Bouncing Tachometer

[Rich P]

I'm after some help with my tacho. I've read various threads about this but am still unclear.
I have a 1974 Targa. I recently did an ignition tune up and changed the plugs/ plug wires/ distributer cap and arm/ and a new MSD blaster coil. the only part of the maintenance kit I didn't use was the new points I also did a service with fuel filter/ fuel accumulator (due to a hot start issue)/ oil filter and oil change. I used Wayne Dempsey's excellent 101 projects book for this work.

Everything fired up well on the first few runs but the Tachometer has developed wild swinging during acceleration. At idle it will settle at around the 1000rpm mark.

This was fine prior to me carrying out this work. I can't figure out why. I've checked around the engine where I was working and can't see anything loose.

It would seem that something must have happened during this work- it would be a bit coincidental if the alternator had suddenly failed 10 miles after this work. I'm a little suspicious of the new coil- it's the only part that has been switched in (I did ask at Pelican if it was compatible prior to purchase). Could this new coil have fried something/ caused an issue. I put the old coil back and the problem is still there.

Any thoughts would be gratefully accepted.

Thanks

[GH85Carrera]

The new Bosch coils right from a dealer for my 85 911 are junk. I don't know if that hold true for the older coils.

Again on my car the tach bouncing around a lot is often a alternator issue. Find a good 12 volt voltage gauge and see it it shows a solid charge or 13 to 13.5 volts but not past 14. On my car the tach bouncing was indication that the voltage was way too high. I finally broke down and but a voltage gauge in my 911. Mine is a custom tachometer with a volt gauge in the bottom of it.

I am sorry I can't help more with your 74 model.

[Paulporsche]

I have points on my engine. I get a tach variation if the points are bad or otherwise out of spec.

You may want to install those new ones after all! Or at least check the dwell on the existing ones.

[gazzerr]

Mine starting doing this when my alternator regulator failed. Too much voltage like GH85Carerra said. You need to check the voltage across the battery terminals when the car is running. Too much will overcharge the battery and cause it to fail too.

[SCadaddle]

82 SC: I had the "twitchy" tach at a steady idle. Maybe a "twitch" of 200-300 rpm. Then I started hearing a crunching noise that was determined to be the distributor after putting my hand on it while the engine was running. The crunch and the vibration were in sync. Took the distributor cap off and found I could lift the rotor about 1/4". Swapped it out with a known good distributor and noise and twitchy tach both disappeared.

[Sunroof]

On my 1973.5T/CIS with points, whenever I get a tach bounce, I reset the point gap and that takes care of the problem.

As for the coil? I used the MSD blaster after trying out that silver piece of junk Bosch unit made in Brazil. I had two fail on me. I got rid of the Blaster after I found a Porsche parts vendor who manufactures the black Bosch coil that has worked out great.

[bkreigsr]

The tach is probably at the age when the circuitry is due for replacement or a refresh.
Could be coincidental that it happened when you did the work.
Make sure your point gap is exactly to spec.
Bill K

[diy83sc]

I had this happen to me a few weeks ago. In my case, the lower inner latch on my distributor wasn't locked on, letting the cap float. The rotors swirling was enough to make the tach bounce. It's a quick and easy check.

[kevbo]

I've been experiencing this issue since I fixed my speedometer. I'll check my distributor as well. My tach stops working entirely when I turn on my headlights. It won't start working again until I power down the car and restart it. Any other hints? - I was thinking a short since sometimes it starts when I hit a bump on the road.

[gomezoneill]

Originally Posted by kevbo

I've been experiencing this issue since I fixed my speedometer. I'll check my distributor as well. My tach stops working entirely when I turn on my headlights. It won't start working again until I power down the car and restart it. Any other hints? - I was thinking a short since sometimes it starts when I hit a bump on the road.

Check your ground strap under the engine.

[gomezoneill]

Originally Posted by paulporsche

i have points on my engine. I get a tach variation if the points are bad or otherwise out of spec.

You may want to install those new ones after all! Or at least check the dwell on the existing ones.

this^^^^^