

Do alternators generally give off heat? Any suggestions??????? This sucks...

Anything under the hood will get somewhat hot due to the heat build-up under from the engine. However, that alone should not be causing a voltage drop.
I'm wondering if you've got a wiring problem after the alternator that is causing an excessive current draw, and causing the rectifiers in the alternator to overheat.
In any event, I sure wouldn't trust this car until you've had it more thoroughly checked, because as it stands now, it will leave you stranded somewhere.
PS, where/how are you measuring the voltage drop? (Just curious).

I just bought and put in a brand new alternator. Within 20 minutes it's again too hot to touch and the battery charge begins to drop. Haven't taken it out for a long drive yet as I don't love getting stranded and waiting for a tow truck.

Do alternators generally give off heat? Any suggestions??????? This sucks...

Yes alternators give off heat, they have an internal fan to cool it. The will get too hot to touch after running a while.
I would test the voltage at the battery with a meter, should be around 12 V.
A good test is at 1500 rpm, headlights, heater fan and radio on, should hold around 14 V.

I have a 2001 Toyota Sienna with the same issue. I have gone through 4 alternators in a week. Did you find out what was wrong?

Did anyone find an answer ? If no-one else has solved it I think the answer may be an unusual battery fault. Try a substitute battery.

Stevef99 Brisbane

[quote=Stevef99;7159432]Did anyone find an answer ? If no-one else has solved it I think the answer may be an unusual battery fault. Try a substitute battery.

Stevef99 Brisbane [quote]


post with this is 7 years old .. I am sure it is fixed by now...

[quote=Stevef99;7159432]Did anyone find an answer ? If no-one else has solved it I think the answer may be an unusual battery fault. Try a substitute battery.

Stevef99 Brisbane [quote]


post with this is 7 years old .. I am sure it is fixed by now...

Hi j cAT,

I have the same symptoms but I do not know what the definitive answer was. It is not posted. Human nature dictates that after the original poster has worked out the solution he cannot be bothered to provide further information on the forum ~ he is too busy cruising the highway.
My details are:- Everything is good from a cold start ~ 14.2 volts at the battery from the alternator, but after a few miles, the charging voltage falls lower and lower until the battery ceases to charge. I replaced the alternator but it did not make any difference. I replaced the heavy cables from the alternator to the battery. There are no wiring defects, drive belt is tight etc. A mystery, but I do not have a spare battery to try. So I was looking for the 7 year old answer.

Steve Brisbane, Australia

to begin with why did you replace your alternator?

he is too busy cruising the highway.

post with this is 7 years old .. I am sure it is fixed by now...
LOL I thought this was a new problem

to begin with why did you replace your alternator?




LOL I thought this was a new problem


Hi la1,

It is a NEW and CURRENT problem for me. Some time back my previous alternator failed. When I took it apart, I saw that one of the diodes was dud. It looked like the entire rectifier assembly had been overheating, but I put that down to the shorted diode. My car is a 1979 V12 Jaguar. I replaced the faulty rectifier, and changed the voltage regulator. But the symptom that I described had not gone away. I dismantled the alternator again, and concluded that there may be shorted turns in one of the stator windings. I could not source a replacement stator coil, (NLA), so I had to fit a replacement alternator (later design, as original pattern alternator was obsolete)

Hi la1,

It is a NEW and CURRENT problem for me. Some time back my previous alternator failed. When I took it apart, I saw that one of the diodes was dud. It looked like the entire rectifier assembly had been overheating, but I put that down to the shorted diode. My car is a 1979 V12 Jaguar. I replaced the faulty rectifier, and changed the voltage regulator. But the symptom that I described had not gone away. I dismantled the alternator again, and concluded that there may be shorted turns in one of the stator windings. I could not source a replacement stator coil, (NLA), so I had to fit a replacement alternator (later design, as original pattern alternator was obsolete)

what you need to do is measure the alternator output current. this is done with a meter attachment that clamps around the wire output of the alternator. see how much current it is producing. see if the amount of current is to high. most operate at 60% at the rated alternator current spec, with a 130 AMP and engine running have all power assy OFF. should be 50-70amps.then put on one assy at a time see if the current surges. if this is a 79 JAG this is a wiring mess. I do not like these vehicles electrical systems very poorly designed.... you could have a short circuit in the door that feeds the window motor as an example .........................

what you need to do is measure the alternator output current. this is done with a meter attachment that clamps around the wire output of the alternator. see how much current it is producing. see if the amount of current is to high. most operate at 60% at the rated alternator current spec, with a 130 AMP and engine running have all power assy OFF. should be 50-70amps.then put on one assy at a time see if the current surges. if this is a 79 JAG this is a wiring mess. I do not like these vehicles electrical systems very poorly designed.... you could have a short circuit in the door that feeds the window motor as an example .........................

Hi jc AT,

Thanks again for your post.
A short in the door (window motor circuit) would trip the thermal cut-out, and only when the window switch is operated. Same for door lock solenoids.
I will have to obtain an ammeter.
I am on vacation for 5 weeks, and will not be able to post again until May.
The XJ Jag wiring was designed in the sixties, so I guess we cannot expect too much....

Steve ~ Brisbane, Australia

Hi jc AT,

Thanks again for your post.
A short in the door (window motor circuit) would trip the thermal cut-out, and only when the window switch is operated. Same for door lock solenoids.
I will have to obtain an ammeter.
I am on vacation for 5 weeks, and will not be able to post again until May.
The XJ Jag wiring was designed in the sixties, so I guess we cannot expect too much....

Steve ~ Brisbane, Australia

the XJ6 I worked on always had 12 volts to the window motor. the return ground was used to complete the circuit. so 12 volts always on.. yes it will blow circuit if a good solid connection ..also wiring has no protection , dash is a nightmare..

All these stories reminds me of Lucas Electric, prince of darkness.

Late to the thread but researching a similar problem with alternator output.



Sad; we said the same thing: "Lucas, prince of darkness" in the 60's. Some of the guys in our college Sports Car Club would suddenly loose all their lights, others would have to stop and switch off all electrical to let the generator recharge the battery because the extra lights would draw more than the generator (not alternator) would provide.

No dawg... I just installed a mini street alternator with one wire setup. Well first I have it running CCW. Was not charging. Almost gave up, but thens it hit me... try to excite the alternator. So I made a jumper wire and and touched to the first terminal... nothing changed. So then I touched it to the other terminal and the tractor volt gauge jumped to 14v and could hear the engine baug downa little. So no kidding the 1 wire self exciting gm style alternator needs to still be excited if not charging and have good ground. However a strong charge now the alternator is very hot after running 30 or so mins

the field coils are shorted out .. the alternator is rebuild ...I replaced my alternator and bought it ,rebuild ... alternator 78amp...the voltage is not a lower voltage around 13 volts ... so the alternator take it out to go back to the auto store ..
then I put my hand on it was hot as hell ...............