911T 2.4 strange rev counter surging

[sfh3l]

I made the right call and went out in the 911 today, to be paid back by a sunny couple of hours and a great drive up to Warwickshire and back. One thing I noticed, while poddling along in traffic was something rather strange from the rev counter. When at approximately 2500 rpm in 4th on a very small throttle opening, the rev counter seemed to be gaining and losing perhaps as much as 300 rpm. I should add that the engine spead/note was not changing and I had not fallen asleep with my foot on the clutch or anything silly.

If I lift off and close thr throttle completley the reading seems to steady, as it does if I accelerate properly. Curious that the reading should appear to be throttle dependant?

My car runs Webers, standard CDS system with Pertronix 'implant' in the distributor and a standard Bosch coil.

Am I going mad or does anyone have any ideas? Only other rev counter issue I have observed in the month or so that I've had the little darling is that if you are accelerating up through the gears on the keen side, the counter does seem to swing about a bit violently - somewhat more than the true change in engine speed.

Thanks in advance for any ideas.

All the best, Sam.

[butzsi]

Check your output voltage from the alternator to the battery at the rev range that you notice the problem - it could be that the voltage regulator is faulty and allowing the voltage to rise to a higher level than it should be. You can also test this when driving by switching on a high current load like the heated rear screen or headlights and see if the rev counter reading stabilises.

Chris

[ross.mcw]

Hi Sam,

Did you get to the bottom of this problem, mine has exactly the same issue as you describe and I'm hoping to to get this sorted soon along with a load of other general 'get it back on the road' work over the next couple of months.

Cheers, Ross.

[RENNSPORT CLASSICS]

Straight from Pertonix technical USA.

Tachometer fixes for 12V signal style

If the cars tachometer is reading to a higher RPM then engine is running. Or the
tachometer works ok, to a certain RPM then stops or starts reading incorrectly.
We recommend installing a resister in line with the tachometer wire and coil
negative. This helps knock down the voltage spike/fire line on coil negative to
look more like a set of points. Allowing the tachometer to read the Ignitors signal
correctly.
Depending on the tachometer will depend on the resister needed. We have found
that a 10K ½ Watt resister fixes most problems and suggest starting with this
resister. If you find it doesn’t fix your problem you might need to use a larger
value. Like a 15K ½ watt resister or 20K ½ watt resister.
In some cases the resister might not work, so you can try a capacitor. We
recommend using a .01 Micorfarad 1000VDC capacitor or you can come down to a

Install resister or capacitor in the Tachometer line running from the back of the tach
to coil negative.

This was the reply I received for a similar problem. If yours was working correctly and has just started manifesting the problem I would suggest this is not the solution but it points to something else that has now changed. Good luck.

[ross.mcw]

Thanks for the extra info! I should have mentioned that I'm using the standard points set-up though.

Cheers, Ross.

[RENNSPORT CLASSICS]

OK you stated you had a Pertronix implant.

The only fault I have had with standard points is a bouncing tacho needle when the points close.

Good luck finding a solution.

[sfh3l]

Actually,in my original post I did say that I have Pertronix. Thank you so much for the tip on this. I shall investigate. The ignition behaves perfectly, with no outward signs of any running problems, so it may well be an unhappy interaction between the Pertronix and the old rev counter.

I wonder where would be the best place to get such a resistor though? It occurs to me that I have a spare ballast resistor from my old (pre Pertonix) set up on the BMW. I'll check the rating for that in the BMW manual as it might be in the range you suggest.

Thanks again and when I get my @rse in gear to do something about it I'll feed back the results.

[Chopperman]

At a bit of a tangent, I have a red light showing on my Rev counter as well as the flashing red without the hazard pulled-suspect the wires have been replaced incorrectly.
I have had the alternator serviced but the battery is still not charging, obviously suspecting the voltage regulator.
Does the rev counter get its signal via the voltage regulator or through the LT negative?
1972 911T Targa

[stuby]

I am pretty sure the rev counter feed s off the Lt side of the coil - (mine does anyway and appears to work )

Stu