MFI micro switch missing plunger?
 

After looking at a number of photos I think a cause of my backfiring is the fact that the micro switch is missing the plunger or at least I think it is. I have posted a picture and would appreciate comments. I have seen that there are some alternative switches from Allied but I could not find a source to actually order, any help would be appreciated.

http://forums.pelicanparts.com/uploa...1189873983.jpg

Ive never had mine apart.

here's how it looks on a 2.4

Gino

http://forums.pelicanparts.com/uploa...1189877105.jpg

Can't quite tell from your photo, but my '70 E looks the same as Gino's 2.4.

If I understand how this should work in it's resting position the plunger is depressed and once you accelerate the pressure is removed from the plunger?

Correct.
My picture was taken with no throttle movement.

Gino

IF (RPM > 1800) AND (Microswitch ON) THEN (Solenoid ON)
which moves the rack and cuts the fuel flow

AND

IF (RPM < 1300) AND (Microswitch ON) THEN (Solenoid OFF)
which releases the rack and starts fuel again

So if the RPM is over 1800, power gets sent to the switch. If the switch is closed, e.g. the throttle is closed, then power flows to the solenoid, cutting off the fuel.

Therefore, the switch must close only when the throttle has returned to the stop. Hence the fine adjusting screw. Your switch should have a rubber gaiter to protect it.

IIRC that switch enables the cold start injector, isn't that the extent of it? The lever on the floor lifts the throttle off the switch, the contacts close and provided the thermal switch is also closed you get fuel from the cold start injector when the starter is engaged.

So if you never close the switch the cold start injector will work everytime you start the from cold.

I think.

The cold start system actually runs off the yellow wire from the starter.

John,

That is very appropriate BOOLEEN logic for the operation of the rpm overrun. Good post.

PBH, John is correct. The micro switch is only part of the overrun circuit and not cold start or cold run. That is not an uncommon error. This really shows how simple the MFI system is. If you want to email me, I’ll lead you through the process.

The micro switch has a common US replacement (I think the original part was a US Micro Switch). It is a normal commercial / industrial switch and readily available. Yes, it should have a plunger.

Best,
Grady
gradyclay@hotmail.com

Grady is the expert, but I'm a practitioner. The switch needs to work or you WILL get backfiring on decel. The major problem with these things is getting enough linkage pressure to fully close the switch. Any replacement needs to be very sensitive to the point of not holding the linkage open and causing a high idle. As mentioned, there are overrides and a stiff switch will not help. the system doesn't understand itself since it's all mechanical.

And, a big throttle return spring just makes the car harder to drive, IMHO.

david482 -
I've got an extra microswitch if you need it. Sent you a pm.
Bill

David,

Yes, your plunger and rubber weather-sealing bellows seem to have departed to another location in the engine compartment, maybe elsewhere in the world!

Here is a Honeywell/Microswitch Application Bulletin #14 that may be useful to read for many MFI enthusiasts here:

http://sensing.honeywell.com/index.cfm/ci_id/140788/la_id/1/document/1/re_id/0

http://www.alliedelec.com/Images/Products/Datasheets/BM/HONEYWELL_MICROSWITCH/Honeywell--Microswitch_Industrial-Control_6421076.pdf