MFI Speed Switch

[robinfly]

I just came back from a 15 min drive and totally enjoyed the car's handling and the smooth power of the engine. However, when I opened the engine lid after the drive, I discovered one lead on my speed switch is broken. What can I expect with a nonfunctioning speed switch?

Thanks in advance,

Robin Hou
911E w/MFI

[Flieger]

less engine braking and more backfiring when you let off the gas. some people think it sounds cool.

I like mine functional

[Plays with cars]

The speed switch is non-functional on both of my cars right now and I don't have any ill effects. I have not experienced the backfiring that some people have. I have however confirmed that the mixture goes rich when decelerating and suspect that over the long run this will reduce engine life as the cylinders walls are being washed with raw fuel and removing the oil coating.

When you say that you "opened the engine lid and could see that a lead was broken", did you mean one of the leads going to the micro switch mounted on the throttle linkage? Or the one going to the solenoid on the MFI pump? I don't remember being able to see the leads going to the control module. The leads for both the micro switch and the solenoid are just basic 14ga wire, very simple to repair if damaged.

[tobluforu]

Pretty easy fix if its just a broken connector. Since your car runs great without, have you tested to see if the transducer even works?

[robinfly]

Thanks guys.

Looks like a broken terminal so a replacement switch is needed. I will test my transducer while working on replacing the switch.

[robinfly]

I repaired my broken terminal on the micro switch, but also found my rpm transducer is not functioning.

I cleaned all 4 connectors real well, cleaned the plug with contact cleaner, verified connectivity to power, ground, CDS and micro switch, put a test light on output to micro switch but still no joy.

Where do you send your rpm transducer for overhaul nowadays? What is a reasonable cost?

Thanks,

Robin

[Flieger]

You can do it yourself if you have some basic electronics stuff (or if you want an excuse to buy them). Actually I think you could do it without them if you aren't too concerned about tuning the rpm cutoff value too much. I think there is a link to the DIY thread in the Ultimate MFI Resources thread. It is almost always the leads on the transistors that have gotten brittle and snapped. Just replace them and it should be fine.

[BSNMOE]

contact Mark Jung MFI Werks he may have one

[robinfly]

Thank you guys for your suggestions.

I ended up sending my rpm transducer to http://www.systemsc.com

Fair price and quick turnaround. Highly recommended.

I reinstalled my rebuilt transducer this morning and it worked great!

[BasherBell]

Not Trying to hijack the post , thought this pertinent.
Ive logged some previous posts on problems with my 911T 1972
I imported it from California to the UK a few years ago.
Ran very rich, checked the oil- courtesy the oil lab and found it had loads of gas in it- near explosive. Tried to dial it in correctly, checked compression and Oh Oh!
Had the engine stripped and found oval barrels , nearly oval pistons Ha. Couple of teeth missing on various cogs and stuff incorrectly repaired by someone in the past. Had the MFI pump refurbed professionally and all put back together with new barrels and pistons etc.
Ran like a dog, couldn't get the part load, idle etc set. Little power at 3k rpm and above 4k , between seat lever needed to rescue dying revs at various stops and junctions , popped and farted its way along the road killing old ladies and small dogs with fright, and embarrassing the hell out of me. Overall not enjoyable to drive to the point of thinking about getting rid. Anyway brought it home to garage and think on for a while and research. Reading here was leaning towards the CDI and RPM Transducer as potential culprits.
Finally pulled the rpm transducer and took it for a spin (thanks to all those posting on that subject I know a few people have pulled and kept these small tin boxes of 72 electronics out and kept them out, also know they can be repaired) Completely different car. Still not perfect but accelerates through the gears , nice cruising motoring speeds , very little backfire (That can be sorted at the microswitch now I'm not relying on the idle lever). Looks like a duff RPM Transducer kicking in when it wanted to strangling the engine. Anyway we shall see. Frightened myself at official parts replacement (90161511100 Relay for injection pump £300/$360) I sourced a replacement part Bosch F 026 T00 002 new for the princely sum of £85/$100 . Thought it worth a punt - direct replacement evidently . The carsnow in the shop getting its annual Road Test , having the ignition , timing, advance, dwell, and part loads checked and seeing if the new RPM Transducer has any effect. They will try set it up without the transducer and if it runs well, then add it back in and see what happens. Thought I would post as the Bosch unit is a huge saving in money if it works .
I will report back on that subject.
I'm still a bit worried about the 3 pin Permatune CDI thats installed in the original case . Not convinced its doing its job linked to the Permatune Coil . Would welcome views on a modern replacement . As the 2.4T MFI is a pig to set up and fewer with the knowledge are around , thinking about more controllable CDI units and distributors and installing an AFR system to monitor the fuel gas mix via a Lambda sensor installed in a boss in the exhaust .

Overall a vast improvement in performance by pulling one component and finding it at fault which has restored my faith in a very solid car which I'm now going to lavish some bucks on interior renovations .
Thanks to all to Pelican Parts Tech posts and feedback - couldn't do it without you all.
Cheers
Dave

[nickelplated5s]

Old thread but does anyone know the thread size for the set screw on the switch? Sort of funny that was what a micro switch was 50 years ago. The other question is it just supposed to ground or depress the contact?

Originally Posted by robinfly

Thanks guys.

Looks like a broken terminal so a replacement switch is needed. I will test my transducer while working on replacing the switch.

[nickelplated5s]

If anyone ever needs to know it's an M7. There is a click when the switch is depressed. Ballparked the setting and got rid of the 3-4 K pop. Still have a burble around 2.2 but haven't tried to fine tune it. Problem was when a shop 'touched' my MFI without permission. If you can get in their's a shop in the PNW with a dyno that knows what they are doing.