1989 Turbo

[AL J.]

I have an 1989 turbo cab. The car will shut off for no apparent reason. I have replaced the CDI unit, coil, fuel pumps and the problem still exists. First indication that the car will stall out is the tack starts to jump, car shuts down, after 5 to 10 minutes I can restart and the car is okay for the rest of the day.

[Loren]

:welcome:

Have you checked the grounds - especially engine/transmission to chassis?

[magcuda]

the tach jumping in older 911s is a sign of a charging system problem

it could be over charging to the point where it shuts it down

you need to rig a volt meter to see what is going on when the tach starts jumping

[wwest]

On my '78 the tach jumping, and then later the engine overspeed circuit latching up was the result of a failing alternator. Turn the key completely off long enough to remove the overspeed latchup and the car would restart and run for....unpredictable....

New alternator w/regulator built in and the rebuilt CDI ignition seems to have solved the problem.

The factory original CDI ignition and over-rev protection circuit both seem to be overly sensible to over-voltages. The tach signal comes from the CDI so when it starts misfiring(***) due to the over-voltage the tach jumps and eventually the rev limiter latches up.

With the battery all the way up in the front and the alternator/regulator clear in the back then both the battery feed + wiring and the ground return wiring front to back become EXTREMELY critical. Voltage spikes, UPWARD voltage spikes, due to intermittent internal short circuits within the alternator can destroy CDI system in very short order.

When I disassembled the factory original alternator I could not believe it had been operational, every winding showed signs of (frame) shorting and overheating.

*** With the supply voltage well above 12 volts the CDI's internal voltage DC-DC upconverter's output voltage goes well above the ~250 volt "standard" causing the 300 volt rated SCR to break down and "fire" spontaneously.

Edited May 2, 2010 by wwest