alternator output v jumps after rev

bjornson · 08-05-2002, 01:54 PM

charging system on 79 928 puts out 10 volts until u rev 4000rpm
then meter jumps to 12v any ideas?
thanks, dave

Joe Ricard · Joe Ricard's Garage

That is just the way it is. Well actually someone once explained that it has to do with the exitation voltage. With an actual digital volt meter I watched this happen. I have been driving my 78 5 speed for 12 years on this altenator and voltage regulator. Starts everytime and never kills a battery.

maachan · 08-10-2002, 08:14 PM

it sounds odd to me.

Upchuck · 08-24-2002, 07:06 PM

I have an 80 Automatic, same thing but voltage comes up about 2K rpm and stays. Never fails to start and never runs the batt down

maachan · 08-24-2002, 08:00 PM

When I posted on this thread I was not aware tha t I was on 928 BBS. I am usually on 924/944 BBS.

Well anyways, I do not have much of experience with automobile charging system, but with reciprocating engined airplanes. some of them have D/C generators but most of airplanes are equiped with A/C alternators. They produce charging voltage (13.75v ~ 14.25v) even from idle speed. I thought auto charging system would do the same. If it requires 4000rpm to produce 12v, there is something wrong I think. Let it be charging system or indicating system.

That is why I say "it sounds odd to me" .

Maachan

Joe Ricard · Joe Ricard's Garage

You can make it better by cleaning all the ground connections. I replaced the whole battery cable. The original was pretty shot scraps and corrosion everywhere. I went to a boating store and bought 13 feet of 00 gauge 600 Volt cable. End of most battery problems. New Negative side cable too. Then every brown wire connection on the car should be scrubbed shiny new.

KBlair · 09-26-2002, 12:34 PM

I've been told that it is a faulty diode somwhere. My 83 does the same thing till about 3k RPMs, then fine. No problems with not starting or running down battery.

maachan · 10-09-2002, 07:14 PM

There are 3 diodes in the system to convert alternating current into direct current. Our auto system is DC (direct current). If you have a faulty diode your system is creating Ripple Current. It is almost like alternating current but not quite. This Ripple Current will have adverse effect on your battery since it is of DC. You may facing premature failure of the battery.

This is my best educated guess.

Maachan

The Chief · The Chief's Garage

Driving home this afternoon...

Sat at a red light, noted the "BAT" light illuminated, and the charge gage in the red zone. Decided to pull off at the first available shade, and the gage popped up to about 12.5 volts, light went out.

A few minutes later, same thing. "BAT" light on, gage died. Seeing as I was on the Interstate, wanted to get as close to an exit as I could before I shut down; ran for about a minute or two, light went out, gage started working again. Every time I have wanted to start the car, it cranked right up.

I have also noticed that, occasionally at idle, the charge gage will go into the red zone, but will go back to "normal" when the RPM comes up.

Voltage regulator, perhaps?

Danglerb · 07-21-2007, 01:08 PM

The 928 spins the alternator at 2x crank speed, which is low by most standards, but the engine is designed for long periods of over 5000 rpm, all the little planes I have flown were less than 4000 max if I remember correctly. Good chance they spin the alternator faster relative to crank speed.

Jegs/Summit make a battery relocation kit that many have had success using to replacing the existing main cables. Its 18 ft of 1 gauge red wire and 3 feet of block with crimp connectors, and about $65 I think.

If I had any questions about the alternator, I think I would just replace it with a good rebuilt from Eagle Electric on ebay.