MFI deceleration microswitch with MSD box

[eapcpa]

I am using twin MSD on my twin plug ignition. I am also using the MSD tach adapter model 8920 because the JB racing distributor has magnetic pickups. Tach works great, in fact I have an on/on switch so I can switch the tach signal back and forth to monitor output from each MSD box. My theory is that if a box fails, the tach signal will fail and I won't be stuck running on six plugs. The problem is that I cannot get the deceleration microswitch working so I get a steady snapping and popping noise when I let off the throttle. It does sound good to some but it gets pretty annoying. I have picked up the black and purple wire going from the 14 pin plug on the rear relay panel directly to the tach. I tried picking up the black and purple wire from the other 14 pin connector from the engine harness as that runs through the speed sensor and and microswitch and then then up to the tach. That causes the tach to not work. Unfortunately I did not check to see if that allowed the microswitch to work. I thought I would ask for someone more knowledgeable to advise me. Do I need to split the signal from the MSD tach adapter to the speed sensor and the tach or will this cause a problem. I do not want to let the smoke out of any of these little electronic devices as that usually causes them to stop working and once released you can't put the smoke back into them, so I am told. I already let the smoke out of the first MSD tach adapter but that was another story. It only cost another $60 to replace. Any help is appreciated.

[Flieger]

Firstly, do you know for sure that your speed switch is in working order, with no broken transitor leads?

[eapcpa]

worked fine when I pulled the 2.2 motor out. Anyone know what kind of power it gets from the negative side of the oem CDI coil. I believe that is where it's signal originates from. Right now it is being bypassed. Like I said, I do not want to burn anything up. A friend sent me this but he could not find a bottle of Bosch smoke.

[Flieger]

This helped when I repaired my RPM transducer.

Originally Posted by Early_S_Man

[eapcpa]

Flieger

Thanks for the information. I went back over the other wiring diagrams as well and it appears that I will need to take the tach lead from the MSD tach adapter to pole 4 on the speed sensor to simulate the coil trigger.

[HawgRyder]

Ed...by the look of the diagram...the R1 resistor is acting as a load for the input of the box.
I would check your tach output with a similar load to ground just to make sure the speed sensor box is not goint to load down the MSD box and cause a problem.
If the speed box does load down the MSD box....the ignition system may become intermittant at higher RPM's or overheat and fail.
You might find that putting in another resistor in series with the R1 (12K Ohm) will still work.
If it were me....I would stick a Pot ( variable resistor) in series with the input, and dial it up until the speed box stops working, then back it off until it's stable, and measure it and replace with the closest value resistor at that time.
This would still allow the speed box to work, and not load down the MSD box.
My $.02
Bob

[jluetjen]

1) Unplug the switch from the loom. If you push down on the microswitch with your finger (so that it clicks), can you see the switch open or close on an ohm-meter? If it doesn't -- it's dead and needs to be repaired or replaced.

2) If the micro-switch works, have you confirmed that it is adjusted correctly so that closing the throttles trips the switch?

3) If one and 2 both passed, confirm that the throttles are resting on the stops when they are closed. In my case someone in the car's past had adjusted the linkages so that throttles wouldn't completely close in order to keep the car's idle at 1200 RPM. The result was lots of pops and bangs out the exhaust on over run.

PS: What does any of this have to do with the tach signal. That's a completely different circuit.

[Flieger]

The RPM transducer needs an RPM signal, which it gets from the distributor via the wire going to the tachometer.

I did the potentiometer trick when I rebuilt my RPM transducer circuit. Each unit is hand tuned from the factory so I put in a Zener diode and a couple new transistors and then dialed in the RPM I wanted it to function in. Then I went and soldered in the closest 2 resistors to that ohm value.

Sorry I cannot help with the tach issue. I am running the stock Bosch CDI, distributor, and black coil.

[eapcpa]

jluetjen : What does any of this have to do with the tach signal. That's a completely different circuit.

Well, I am using a distributor with magnetic pickups. Therefore, no signal to the RPM transducer. The MSD boxes have to be run through a MSD Tach adapter to get the tach to work but the transducer still does not get a signal. And as I read my diagram the circuit to the tach runs through the transducer and then on to the tach as I recall from the wiring diagrams. Micro switch and all other parts are functioning as designed. You must not have read the beginning of the thread

HawgRyder & Flieger; Thanks for the info. I will enlist the help of a friend who is also an electrical engineer to review this thread and then we will give it a go. I kinda like the popping from the exhaust on deceleration but it can get to be too much at times.