I'm experiencing voltage fluctuations when I turn on the fans in my 1988 Porsche 928 S4. Could the fan's power amplifier be the problem?
Thx Patrick
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Hello Patrick,
I assume that both fans start running and keep running when you turn on AC or when the engine is really hot?
(If one fan stops spinning, while the other runs full power and fluctuates in rpm, you may have a power amp problem)
The large fans behind the radiator draw quite a bit of power. It is like 17 Ampere when running and even a bit more when powered up. So it depends on what the alternator and the battery can do. Small fluctuations in voltage can be result of alternator problems, battery, or wiring problems. Voltage fluctuations do not sound like a problem with the power amp.
Regards Theo
Theo Jenniskens
1992 928GTS midnight blue
http://928gts.jenniskens.eu----
Hello, thank you for your reply. I've already replaced the alternator and battery with new ones. I've redone all the cable crimps and replaced the ground cable at the battery. Both fans run smoothly and spin perfectly. In the instrument cluster without the fans running, I measure 13.5 volts, and in the engine compartment, I measure 14.4 volts. When the air conditioning is activated or the engine is hot, the voltage drops to 12 volts, and in the instrument cluster, it drops to 10 volts. After driving for a while, the engine starts to surge and stalls. The lambda sensor is new, the MAF sensor has been repaired, and the idle switch is correctly adjusted. My last guess is that the power module for the fans is causing this voltage drop. What's your opinion as a professional?
Thx Patrick
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Hello Patrick,
No, the power stage (fan power amplifier) does not do that. It modulates the power to the fans, but cannot draw more power than the fans consume. The fans have an initial load of like 20A which settles at 17A when they are spinning at normal speed. Only (!!) if the fans run badly due to bearing issues, dragging, the fans do not reach normal speed and draw more current, but I’d not expect much over 20A. The alternator can handle that. If the fans turn freely by hand, this is not the problem.
It's not uncommon to see 12.5 volts (or lower on a hot summer day with the A/C running.) A lower voltage may cause misfire and even stalling and not starting anymore.
If the instrument cluster says 10v, be aware… this instrument is hardly precise. Trust your own voltmeter instead.
To me it sounds like the alternator is not delivering the required power. Also, if the engine rpm drops to a low rpm, the alternator will also deliver less power, thus voltage may drop. Maybe that is what happens?
You replaced the alternator you say, was there something wrong with the original? Did the new alternator change the symptoms? Does the alternator provide the same power (specs) that the original unit does (I think 115 amps)? I think the alternator voltage on the terminals is close to 14.5v.
Regards Theo
Theo Jenniskens
1992 928GTS midnight blue
http://928gts.jenniskens.eu----
Hello, the old alternator was fine, and I only replaced it preventively because I was at my wit's end. That means I'm experiencing the same symptoms with the new alternator. I also replaced all the cable clamps. Only after driving for a while does the voltage drop even further, and the engine starts to surge and eventually stalls.
Thx Patrick
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Hi,
Sounds a bit like something is getting hot. Maybe the cooling of the alternator is not working? Or a problem with the ground-cable that connects the engine to the chassis?
Regards Theo
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Hi,
I've now replaced the ground cable on the motor with a 50mm˛ cable, and it's running much more smoothly. Thank you so much for your help.
Thx Patrick