Need help diagnosing random shut down.

chrisp · 04-28-2008, 05:31 AM

The CDI box on my '83 SC race car is randomly shutting down. I swapped with another CDI unit and the same thing happens. So it's not the box itself. I know the box is shutting off because the humming noise is missing. If I stall the car with the key on I hear the humming noise. When it mysteriously shuts down, the humming is non-existent.

It will usually shut down within a couple of minutes after starting (while idling). It also shuts down on-track in both left and right hand corners. It also shuts off on the straights. It's a little hairy when the car shuts down mid-corner.

I recently took the tank out of the car to install a new front sway bar and although it's been happening since I don't think it's related. I can smell fuel as I am cranking the engine over.

What's the list of things I should check...things that are up stream of the CDI box that would cause it to shut down?

equality72521 · 04-28-2008, 05:38 AM

Sounds like an intermittant bad electrical connection if the CDI doesn't whine when the shutdown occurs. Check all connections in the CDI circuit.

chrisp · 04-28-2008, 05:57 AM

check how? I've installed and reinstalled the plug and checked the wires leading into the plug.

equality72521 · 04-28-2008, 06:01 AM

First, check to make sure every connection in the CDI circuit is clean. Power wires, ground wires,... Also make sure the wire is not broken where it connects to each pin in the connecter. Very tedious but necessary. You will need a schematic so you can figure out the CDI circuit. It's not just the wires in the plug at the CDI, it's every wire that is in the circuit.

Flieger · Flieger's Garage

Fuses. Those little so-and-sos get corroded easily and quickly. Rotate/tighten/clean them along with the battery connections.

chrisp · 04-29-2008, 04:57 AM

That brings up a good point. Bentley refers to a fuse for the CDI box in the #22 location. My panel (and the labeling inside the panel cover) only goes up to #21. Where is the CDI fuse located?

boyt911sc · boyt911sc's Garage

Quote:

Originally Posted by chrisp

That brings up a good point. Bentley refers to a fuse for the CDI box in the #22 location. My panel (and the labeling inside the panel cover) only goes up to #21. Where is the CDI fuse located?

Chris,

Your CDI gets power directly from the ignition switch when you turn it to RUN (teminal #15). The subsequent connections are the following: power supply terminal to fuses #12 and #11 (power does not travel via fuse), then to connector T2a. T2a splits into two (2), one line goes to S22 (heater blower relay for '78-'80) or S24 ('81-'83) and the other one goes to CDI (terminal #15). The three (3) fuses in the engine bay are designated as S22, S23, and S24 accordgingly.

Focus on three things:
1). Continous power to CDI @ terminal #15 @ RUN.
2). Continous power to fuses mentioned above and after connector T2a.
3). Uninterrupted ground connection from terminal 3/1 @ CDI.

Installing temporary low wattage pilot light/s at suspect point/s will give you instant feedback when problem occurs or to replicate the anomaly. Keep us posted. Good luck.

Tony

chrisp · 04-29-2008, 12:58 PM

thanks. this is very helpful.

hmmmm....maybe it's related to my ongoing issue of my key switch not shutting the car down. I tried a new switch last year and it didn't solve that problem. I wonder if there's a steady degradation of some part or connection and first it was the shut down issue and now the CDI problem has added itself to the list.