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Peter_in_AU · Joined: 29 Jul 2001 Posts: 2743 Location: Sydney, Australia

after finally sorting out nearly all the electrical probs in my '79 NA and feeling excessively smug about it my alternator has gone bad on me.

The symptoms are a whine raising and falling with revs (sounds like a supercharger whine which would be cool if it was) and a drop in voltage of about 1.5 volts. The charge light comes on at idle and I'm only getting about 12.5 volts at 3,000+rpm.

Before I dive in and get all greasy does anyone know if this is a fixable problem or should I start negotiating on a rebuilt alt?

I initially thought it was a slipping belt and the belt was a bit loose but tightening it has made no difference.
_________________
1979 924 (Gone to a better place)
1974 Lotus 7 S4 "Big Valve" Twin-cam (waiting)
1982 924 (As featured on Wikipedia)

Learn to love your multimeter and may the search be with you

Joes924Racer · Posted: Mon Jan 27, 2003 11:23 am Post subject:

Your sure its the alt. ited be a good idea seems thought the whine comes from a to tight belt...
_________________
1979 porsche 924 Na
1980 porsche Turbo 931GT Replica
Have u ever driven a turbo.

dpw928 · Joined: 02 Nov 2002 Posts: 1860 Location: owasso, ok 74055

Peter,

The low voltage and whine could be due to worn out brushes on the regulator. Removing it for inspection is fairly simple and may even be done without removing the alternator. BTW make sure you remove the negative battery cable before starting.

Dennis
_________________
81 931 5 sp
78 928 5 sp Silver
78 928 AT Euro Black

Peter_in_AU · Joined: 29 Jul 2001 Posts: 2743 Location: Sydney, Australia

I first thought it was the belt. I readjusted it as per the bush mechanic instructions in Haynes - 3/8" movement between the crank and alt pulleys when pressed with an ISO thumb at some pressure.

Incorrect belt tension would not cause the drop in voltage unless it was slipping. I loaded up the alternator pulley at idle using a rag and my measuring thumb and it didn't slip.

Regulator bushes. Yeah, they're reasonably easy to check.
_________________
1979 924 (Gone to a better place)
1974 Lotus 7 S4 "Big Valve" Twin-cam (waiting)
1982 924 (As featured on Wikipedia)

Learn to love your multimeter and may the search be with you

Richard · Joined: 04 Nov 2002 Posts: 617 Location: Pacific N.W.

I vote for alternator bearings, although brushes will make a whine. How does the alternator sound with the belt removed and just spin the pulley?

Peter_in_AU · Joined: 29 Jul 2001 Posts: 2743 Location: Sydney, Australia

I didn't try spinning the alternator when I had it loose to adjust the belt.

Looks like it'll need to come out so I can check the bushes and bearings. Some greasy entertainment for tomorrow.
_________________
1979 924 (Gone to a better place)
1974 Lotus 7 S4 "Big Valve" Twin-cam (waiting)
1982 924 (As featured on Wikipedia)

Learn to love your multimeter and may the search be with you

Peter_in_AU · Joined: 29 Jul 2001 Posts: 2743 Location: Sydney, Australia

curiosity got the better of me so I ripped the alternator out this afternoon.

No obvious problems, spins just fine.

Go to split the alt case and "hello what's this?". Out of the 4 bolts holding the alternator case together, 3 are missing and the last one is loose. That explains the smashed vanes on the cooling thingy the pulley is against.

That probably explains the whining, the case was twisting under the pressure of the belt. Don't see how that would cause the voltage drop.

Time to split the regulator case and check the diodes.
_________________
1979 924 (Gone to a better place)
1974 Lotus 7 S4 "Big Valve" Twin-cam (waiting)
1982 924 (As featured on Wikipedia)

Learn to love your multimeter and may the search be with you

924RACR · Posted: Mon Jan 27, 2003 10:54 pm Post subject:

Just replace the regulator outright, comes with new brushes too, instant new alt., fairly inexpensive. You should be able to get it at either an autoparts store or alternator/starter service shop.
_________________
Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype

Peter_in_AU · Joined: 29 Jul 2001 Posts: 2743 Location: Sydney, Australia

awww where's the challenge in that?

Actually you're right. The damn thing's potted so I can't get at the diodes without digging out a heap of old epoxy.
_________________
1979 924 (Gone to a better place)
1974 Lotus 7 S4 "Big Valve" Twin-cam (waiting)
1982 924 (As featured on Wikipedia)

Learn to love your multimeter and may the search be with you

geezergary · Joined: 07 Nov 2002 Posts: 80 Location: sask.ca

Peter - I've had that problem in other units and it was one of the diodes
blown open redoucing it from the 3phase bridge rectifier to something less
resulting in your 12.5 vdc out instead of 13.5+. The way to check that
( I read someware ) is- with the engine runnig turn on your radio to an
empty space between stations and you will hear that blown diode in the
speaker. The howl will rize and fall with the rpm ! Then its time to look inside but I see you're there already. My 2cents worth.
Hope you clear of the fires, We hit -36c yesterday( -30 f )here in
mid western canada. It's warming up today , yay.

Even I love your multimeter!
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Any fool can say 'it won't work"

Peter_in_AU · Joined: 29 Jul 2001 Posts: 2743 Location: Sydney, Australia

I never thought of using the radio as an alternator diagnostic tool, almost makes me want to put everything back in and see if the idea works.

The temperatures have been a bit cooler here is Sydney and there's even been a few spots of rain so it looks like the fires are less of a threat for us now. Our house backs on to a large area of bush so summer is a bit of an anxious time.
_________________
1979 924 (Gone to a better place)
1974 Lotus 7 S4 "Big Valve" Twin-cam (waiting)
1982 924 (As featured on Wikipedia)

Learn to love your multimeter and may the search be with you

Zuffen · Posted: Tue Jan 28, 2003 8:47 am Post subject:

"Learn to love your multimeter"

Is that a typo and it supposed to read

"Learn to love your multipeter"

?
_________________
Bob Dodd - 924turbo@cox.net
931 1982, 944 1982 euro, 924S 1988SE, 93 968 tip 06 Silver Cayenne S, 06 Black Cayenne S

I have Way too many cars, parts for the 931,944 and 951

numbbers · Posted: Tue Jan 28, 2003 12:18 pm Post subject:

Gezzer has got it. You are missing one leg on your rectifier. Us old analog types can teach you digital wiz's a few things.
_________________
1980 924 Turbo

Joes924Racer · Posted: Tue Jan 28, 2003 12:32 pm Post subject:

hhhmmm my rectifier went out a few months ago because of a bad battery
I was able to have it replaced for frr though as the alt. was on a warrenty cool huh.
_________________
1979 porsche 924 Na
1980 porsche Turbo 931GT Replica
Have u ever driven a turbo.

Peter_in_AU · Joined: 29 Jul 2001 Posts: 2743 Location: Sydney, Australia

numbbers, good to know that you "old analog types" are still out there

My first reaction to a control problem is the throw a $10 microcontroller at it when a $1 comparator and a couple of resistors would do just as well.
_________________
1979 924 (Gone to a better place)
1974 Lotus 7 S4 "Big Valve" Twin-cam (waiting)
1982 924 (As featured on Wikipedia)

Learn to love your multimeter and may the search be with you