MFI Microswitch Wiring Question

[justacoupe]

I am frequently "challenged" when it comes to the electrical side of things. I know there are "no dumb questions" but these may be as close as you get to dumb. I need help regarding the wiring of a replacement overrun fuel cutoff microswitch ('73E MFI 2.4). The new switch has three wire connectors: common, norm open and norm closed.



My questions:

1. The factory switch only had two connections (one on each end of the switch body... no terminal in the middle). Considering that, and from looking at threads on the microswitch, I originally believed I should wire up to the common and the "norm closed." One person thought I was wrong... he thought it should be "norm open." Can someone here confirm that it should be "norm open?"

2. Also, the 73E MFI wiring diagram shows two different colored wires connected to the microswitch. However, our car has the factory original harness (we are original owners) and both of the spaded connectors are red/gray striped wires. I marked the wires before removing them, so I know which one was mounted to the terminal below the actual "plunger" on the original switch. Am I right to assume it doesn't matter?





Thanks in advance.

Bill
Grand Rapids, MI
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[NorthernThrux]

The microswitch circuit should be closed at idle. It provides for actuation of the stop solenoid. And yes, it doesn't matter which wire goes to the 2 relevant terminals. All you are doing is closing a circuit. I am not sure with the new microswitch whether normally open refers to the circuit condition when the microswitch is free or depressed. But that is easy to figure out with an ohm meter. Just find the two terminals that are shorted when the switch is depressed.

My guess, given standard conventions is N/O and Common are what you want.

Ravi

[Flieger]

Actually, (splitting hairs here) the microswitch circuit is open at idle unless you have worn throttle bodies which causes the idle to rise to the point that the speed switch sends power to the microswitch. The switch should be depressed (closed) at idle, but there should not be any current flowing through it. That only happens when the rpms are greater than about 1500 and the switch closed.

(I was the dissenting voice he speaks of in the original post)

I agree about the multimeter. Just use the ohmmeter function and see which terminals give you a closed circuit only with the switch depressed.

[NorthernThrux]

Yes, good call. Current flows in the circuit above 1500 rpm at idle and stops below 1300 at idle, as per pg. 38 of the bible, check, measure, adjust.

[justacoupe]

My thanks to all of you who replied. For the benefit of others who might search the forum and come across this thread, I've included a copy of CMA page 38 mentioned above. Also, following is Wayne Dempsey's description of the switch and its importance & adjustment:

"The shut-off solenoid is used to stop fuel delivery when the engine’s RPM is high, yet the accelerator pedal is not pressed. This occurs when coasting along in gear, most commonly down a hill or along the highway. The shut-off solenoid significantly reduces the delivery of fuel to the engine during this moment. Without the shut-off solenoid in operation, raw fuel would be dumped down into the cylinders with the throttle bodies closed. In this condition, stepping on the accelerator can generate a huge backfire in all cylinders. The shut-off solenoid is controlled by a small microswitch linked to the main throttle rod. Make sure that this switch is properly adjusted to be electrically closed when the throttle is completely closed (foot off of gas pedal)."



The switch I am using is on the first page of the Honeywell Microswitch catalog .pdf at this link: http://www.alliedelec.com/Images/Products/Datasheets/BM/HONEYWELL_MICROSWITCH/Honeywell--Microswitch_Industrial-Control_6421076.pdf

Regards,

Bill