'88 Testarossa Battery light off at idle yet illuminates above 3k rpm

Did you measure the battery (alternator) voltage when that warning light was wrongly illuminated? That warning light is really only meant to detect a broken alternator belt. It will often wrongly illuminate when something inside the alternator goes haywire, but that is not its intended function.

 

I’ve had two alternators failing on me with the same symptoms, one time I was left stranded on the highway when the battery ran out and the second time I was actually coming from the shop to see why the light was flickering, they tested the alternator and it was working “fine”, just to had actually smoke coming out of the alternator on my way home and I was able to make it back to the shop with the battery almost dead.
Before the second one failed I did several tests, like measuring the voltage with a radar detector plugged to my cigarette lighter while driving and I got good voltage.
For me if I see that light again I know my alternator is about to die and I wouldn’t risk it, even if I get good voltage.
Somehow my car likes to eat alternators so the mechanic told me back then that the only permanent solution was to upgrade the alternator to the one in a F430 if I recall correctly (AC Delco vs Denso???)
These are the videos I took days before the alternator suddenly died with smoke and everything.







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When I did that I ended up stranded on the side of the highway.
I would go back to the shop that rebuilt the alternator and tell them something is off. As Steve said if the light is on is because something inside the alternator has gone haywire.
And also the blip to get the light off at cold start is not normal either, do a test, start the car and don’t blip it and measure the voltage with the light on, I would guess the voltage is not right until you do the blip.
Good luck


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The light at start up which goes off when you rev it the first time is a form of test, to prove the alternator light and its associated circuitry works. Normal.

Does the voltage drop above 3k rpm?

 

I know it is common and some cars here do that (battery light until you blip it) but in my experience in a perfectly working alternator there should not be any light at cold start than needs blipping the throttle to make it go away.
Of the 3 alternators I’ve had, the one that lasted the least was the one that was doing that, after it was rebuilt there is no need to blip the throttle to make it go away, it goes off automatically without any throttle input.
Actually when that happened there’s a very particular noise you don’t hear until you blip the throttle that makes me believe the alternator is not really working until you blip it.
Basically I don’t think there should be a throttle blip by default to make the light go away or (if my theory is correct) to make the alternator start working.






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this alternator bulb gets plus ( + ) by the ignition key when switch to "ON" and minus ( - ) by the alternator when engine is not running. as soon as the engine runs the alternator brings plus ( + ) and the bulb turns off because getting plus from both sides. so may be the alternator brings much more voltage than those normal 14,4 V then the bulb also will go on. or you have problem with the plus ( + ) from the ignition switch?

 

I have this situation for several years on my mondial QV. The alternator was tested twice and did not show any problem. So I live with it because I never drive at night with that car.

 

I have that slow Alt. on other European cars. I believe the execitor in the Alt. needs a voltage boost at 1st.

 

Exactly my point
That is NOT a normal behavior, the alternator doesn’t really start working until you blip it, the belt IS turning but not enough to make it start, you can actually hear it working when you do the blip, it is a very particular sound. When that happened to me, the alternator eventually did the same as yours with the blinking light, just to suddenly go out with smoke and everything.
Not saying the shop doesn’t know what they are doing but if I were you I will look for a new alternator to fix this issue once and for all.


I actually found the videos in the conversation with the mechanic
This is a video he sent me telling me “everything is fine come get your car”

https://youtube.com/shorts/_024j02PRIo?si=TceJhdFpl2k5cQKK

2 miles away I saw smoke coming out of the back of the car and I barely made it back to the shop with an almost dead battery

https://youtube.com/shorts/3x8KhhZjM7E?si=h08iV-NSxLsQFZFI



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Yes it is a commonly accepted feature with older European cars that the alternator doesn’t get excited until it reaches certain rpms threshold but I don’t think it is a normal behavior with the TR alternator, I believe the alternator should start working right away without any throttle blip, I have no idea what would be the issue (voltage regulator, bad brushes??) but an electrician expert like Steve M could say if that’s normal or not.



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Every post here is a gold mine of information. So I have a 1987 328GTB that the alternator light stays on until you rev the engine. You can be driving down the road and the light will NOT go out until you rev the engine. Obviously is NOT charging until the engine is revved. I KNOW the idiosyncrasy and have lived with it for 10 years. Never been a problem until I let a friend drive the car and forgot to tell him above revving the engine. Whoops.

Pretty sure it's an original alternator as the car only has 15K on the odometer. It IS a Bosch alternator.

 

My 328GTS 1987 does the same thing. Already asked Steve if he knows what the problem could be but even he does not know. I just live with it ....

 

I'd say that it's not a super-uncommon behavior but also not a common behavior. Never had that symptom on my ex-F alternators -- the warning light always went "off" at engine start-up. If the RPM required to "activate" the alternator was low enough that would always be exceeded just in normal cold-running use (like say 1500-2000 RPM), I wouldn't worry about it. If it required a high RPM, like 3000-4000 RPM, that would be worth trying to fix IMO.

 

I wonder if people experiencing this have AGM batteries, that require a higher charging voltage than these alternators are designed for?

 

Standard battery in my case



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Optima type batteries. If the battery is dead below 10.5 vdc, Don't charge with alternator or old type battery chargers. You need a lead acid battery between. Ask me how I know? I burned up an alternator on my hot rod after jump start dead battery. The newer battery chargers are designed to charge newer type of batteries.

 

This is a cold start of my car
Rpms barely reaching 1k and the light went off automatically
I think a lazy alternator that requires a blip of the throttle to be excited (while accepted as normal-ish behavior) is an indication that an alternator is not working perfectly. My 02 cents

https://youtube.com/shorts/VYuFpc2qLT0?si=rPNHl2GfWAzJOuaC




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