924Board.org

[seanski44]

hi all,

Quick query - I have had a problem or two recently, (posted here) concerning electrics, first a burned out loom, the front harness to the alternator and starter motor (replaced with a brand new part) and second in the earths from the cold start gear, causing the car to be permanently 'on choke'

Anyway, the background to one side, now those probs are fixed, the car runs fine (and Volts at about 13V constant), but on the last few drives the voltmeter was constantly about 11V. Occasionally it'll go above 12V and behave erratically. Car still seems to run OK, but lights slightly dimmed. Revving engine doesn't seem to increase Volts. Alternator belt is only a thousand miles old. Turning on indicators makes the Volt meter 'tick' in time with them in the same way as it used to, just lower down the scale!

My q, is this indicative of a dying alternator? or could it be more bad earths?

What's the easiest way to check.

TIA

Sean
_________________
1998 BMW Z3 2.8 arctic silver & red

1981 168HP 931 S2 blue/tan leather - SOLD
1985 121HP 924 2.0l na modded - SOLD!
1992 968 lux SOLD
1989 944 2.7 lux SOLD

[Ozzie]

Make sure you have a good earth from the alternator to the motor and from the battery to the motor.
Another common problem is the brushes in the back of the alternator wear out. This is a black box screwed onto the back of the alt and holds the brushes and is a common bosch part found anywhere.
_________________
Porsche 924 1984 (UK import) NA
Its AUTO and its BLACK
Montego Black on black/red
Engineer of Electro/Mechanical Systems Maintenance

[Peter_in_AU]

there's been a lot written about the voltmeter.

Basically, the way Porsche wired it it's really just a bit of eye-candy after all these years. The problem is that all it reads is the voltage in the wires connected to the surrounding instruments.

If you want an accurate reading you need to run some wires from the voltmeter to a chunky switched cable electrically close to the battery. Have a look at the current-flow diagram and you'll see what I mean.

The voltmeter dancing in time with the indicators is a standard feature that stops you getting bored while waiting to turn. Remember these cars were made before electro-luminscent CD head-units.

The dimmed lights are probably an earthing issue.
_________________
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

[seanski44]

[seanski44]

[Ozzie]

Why would the earth be perfect. The only wire in that loom that went to the alt. was the red +.
On the alt itself is a large braided wire connected to the block or should be.

I think you would have a hard time gettting to the back of the alt without removing it.
_________________
Porsche 924 1984 (UK import) NA
Its AUTO and its BLACK
Montego Black on black/red
Engineer of Electro/Mechanical Systems Maintenance

[D Hook]

Have you read the battery voltage at the battery with a DVM? What's that read?

Clean every ground you can find, especially above the fuse panel. Unplug and replug everything to scrape off any corrosion on the gauges, including the fuel/temp gauge.

You'll be amazed how much better the car will run, too.
_________________
'80 924 n/a SOLD

[seanski44]

[D Hook]

You might want to buy an inexpensive one that is meant to read DC volts and has leads on it. It's probably a good tool to have for any owner of these cars.

Also check the connections on the firewall that are part of the mounting bracket for the coil. There's usually a ground that hooks there, too.

Clean every ground you can find, shine them up so you get a good solid connection. Then the plugs to all the gauges. Condensation in the cabin from water leaks will cause corrosion on them and the connections will start to fail.

Good luck!
_________________
'80 924 n/a SOLD

[Ozzie]

I've heard bicarb/soda works well on copper corrosion.
_________________
Porsche 924 1984 (UK import) NA
Its AUTO and its BLACK
Montego Black on black/red
Engineer of Electro/Mechanical Systems Maintenance

[gohim]

Run a 12 gauge or larger stranded copper wire (some people use battery cable) from the alternator case to the engine block, and then, another piece of the same cable from the engine block to the chassis. Soldered terminals on the cable ends will give you the best electrical connection. Clean the mounting hardware you use, and spray an anti-corrosive chemical (made for use on battery terminals) on the hardware before assembling, and again over the top when the connections have been secured. Make sure that the battery cable that connects from the battery to the chassis is in excellent, uncorroded condition.

The little black plastic box on the back of the alternator that contains the brushes is actually the voltage regulator for the alternator. Sometimes they go bad, and a replacement is not included with rebuilt alternators. You buy them separately, so you could try buying a new one and installing it to see if it cures your problem. The brushes that are part of the voltage regulator are not replaceable. When the brushes wear out, the voltage regulator needs to be replaced. If you suspect that the voltage regulator is bad, you could try getting a complete alternator from a salvage yard. At the local Pick A Part salvage yards, you can buy a complete alternator for less than the cost of a voltage regulator at the FLAPS.

[Khal]

[Ozzie]

The volt regulator/brush gear only cost about $30au last time I bought one.
Was a while ago though.
_________________
Porsche 924 1984 (UK import) NA
Its AUTO and its BLACK
Montego Black on black/red
Engineer of Electro/Mechanical Systems Maintenance

[gohim]

Complete used alternators are $20 at Pick A Part yards here.

[Ozzie]

Bugger- lucky to get away with $50 here. and then you'd still have to replace the bearings or brushes
_________________
Porsche 924 1984 (UK import) NA
Its AUTO and its BLACK
Montego Black on black/red
Engineer of Electro/Mechanical Systems Maintenance