924Board.org

kc77 · Joined: 26 Nov 2003 Posts: 23 Location: Kansas City

My ornery '77 924 NA wouldn't start today. Yesterday it died 3 or 4 times on the road but now it will crank and ignite when you turn the key but it dies immediately. The driver's side ballast resistor (next to the ignition coil) is literally smoking and hot to the touch. I assume this means it has blown, but I'm not sure if it is the cause or effect. I installed a new, slightly more powerful starter very recently and replaced the alternator cable, could that have caused the problem if I messed something up? I can't find a replacement part anywhere; someone told me they don't make them anymore, even in Germany. (Of course that would be the case!) Does anyone have any ideas 1. Where to find a suitable replacement .4 and .6 ohm resistor that will fit, and 2. if there is an underlying problem that's causing the resistor to blow?

is anyone parting out a pre-78 924 that might have the resistors? Are my resistors even the problem in the first place?

any help is greatly appreciated.

Rasta Monsta · Joined: 12 Jul 2006 Posts: 11733 Location: PacNW

You can bypass the resistor and fire up the car briefly to see if it runs. As to them not being made anymore, I would wager that you could find a generic one with the appropriate specs at FLAPS.

rasta
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Toofah King Bad

WeiBe (1987 924S 2.5t) - 931 S3

!tom · Posted: Thu Mar 29, 2007 5:10 am Post subject:

Sounds to me like you have a short, which is causing the ballast resistor to overheat, a symptom of the real problem which is causing your car to not run. I can't see how replacing the starter or alternator cables could have caused a problem.

A blown resistor will be cold to the touch, since fried resistors tend to fail open.

Perhaps your coil is on its way out; it still works somewhat since it works when the ballast resistor is bypassed. Check its resistance ayway, the coil is the most likely cause of your problems.
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78 924 NA
5-lug

gohim · Joined: 02 Nov 2002 Posts: 4459 Location: Rialto, CA

If the ballast resistors are screwed up, it's usually because of a problem with the alternator wiring harness.

The alternator wiring harness includes the wiring from the battery, the alternator, the starter, and the balast resistors. I don't remember off-hand how it is wired, but what happens is battery acid runs out of the tray, and directly onto the harness, and under the insulating sleeve.

There, it attacks the wiring, and the wiring for the alternator and starter. You get a bad/high resistance connection at the starter, and that is where the ignition (ballast resistor) gets power for the ignition system. Low voltage due to damaged electrical connection at the starter causes a high current situation at the ballast resistor, which causes it to burn out.

Earlier cars have one ballast resistor for the starting circuit, and a second one for the running circuit. It is possible to burn up the running circuit resistor, so the car will start and run when the ignition key is in the "START" position, and die when the key is released to allow the ignition switch to drop into the "RUN" position.

So if you have a not ballast resistor, I suggest that you check the wiring at the starter for a bad connection.

Have you tried a Porsche Dealer for a replacement wiring harness and ballast resistor? Your local FLAPS is not going to have Porsche specific parts, and the local salvage yard 924 cars are probably going to have wiring harnesses in the same condition as yours.

The wiring alternator/starter wiring harness is not that complicated, and that is why they probably only cost $21 the last time I bought one from Porsche.

kc77 · Joined: 26 Nov 2003 Posts: 23 Location: Kansas City

kc77 · Joined: 26 Nov 2003 Posts: 23 Location: Kansas City

Now there's a short somewhere that wasn't there before. Whenever I put the battery cable back on the terminals it sparks real bad and the alternator starts to smoke. Who invented the electrics on this car?????

gohim · Joined: 02 Nov 2002 Posts: 4459 Location: Rialto, CA

You probably made some mistakes in the wiring when you "rigged" the alternator cable.

With the amount of power that runs through the wiring, any mistake would cause smoke.

Here are some common mistakes...

#1 Did you use new, same gauge (size) or larger wire for any wiring that you replaced? Alot of people figure that wire is wire. Wrong. The larger the wire, and the larger the number of strands in each wire, the more current (in amps) the wire can pass with generating excessive heat and burning.

#2 Did you replace the entire length of each wire that was repaired, or did you spliced? Each splice causes causes power loss, because each spice causes additional resistance to the flow of current.

#3 Did you solder where you spliced? Solderless crimp type connectors have a high power loss, and high resistance to the flow of current. This causes the connector itself to heat up, and possibly cause a fire if a high amount of amps is passed through the wire.