Tach Bounce and Ignition Shut Down

[Davis Montham]

79 SC. I have a tach bounce at idle - 1000 to 1300 rpms - that shows up after the car warms up - after 5- 10 mins of driving - and it stays that way and always at idle, there's no bounce while driving. The motor does not surge and holds a steady idle. The car has stock ignition. When the car is fully warmed up (20-30 minutes) it has been randomly shutting off at idle and also while I'm driving (rpm needle drops to zero and it's dead) but will immediately start right back up as if nothing happened. Sometimes it will start to die and then catch itself and all is well again. I have borrowed and switched out the cd box, coil, ignition switch, and tach and I changed the voltage regulator (even though voltage always tested no more than 14v) - all to no avail. I have checked the grounds and 14 pin connector as well. I am at the end of my rope. Any ideas are appreciated. Thanks.

[al lkosmal]

I would check the BLK/VIO tach signal wire that runs from the CDI unit to the tach. It may be loose and/or grounding. Check the insulation and connections and if you have a multi-meter, I would disconnect this wire from the CDI and the tach to check continuity from one end to the other and also see if it is shorted to ground. This problem sounds like it's intermittent, so shake/wiggle the wire and connections to see if you can recreate the fault condition.

Good luck,
Al

[wwest]

You need to test the voltage in the engine compartment. When the tach bounces try switching on as much load as possible, headlights, etc, and see if doesn't clear up.

[Davis Montham]

Check continuity on tach wire from 6 pin to tach and it is good. I found a frayed wire that goes into the 6 pin from the distributor I believe - its the wire for the 31d slot on the cd box and I cleaned it up but no difference. Tried the "all accessories on" and it did not change tach flutter. Car died tonight and instead of a rapid shut off it was like the power slowly bled off until I was hardly moving and then it died. Hopefully that will trigger an idea with someone. Thanks for the help.

[porterdog]

When my Jetta did that the garage determined that the wiring to the hall sensor in the distributor was intermittent. I don't know if that early of an SC has a similar setup but it may be worth a look.

$0.02

[targa80]

I have been away from this forum for at least 8 years so I have not replied to any posts in quite a while. If my memory serves me well you have an issue with your fuel injection causing the tachometer to bounce at idle once it is warmed up. The engine RPM's will change and if it drops too low the car stalls. This is directly related to the CO2 sensor in the exhaust pipe reading a rich mixture and leaning out then reading a lean mixture and adding fuel. The reason this happens once the car is warmed up is because during cold start the fuel CO2 system is in a default state and sends a specific amount of fuel and ignores the CO2 sensor. There are old posts with ways to correct this issue.

[targa80]

Davis,

This is from an old post I responded to in the archives and is specific to the Kjetronics CIS with Lambda. Since your vehicle is a 1979 your system would have a different controlling device.

This can also be a problem with the mixture control. Once warm the idle can surge high and then drop low to the point of stalling. Warm the engine and disconnect the O2 sensor plug and see if idle settles out. If it does either your O2 sensor is bad or you need to readjust the fuel /air mixture.
Recently I posted info on the sequence of the cold start to warm engine theory.

Here is the theory of the Kjetronics with lambda operation from cold start to warm operation. It might help you in figuring out your problem. (The Kjetronics with Lambda was first installed in the 1980 model)

The Lambda (O2) sensor requires high temperature to operate properly. With the O2 sensor connected and the engine is cold the Kjetronics with Lambda system operates in the open-loop mode. The lambda control unit does not have a valid signal from the O2 sensor at cold start To correct drivability problems at cold start a lambda thermoswitch is installed to give the lambda control unit an additional input and insure proper fuel metering during warm-up. When the engine is cold, the thermoswitch is closed. the lambda control unit sends a fixed, slightly-rich duty cycle signal of 60% to the lamba control valve (frequency valve). when the thermoswitch warms enough to open, the control unit sends a fixed middle signal, 50% duty cycle to the frequency valve. When the lamba (O2) sensor reaches operating temperature so its signals are valid, the control unit switches to closed-loop operation. In closed-loop oeration the duty cycle signals are constantly changing cycling back and forth from 45% and 55%. so the air-fuel ratio is maintained near stoichiometric; lambda=1.

So disconnecting the O2 sensor at a cold start would do nothing to effect the running of the engine. A bad thermoswitch would effect the running of a cold engine since it would not provide the necessary signal for the control unit to run at a 60% duty cycle. Instead the control unit would put out a 50% duty cycle.

When the engine is warm and the O2 sensor is disconnected (open-loop) the contol unit would put out a constant 50% duty cycle.

The thermoswitch is something you can check for proper operation with a meter.
__________________
Pat Henry
Targa80
1980SC Targa (Mocha Brown)

[DaddyGlenn]

I had the exact same symptoms on my 78SC. Classic CDI box failure. Replace the box. I went with a MSD unit and matching coil. Problem solved.

[wwest]

I am of the rather firm opinion that the reason the CDI "box" fails is because one of the solid state components cannot survive long term operation at elevated supply voltages, >15 VDC.

With my '78 I am quite certain that the base cause of the tach bouncing was intermittent shorting of the alternator stator winding(s). The engine suddenly stopping most definitely correlated to a roadway bump.

[timmy2]

Targa80, no O2 sensor on a '79...

[DaddyGlenn]

Originally Posted by DaddyGlenn

I had the exact same symptoms on my 78SC. Classic CDI box failure. Replace the box. I went with a MSD unit and matching coil. Problem solved.

Crap! Mixed my issues .... The tach bounce was alternator/voltage regulator. The random dieing was the CDI box.

Sorry.

[targa80]

Originally Posted by timmy2

Targa80, no O2 sensor on a '79...

Timmy2, Thanks for the correction, I would not want to be giving out bad advice.