Tach bouncing..engine quits.

[wwest]

New alternator and rebuilt CDI solved the problems but what was the actual causative factor(s)...??"

1978 Targa, ~90,000 miles.

Symptoms.

1. ~2004, Engine would just up and die, but could be restarted.

Installed new Costco battery.

2. 2004 and on, e-brake light stays on, disconnect battery to clear.

Long period of not being driven very often, or very much.

Rare 1, 2 events.

3. 2008, car loaned to nephew living in McCall Id.

Nephew picked up the Targa, drive to ~50 miles south of Moscow, ID, car quits, DEAD. Flat-bedded back to Moscow Sunday evening, to be worked on AM. Based upon nephew description of symptoms I concluded that the battery was being overcharged. Convinced my nephew to buy a new battery and continue on to McCall. Procedure: Unplug the regulator, drive until the engine begins missing, plug the regulator back in, drive until comfortable the battery is fully charged, repeat as required. 2 cycles got him home to McCall.

He installed a new regulator, eveything fine........seemingly

Until come spring in McCall. engine quits randomly..then DEAD. left across town in friend's driveway.

Wife and I arrive on the scene.

Once the engine quits the ignition key must be switched completely off, count to 10 slowly, it will restart..tach bouncing throughout. Obvious something is latching up due to teh system overvoltage...my conclusion was that it was the rev-limiter latching up as a result of a random tach HIGH bounce.

Remove the voltage sustaining the latch-up and the car restarts.

Strange, the car will start and drive....until a bump in the road (I swear..)

Called Squire, he insists, STOP vacillating and replace the alternator.

Ordered a rebuild from Pelican and installed. Primary symptoms gone. But car will now not run for long periods, 10-15 miles, and cannot be restarted. Discovered that a bit of cool spray on the CDI and the car would restart immediately.

Shipped the CDI to Loren for repair and nephew installed.

Problem(s) SOLVED.

But why..???

The tach signal is derived from the CDI internally, in the input circuit from the distributor magnetic pickup coil. Even if the SCR/Thyristor is breaking down and provided a multiple, group, firing sequence that should not reflect back into the input trigger circuit. Plus which that would not be a normal failure symptom for an SCR in a CDI type circuit.

So just what would cause multiple, a grouping, of firing pulses, enough so as to seemingly over-rev the engine, NOT, no way, actual over-revving.

Having seen the internals, stator windings, BURNED wire insolators, of the old alternator gave me the best clue. Bumps in the road would provide enough vibration that those stator winding would short out. That, in turn, resulted in enough magnetic fields, PULSES of magnetism, that coupled into the nearby distributor magnetic pickup.

My conclusions are: 1.) The tach bouncing "itself" is the result of the alternator internal shorting, caused mostly by alternator vibration. 2.) The e-brake and rev-limiter latchup is the result of over-charging the battery. The heat sensitivity of the CDI probably also relates directly to the battery overcharging.

Any ideas, thoughts?

[Ronnie's.930]

I think you could have saved yourself a lot of aggravating parts-swapping by simply mounting a couple of SPAL fans aimed at the alternator and CDI box and called it a day!

[stormcrow]

I had a similar problem with my 78 911 SC Targa - the car would start up fine, run fine.

However, when driving on the highway it would just quit. Turn the key to the off position, push in the clutch and engine would start -sometimes backfire through the exhaust pipe.

Well this went on for quite some time - wouldn't do it all the time but driving with the idea that it could happen anytime was nerve racking.

Several months ago I decided it was time to do a tune up. Went to NAPA and bought a Bosch distributor cap and rotor along with standard plugs.

I pull off the old distributor cap and rotor, and inspected both, usually looking at the terminals to see how burned they are - Sometimes I will save them as a spare - just in case. What I noticed was that the terminals had very little corrosion on them - just some minor corrosion along the bottom edge. I also inspected the rotor and noticed the same thing - just the very leading edge of the rotor was corroded. I had driven the P-car around 6,000 miles.

Usually the terminals in the distributor cap have about 1/8" or more of corrosion along the bottom edge, and the rotor tip is usually corroded all the way across the front. Just out of curosity,

I decided to compare both the new rotor and cap and the old rotor and cap to see if there was any difference between the two, and what I found sure in hell surprised me.

The terminals that protrude inside the old distributor cap were shorter then the terminals on the new distributor cap by about 1/8" or more. The rotors were identical in length and there weren't any noticable differences.

After installing the new distributor cap, rotor and plugs and have driven the P-car around 600 miles, to my surprise I never had another problem with the engine quitting.

Looking back, it is apparent that the problem was caused by the terminals on the inside of the old distributor cap being too short - they weren't making proper contact with the rotor and as a result were causing the engine to shut down. I am also certain that the old rotor and distributor cap was not a "bosch" and was purchased from one of the major auto parts companies.

So in the future I will definitely make sure that I do purchase Bosch and inspect the terminals to be certain they are correct.

Hope this info helps - It sure did help me in determining the problem. And I am certain it save me from having my CDI unit rebuilt.