Diagnosing the MFI RPM Transducer/Switch Help

[Gearbox]

Trying to eliminate possible causes for a slight bruply back fire on deacceleration. I have a 71 911S and when cold, I get a pop pop pop from the exhaust. It gets a little bit better when warm, but it's still there.

I understand the MFI RPM transducer switch may be at fault. From what I have read, the switch cuts off (reduces?) the fuel to the pump on deacceleration. I like to know the location of the switch (I think it's the one activated by the throttle linkage on the rear driver side bank of trumpets) what does it do, and how do I test it to see if its working? I assume the switch is just a on or off switch, but how do I test to see if the fuel is being shut off? Thanks Allan

[lwalwa]

There are two switches associated with this system.

There is a little silver box, I believe its called the RPM transducer, in the engine compartment on the drivers side with the other electronic stuff. If you have greater than 1600 RPM or so, it will output 12 volts. If not, it will output 0 volts.

There is also a microswitch, a mechanical switch, hanging off the driver's side intake, actuated by the intake.

To make some basic tests, you need a DMM. Disconnect the two wires from the microswitch. Use the resistance function to measure the switch. It should read 0 ohms when actuated and maximum when not.

One of the two wires comes from the silver box. The other goes two a solinoid on the injection pump. At idle, less than 1600 RPM, both should read 0 volts. At greater than 1600 RPM one should read 12 or 13 volts. If you can't find the 12 or 13 volts, the silver box is shot or your wiring is bad. Over on Pelican there are some threads describing how to rebuild this component. If you find the 12 or 13 volts then the solinoid is suspect.

good luck,

Larry

[Gearbox]

Thanks Larry, wonderful description and trouble shooting flow chart. Exactly what I was looking for, thanks again, I'll let you know how I make out, Allan

[Grady Clay]

Allan,

You can add to Larry’s good procedure. You can test the solenoid on the MFI pump by applying 12 V while at idle. It should audibly “click” and the engine should immediately die.

This part of the MFI system has three parts: The electro-mechanical that physically moves the pump rack to shut off the fuel, the electronic that detects rpm and throttle position and all the interconnecting wiring.

The advice you got on Pelican may have been more cursory. The reason (I think) is you were being encouraged to learn the MFI system for yourself and not just a small part of trouble-shooting. The CMA (and CMA2) procedure is critical to being able to maintain your MFI yourself.

I encourage you to search all the forums (particularly Pelican) and make yourself a resource of all the MFI information. It is worthwhile to edit much and print yourself your own comprehensive manual. In the process, you will learn Porsche MFI. Owning and maintaining MFI is a significant responsibility in the Porsche world. The best part is the performance you get.

Best,
Grady

[edmayo]

The quick test I use, is with engine running open throttles to engine speed of around 2500 RPM, then manually push the microswitch button down, if all is working the engine will drop to about 1500 RPM, at which time it will jump back up to the higher RPM and then fall back again to 1500 and will keep cycling like that as long as you hold the microswitch button down.

[Grady Clay]

Ed,

Right on.

Best,
Grady

[Gearbox]

Thanks for the info guys, I am compiling a data base on anything I can get my hands on following my philosophy that information is dangerious if you don't have enough. Stole that from Hercules at the stables, but I think it was said a bit more eloquently.

Looked over the microswitch and found the adjustment screw all the way in and still not activating the switch at closed throttle. I assume the arm needs to be bent up a bit. But with the engine at 2000 RPMs and manually cycling the switch, it did nothing. So before I dive into this I just wanted to make sure of the components. Picture one shows the RPM transducer arrowed in yellow. microswitch in pic 2, and RPM soleniod in pic 3, yes?

Also, searched through the Pelican site and couldn't come up with the RPM transducer rebuilding thread. Any help would be great. Additionally, after going through the Pelican site, I noticed the mention of a speed sensor that suppose to be incorporated into this system. Something about energizing the RPM transducer with 12v after it reaches above 1600 rpm. Any one have some more info on this?

Thanks Allan PS sorry for the sad state of my engine compartment, but have the car done and that will be next.

[lwalwa]

More detail on the RPM transducer can be found here:

http://forums.pelicanparts.com/showt...hlight=mfi+rpm

The microswitch you have is not the original style, so that explains why it is not actuating. I found a replacement at Parts Heaven (Hayward, CA).

Larry

[Gearbox]

Thanks for thehelp. Anyone have a picture of the correct microswitch for a 71 911S? Thanks again Allan

[Gearbox]

Finally got out to do some probing and found the following:

A) Transducer plug
1) Pin 1 grounds when the throttle is opened
2) Pin 2 is 12V all the time
3) Pin 3 is ground all the time
4) Pin 4 is 12v all the time

B) Soleniod injection pump
1) Shuts engine off when 12V is applied when at idle

C) Microswitch
1) Open circut when at idle, closed when throtlle is open

Based on these findings, I assume the microswitch and soleniod are working correctly and the transducer box is getting the correct signals and power/grounds. So can I assume the box is no good? I took it apart and didn't find anything rattling around, rusted or broken. Tested the resistors and they all read about the correct ohms save R10 which was 175 ohms and I think it should be 220. As for the transistors and diodes, I don't have an osiloscope. Are these things still available? Thanks Allan