Will Pertronix points send the correct signal to the RPM Transducer on a MFI engine?

[skinnerd]

Just built a 2.7 MFI engine.
All stock except I used Pertronix magnetic points in the distributor plus went with the Parts Klassik solid state CDI box and accompanying coil.
The engine runs fine and makes good power (230hp on Rothsport dyno).
The fuel cutoff circuit does not appear to be working on deacceleration.

Is there anything in the above that would prevent the correct signal from getting to the RPM transducer?
When I rev the engine to about 3000rpm and depress the micro switch....nothing happens.
I believe the engine revs should drop to 1300 rpm and then oscillate between 1300 and 1500 rpm (approx) if all is well.
Not seeing that.

Power to the fuel cutoff solenoid definitely generates an audible click so I believe that's working.
I've got a new micro switch and it works fine.
Also have a couple of new RPM transducers, so it's hard to believe both are not working...guess anything is possible.

I'm going to put a couple of test lights on various parts of the system to see if I'm getting 12V out of the RPM Transducer at some rpm above 1500.
If no output signal there, I know that either the transducer(s) are faulty or the rpm signal coming from the Pertronix to the transducer is not correct.
If I do get 12V out from the RPM Transducer, I'll follow it through the micro switch and to the shut off solenoid.

Any ideas I'm missing?

How do you test for the rpm signal at pin 4 of the RPM Transducer?
This is the same as the black output on the Pertronix.
Is there special equipment needed for this?

Thanks for the help.

[gled49]

Years ago we bought a pile of RPM T’s from Emory, they all had broken legs on the transistors, none of them worked. Open them up and look. Smith bought a bag of transistors from Radio Shak and fixed them.

[skinnerd]

Years ago we bought a pile of RPM T’s from Emory, they all had broken legs on the transistors, none of them worked. Open them up and look. Smith bought a bag of transistors from Radio Shak and fixed them.

Already opened up the one NOS RPM Transducer I have....all the legs look fine.
The other RPM Transducer I have is a new style that's all solid state....no more transistors sticking up on the long, fragile legs anymore.
See pic below:



So the fuel cutoff circuit doesn't work with either transducer installed on the relay board.
This engine is in a 914-6, so you're dealing with that engine bay relay board that the 911's don't have.
I went through the relay board once before installing it into the car and found two of the traces with opens.
I had those repaired (re-soldered) and checked out everything with my multimeter after the repairs.
All the traces on the 914-6 relay board were good.

It's possible I may still have another issue with the 914-6 relay board.
I checked for 12V at one of the inputs to the transducer socket with the key on....had 12v there just as it should.
Also the ground pin on the transducer was indeed hooked up to chassis ground.
The only 2 other pins in the transducer socket are the rpm signal in from the Pertronix...the same signal that is sent to the tach (which is working fine).
The other socket pin is the 12V signal out that goes to the micro switch (engine rpm > 1500) and then to the fuel cutoff solenoid on the rear of the MFI pump.

Again, I'll check to see if I'm getting the 12V signal out from the RPM Transducer at the correct time (above 1500 rpm).
If it's not there, then it's either the transducer (2 new ones of different types both failing....unlikely) or the rpm signal into the transducer is either not
there or not the correct type of signal.
That's why I'm asking about the Pertronix and PartsKlassik CDI/coil I have.
Wondering if either of those changes the rpm signal is some strange way such that the RPM Transducer does not recognize it anymore?

[skinnerd]

Anyone on this board that knows of anyone who can check these RPM Transducers for functionality? or can repair them?

[Bill Simmeth]

Anyone on this board that knows of anyone who can check these RPM Transducers for functionality? or can repair them?

Don't know if he does, but you might check with Bob Ashlock at ashlocktech.com He rebuilds CDI boxes and also offers the Tach-Adapt product (which might be the shortest path to your solution).

[Ratech]

Hi all, yes I do test/repair those little RPM transducers. The most common problem, as was stated earlier in the thread, is the transistor leads "rot" away and the parts fall off the pc-board. There are a few other quirky problems that pop up also, most of them being mechanical in nature. The RPM signal seen by the transducer should not be affected by the Pertronix-1 unit, as commonly used to eliminate the points. Thanks, bob@ashlocktech.com

[frederik]

I repaired mine myself a couple of months ago, it was not difficult. See this thread for more information: http://www.early911sregistry.org/for...nsducer-repair

There are also a couple of threads on Pelican with lots of information. As for testing, you would need a power supply and a test signal generator. I just hooked it up to the car. Check that it's providing 0 V to the stationary switch when the engine is not running or idle. Then run it up to 2000 RPM and it should provide 12 V. For a final test, activate the switch manually when the engine is running at 2000 rpm: this should cause the engine to cycle between 1000 and 2000 RPM every second or so.

Before the repair, my RPM transducer was always outputting 12 V so the switch was disconnected.

[skinnerd]

Updates....

With the new micro switch, correcting some faulty fuel cutoff circuit wiring through the micro switch, and a new solid state RPM transducer.....
It all finally works.
When you rev the engine to about 2000 rpm and depress the micro switch, the engine will hunt in-between 1000 and 1600rpm as it's supposed to.

So the moral of story is that even though you may have an NOS part, the fact that a lot of these NOS parts are now 30, 40 or more years old, NOS
doesn't guarantee function.
The new solid state speed switch should be eons more reliable as there are none of those fragile transistors with the long legs anymore.
I bought mine off of Ebay UK for about $125.

I'll still be sending the faulty old school RPM transducer out to Bob at Ashlocktech for testing and repair, and will be put it on the shelf as a spare.
Thanks to all for your help.

[sithot]

FWIW: Bob Ashlock makes a tach adapter that works with just about any setup. http://www.ashlocktech.com

[skinnerd]

FWIW: Bob Ashlock makes a tach adapter that works with just about any setup. http://www.ashlocktech.com

Tach is working fine....always did.
It was the MFI fuel cutoff circuit I was having issues with.
Bob tests the speed switches for free and repairs them if needed for a very reasonable cost.