Help: Tach under reporting RPMs

[rgupta250]

Hi Everyone:

About to conclude an extensive bare metal restoration project and finishing up the few items.
One of the issues I am tackling with is that my Tachometer is under reporting the RPMs. It tends to do it at the higher RPM level from 4000 to 7000.

Here are the specs of the engine rebuild:
- 2.2S P/C with 9.5CR
- Bosch 2.2S distributor (rebuilt)
- rotor with 7300 rev limiter
- Webers 40IDAP
- Rebuilt CDI box by Bob Ashlock
- Bosch Ballast resistor (tested)

The Tach (original 71T tach) was rebuilt by North Hollywood Speedometer and retested when I brought it back suspecting that there was something wrong with the rebuild.

Any idea what could be wrong?

Thanks!
Ravi

[NorthernThrux]

I had an identical problem with a rebuilt distributor on my 2.4S despite Bob Ashlock rebuilding the CDI box and me checking the resistance and continuity of every wire in my car. I also had Bob put one of his clever circuit boards in my Tach to make it run properly and linearly. It worked fine on the bench, but not in the car.

Only when I switched to a 123Ignition did it go away. My feeling is that there is some high rpm points bounce on the distributor, which at the highest rpms actually felt like a soft rev limiter. Those rev limited rotors can be flakey too, but I replaced mine with a standard one and I still had the issue. Only replacing the distributor solved it (and make the car far nicer to drive TBH).

cheers,
Ravi

[rgupta250]

I have the same issue: tach runs fine on the bench.
I’ll look into the distributor to see if this is the issue.

Thank you! ��

[1QuickS]

My Palo Alto Speedo rebuilt was under-reporting, the trouble was a loose solder connection on the "clockwork" the needle was attached to. Repaired under warranty without issue.

[Richy]

'71T's do not have a ballast resistor , are you referring to the tach intermediate unit ?

[SFE]

When my specialist rebuilt my Tach he set it as a 4 cylinder instead of 6 by mistake...
But in my case my Tach was over reporting RPM.

Howmany wire are coming out of your distributor ?
If you have the intermediate unit (ballast) you should have only one wire, if you haven't, it's 2 wires.
That unit disappear with the 2.2 liters engines I think.

[Richy]

After replacing the tach , it has been verified that the tach itself is the problem .

[Richy]

SFE : '70 and early '71 911's with the intermediate unit have 2 wires , the second wire is for the intermediate unit , later 911's with no intermediate unit have one wire . I just studied several cars .

[SFE]

SFE : '70 and early '71 911's with the intermediate unit have 2 wires , the second wire is for the intermediate unit , later 911's with no intermediate unit have one wire . I just studied several cars .

As you can see 71 and 72 have 2 wires out of the distributor (#5) and no intermediate unit (or "switching SCR")





If you have an intermediate unit , the signal is coming from the coil, not the distributor.

On the 1969 wiring diagram (with intermediate unit) the distributor has 1 wire going to the CDS (HKZ).
On the 1970 wiring diagram (with intermediate unit) the distributor has 2 wires going to the CDS (HKZ) and the transducer (speed switch).

Some cars have a mix of those different wiring and that could be confusing.

[Richy]

SFE : Sorry , I was referring to the coil , not the distributor , so 2 differences in the ignition wiring between '70 and later cars .