Another Bouncing Tach - but problem finally solved

[74-911]

Background: couple of weeks ago the dreaded tach bounce started.

Symptoms: would start after driving for a few minutes, the more I drove, the more frequent and wild the swing of the bounce. When really warmed up the engine would occasionally cut-out and die, particularly on hard acceleration but sometimes just driving down the hiway or pulling away from a stop sign, etc. All other electrics (radio, dash lights,etc.) still on and it would immediately restart when I let the clutch out - did not have to turn the ignition off and back on..

The usual suspects: No indication there was any problem with the charging system. The alternator rebuilt less than a year ago and charge voltage at the battery was as it should be at idle and 2000 RPM. I cleaned and tightened every ignition ground and connection I could find and put on a new transmission/body ground strap.,,, nada

Today I removed the 13 year old Pertronix ignitor from the distributor, got the old set of points out of the glove box and installed them. Started right up and after a spirited 20 minute drive no tach bounce, no engine misfires or dying.

I know some people dislike the Pertronix but after 13 years of flawless operation and never having to screw around with points, I plan on ordering another Ignitor from our host. I'll put the old points back in the glove compartment - just in case.

[wwest]

Wow, THX!

You may have just proved, made, my point.

For quite some time I have been of the belief that the bouncing tach symptom/problem results from some type of electrical noise, EMI/RFI, being coupled into the magnetic pickup used with factory CDI systems.

You might want to put off ordering that new ignitor.

I the case of my '78 the stator wiring insulators inside my factory original alternator were toast, probably resulting in intermittent electrical shorts and huge levels of EMI/RFI.

[wwest]

Originally Posted by 74-911

Background: couple of weeks ago the dreaded tach bounce started.

Symptoms: would start after driving for a few minutes, the more I drove, the more frequent and wild the swing of the bounce. When really warmed up the engine would occasionally cut-out and die, particularly on hard acceleration but sometimes just driving down the hiway or pulling away from a stop sign, etc. All other electrics (radio, dash lights,etc.) still on and it would immediately restart when I let the clutch out - did not have to turn the ignition off and back on..

The usual suspects: No indication there was any problem with the charging system. The alternator rebuilt less than a year ago and charge voltage at the battery was as it should be at idle and 2000 RPM. I cleaned and tightened every ignition ground and connection I could find and put on a new transmission/body ground strap.,,, nada

Today I removed the 13 year old Pertronix ignitor from the distributor, got the old set of points out of the glove box and installed them. Started right up and after a spirited 20 minute drive no tach bounce, no engine misfires or dying.

I know some people dislike the Pertronix but after 13 years of flawless operation and never having to screw around with points, I plan on ordering another Ignitor from our host. I'll put the old points back in the glove compartment - just in case.

Your symptoms fit, perfectly, what happened with my '78.


Once the discovery was made that the battery was being overcharged my nephew drove the car the remaining distance from Moscow Id to McCall by alternately removing the VR (factory, external), drive ~50 miles with all electrical loads off, reconnect the VR, full electrical loads, drive until...


You should smelled boiling electrolyte in any case....

[74-911]

Originally Posted by wwest

Your symptoms fit, perfectly, what happened with my '78.

Once the discovery was made that the battery was being overcharged my nephew drove the car the remaining distance from Moscow Id to McCall by alternately removing the VR (factory, external), drive ~50 miles with all electrical loads off, reconnect the VR, full electrical loads, drive until...

You should smelled boiling electrolyte in any case....

Not sure I am following your logic. I had no charging issues at all that I could ascertain. Just the bouncing tack and engine dying and I felt it was definitely temperature related. Apparently the ignitor was starting to fail when it got hot causing the tach to bounce and the engine to occasionally just die all of a sudden.

[wwest]

With the alternator producing an excessive charging level, does a battery reflect that as a voltage rise or simply act as a huge zener diode until a goody portion of the electrolyte has boiled off, away?

A pot of boiling water stays at 212dF until the water has boiled off.

[LJ851]

Originally Posted by wwest

With the alternator producing an excessive charging level, does a battery reflect that as a voltage rise or simply act as a huge zener diode until a goody portion of the electrolyte has boiled off, away?

A pot of boiling water stays at 212dF until the water has boiled off.

When diagnosing what i thought was a failing voltage regulator on my 85 carrera, i hooked a volt meter to keep an eye on battery voltage levels. I saw 17 volts on the gauge going down the highway...

[tctnd]

No, the battery does not stabilize or limit the charging voltage and the boiling water analogy does not apply, nor does this have anything to do with the original post.

[wwest]

Originally Posted by tctnd

No, the battery does not stabilize or limit the charging voltage and the boiling water analogy does not apply, nor does this have anything to do with the original post.

Strange....

I just took my multi-mode battery charger and charged the battery in my '78 Targa to 100% SOC, 13.16 volts.

Then I put the charger in "start aid" mode, 60 Amps, 14.87 volts after the entire 2 minutes the charger allowed.

Voltage measurements with Fluke model 75 DMM.

Are we sure that the electrolyte doesn't have to boil of at least partially before 60 Amps will cause an extreme voltage rise?

[kodioneill]

Glad you found the problem. Don't bother with the pertonix they're junk. The points will last 30,000 miles+or-.