Which switch is the RPM transducer?

[Eddyvdw]

Sorry, stupid question, which of the switches is the REV transducer back in my 2.4T 72? Micro-switch ok, stop solenoid ok, no signal of rev transducer at 2000rpm. Loads of backfiring when driving and foot of the pedal and killing the engine while cruising at same speed for a while. Guess the Rev transducer is next but where is he hiding?

[tharbert]

It's actaully on the left side of the engine bay - #5.



Inards:



Schematic:

[tharbert]

Flieger has a great write-up on tackling repair but the real groundwork was done by Warren Hall (Early S Man).

Use the search to find VOLUMES of info.

[john walker's workshop]

pull the wire off the pump solenoid for test purposes.

[Flieger]

The RPM transducer signal should not be going to the microswitch at 2000 rpm unless you have set it there. Constant-speed cruising with your foot on the throttle should not be closing the microswitch so any stalling in that manner of driving should not be related to the fuel shutoff circuit.

The "backfiring when driving and foot off the throttle" does, however say that the RPM transducer is not functioning properly.

To test the whole system, open the engine lid with the engine running. Rev the engine manually by pushing the throttle arm. Raise the revs to what sounds like 3000 or so. Now push the microswitch with your other hand. The engine should cut momentarily, the revs will fall and when they reach about 1300 rpm, the engine will come back to life. The MFI will be giving fuel based on throttle position now so the engine will try to rev to 3000. The Speed Switch will cut off fuel when the revs get above the threshold of 1500. This results in an occilating revving of the engine between the upper and lower RPM thresholds while the microswitch is pressed and you are holding the throttle open.

Best

[Eddyvdw]

You're right, the engine troubles after cruising for a while got to do with the cold weather. We test drove while it was freezing.

The car came from New Mexico and the tube from the heat exchanger to the air intake was not connected so the engine 'froze'.

Thanks all, another step closer to a perfectly running engine (after 17 years of sleep).

Ed

[Flieger]

How are the throttle bodies? Mine are worn round the shafts and let too much air in for a smooth idle with the throttle closed. My mechanic leaned out the mixture to spec in the mid/high rev range. I had richened it across the range to get the idle acceptable. Now it is hard to start, even in mild southern california weather. The idle is high and lumpy- many pops/ticks reversion out the intake.

Once above 3000 rpm or so, the S cams start to come in and the dynamic compression picks up enough that the engine zings smooth and runs strong up to redline.

With the high idle though I get a fluctuating idle that lets me know the RPM transducer is working.

I think working on my car is almost as much fun as driving it. I hope you feel similarly. Tschuess

[Flieger]

Originally Posted by Eddyvdw

You're right, the engine troubles after cruising for a while got to do with the cold weather. We test drove while it was freezing.

The car came from New Mexico and the tube from the heat exchanger to the air intake was not connected so the engine 'froze'.

Thanks all, another step closer to a perfectly running engine (after 17 years of sleep).

Ed

If the engine stopped while at operating temperature, I suspect it is too rich due to the (disconnected) thermostat not leaning out the pump.

One thing to remember if the car is backfiring at low revs and decelerating is to take your foot completely off the throttle pedal. Even just touching the throttle pedal to rest your foot is enough force to open the microswitch and allow idle fuel to be delivered while the throttles are almost closed. This causes pooling as well as an inconsistant and rich fuel mixture- causing backfiring.

It sounds like you are sorting the car out, though, and you already know all this.

[tobluforu]

I can't believe this thread came alive right when I was about to post about the same thing, almost. I have had this system disconnected for years and I'm going to put it back on. I found all the parts in a box in storage, tested the micro-switch which works. My car is in hibernation for the winter and I do not have a heated garage, so my question is if there is a way to test the rpm transducer with it being out of the car, no power to it? Pretty sure the answer is no, but I'm not very good with electronics.
Thanks

[Wilhelm]

Originally Posted by tobluforu

My car is in hibernation for the winter and I do not have a heated garage, so my question is if there is a way to test the rpm transducer with it being out of the car, no power to it? Pretty sure the answer is no, but I'm not very good with electronics.
Thanks

Flieger posted some info about testing/repairing these RPM transducers a while back on another thread. It can be done on the bench if you have some electronic test equipment (or know someone who does). Contact Flieger or search the threads.

Hope it warms up back there soon !

[Flieger]

Here is the thread with the process I used to repair the RPM transducer: For MFI Gurus. . .and friends of Warren . . .

You can test each individual component on the bench separately, even the RPM transducer if you have a 12V power supply and a signal generator to input the proper frequency 12V DC square-wave that replicates the distributor's signal to the Speed Switch. The frequency of the signal from the points varies with the RPM. You should also have an occiliscope to know when the relay is open or closed. If you have those tools, you can tune and inspect the RPM transducer on the bench.

To test the whole system, you should have it in the car. You could hook everything up on the bench together and put in the artificial signal. If it works then but not on the car, you have a problem in the wire from the distributor points.

Best

[tobluforu]

Originally Posted by Flieger

Here is the thread with the process I used to repair the RPM transducer: For MFI Gurus. . .and friends of Warren . . .

You can test each individual component on the bench separately, even the RPM transducer if you have a 12V power supply and a signal generator to input the proper frequency 12V DC square-wave that replicates the distributor's signal to the Speed Switch. The frequency of the signal from the points varies with the RPM. You should also have an occiliscope to know when the relay is open or closed. If you have those tools, you can tune and inspect the RPM transducer on the bench.

To test the whole system, you should have it in the car. You could hook everything up on the bench together and put in the artificial signal. If it works then but not on the car, you have a problem in the wire from the distributor points.

Best

I wish I had the knowledge and tools to do this, but alas I don't. Maybe if it reaches above 32 degree's in the next month I will hook it all up on the car and see if she works. If the system does not work, and my wiring is ok, it's off to otto's the transducer goes.
Thanks

[Flieger]

Any MFI'er should be in the Early 911S Registry: Home Page:PORSCHE EARLY 911S REGISTRY

There is a lot of expert information on their board specific to early cars. Early 911S Registry Bulletin Board - Powered by vBulletin

[Flieger]

Is the weather in Chicago this weekend suitable to test the operation of the RPM transducer?

[tobluforu]

Originally Posted by Flieger

Is the weather in Chicago this weekend suitable to test the operation of the RPM transducer?

Hell no!

[Flieger]

Sorry, I am sending the good vibes your way from the West coast.

[Eddyvdw]

The RPM is broken, 2 diodes loose so going to replace them.

My car doesn't have #3 in the picture (RPM sensor). Is that a problem or is this part located somewhere else in the car?

Ed

[Rick's 911E]

hi guys, does anyone know where can I get a clearer image of Warren Hall's schematic of the RPM Transducer?

Thanks, Rick.