MFI speed switch

210bhp · Post by **210bhp** » Fri May 11, 2007 8:13 pm

Does anyone have a 69-73 MFI speed switch they could loan me to prove one way or the other that mine is defective. I have eliminated the microswitch and solenoid as working correctly so the speed switch is now the obvious target. Alternatively, is their anyway of testing the circuit board inside to see which component is defective?

Any help appreciated and obviously happy to cover all costs involved in posting etc.

Regards
Mike

hot66 · Post by **hot66** » Fri May 11, 2007 8:49 pm

I'm looking to check mine whilst its at Mikes. I'll let you know how we get on

Alan @ CanfordClassics · Fri May 11, 2007 9:04 pm

Hi Mike,

This will help

http://www.early911sregistry.org/forum/ ... eed+switch

Alan

impmad2000 · Post by **impmad2000** » Fri May 11, 2007 9:16 pm

I have one ?? I think ??
labele'd "0 336 611 00"
It certainly isn't going back in!!
But since my car was a T running Zeniths it may well be different, all to do with the Over-run enrichment device I think.
Any way, you are welcome to the device If any use to you

Tim

210bhp · Post by **210bhp** » Fri May 11, 2007 9:50 pm

James

Let me know how you get on thanks

Alan

Thanks for the pointer. I have been through the whole circuit from the microswitch to the solenoid and tested what I can. The microswitch and the solenoid work. That means either the speed switch or the wiring is defective. The speed switch is replaceable and available but very expensive. I would like to test mine or swop out test it before I order one otherwise it will be a costly error.

Tim
The inside of my speed switch

There does not seem to be anything lose or fallen off but I don't have the equipment to test the circuit board and it's components. If your unit has the four spade connectors in the same positions it may work but the part number for my car is 90161511100(bosch no. 0333400001). Don't know if I dare risk your unit. What's the potential damage?

Regards
Mike

210bhp · Post by **210bhp** » Fri May 11, 2007 10:01 pm

Thanks to Tim for offering his unit. For anyone else who may be able to help

The speed switch is no. 5 on this diag. The circuit board is inside the 'aluminium can' on your electrical console in the engine bay lhs.

Cheers.

Regards
Mike

Mike · Post by **Mike** » Fri May 11, 2007 11:09 pm

Mike, whats led you to think the speed switch is defective?

210bhp · Post by **210bhp** » Fri May 11, 2007 11:41 pm

Mike

The fuel is cut off at the pump when you back off the throttle but if this doesn't happen you get popping and banging on the overrun which is what I've been getting. The car will run perfectly ok but fuel consumption goes up and excess fuel mixes with oil in the cylinders which over time is not good. A similar effect can happen if an air leak occurs.

When you back off the throttle the linkage touches a microswitch on the left stack. There is an adjustment screw to make sure this is happening properly. When the microswitch closes it signals the speed switch which activates the solenoid on the pump. This shuts off the fuel by moving the rack.

To check if the whole system is doing what it should you get the rpm to about 3000 using the throttle linkage and the press the microswitch with your finger (keep clear of the fan!!). The revs should drop and oscillate between 1300 and 1700 approx until you let go of the microswitch. When I do this nothing happens....the revs stay at 3000.

I tested the microswitch and the solenoid and they are ok. But I am stumped when it comes to circuit boards. Hence I thought the best way to check the speed switch was to slam in another and do the above test again. The speed switch just pulls out once you undoe the retaining bolt............

Regards
Mike

impmad2000 · Post by **impmad2000** » Sat May 12, 2007 8:35 am

Can you manually operate the solenoid, engine off ? You should hear it click.
I'll look closer at my speed switch and see if I can see any differences.
Cheers
Tim

impmad2000 · Post by **impmad2000** » Sat May 12, 2007 9:15 am

Ok, the Zenith system is similar, Here goes....
On overrun conditions over 1350 RPM (Microswitch operates when off throttle) a solenoid device operates.... Blah, blah....
So, on this basis, I'd say the device is similar, but operates at RPMs >1350.
The Early S site suggests >1600 on your one. Sounds pretty close to me. I'd guess that one of the resistors stood up on contacts changes the threshold RPM values. You could try switching these from one unit to the other. I reckon its worth a go

Here's some pictures
Tim

210bhp · Post by **210bhp** » Sat May 12, 2007 10:34 am

Tim

That's great. Thanks for spending time on this.

Yes, I can operate the solenoid externally and the engine dies like it should when 12 volts are applied.

I reckon it would be worth trying your unit even if the operating parameters are slightly different. It will still tell me if my unit is faulty or not.

May I send you my address etc by pm?

Regards
Mike

impmad2000 · Post by **impmad2000** » Sat May 12, 2007 2:05 pm

210bhp wrote: May I send you my address etc by pm?

Please do

Cheers
Tim

210bhp · Post by **210bhp** » Sun May 13, 2007 6:28 pm

Looks like Tim's switch is not going to work despite his best efforts (thanks Tim) so has anyone got a spare MFI speed switch out there which they could send to me for a day or two??

Thanks in advance

Regards
Mike

Lightweight_911 · Post by **Lightweight_911** » Sun May 13, 2007 8:31 pm

Mike - sent you a pm

davidppp · Post by **davidppp** » Sun May 13, 2007 10:05 pm

Hello, Mike.

Happy to lend you mine..

Just let me have your address and I'll post it Tuesday latest...

Kind regards
David