72 MFI RPM Transducer/speed switch ?
 

Today I found the micro switch buried beneath all sorts of crap, installed it and was able to start the old car for the first time in 3 months. Reading a few posts I found a good way to see if the rpm transducer works is to rev the motor to 3000 rpm and press the micro switch which in turn will make the car loose rpms and oscillate until the switch is depressed. Well, when I do this the car just shuts off completely which leads me to believe that the transducer/speed switch needs repairing. Any experts want to concur before I ship it off?
Thanks

It sound like the RPM transducer is working, but your fuel injection/ignition system is out of adjustement. You can check the RPM transducer with a DMM.

One side of the microswitch goes to the RPM transducer. The other side goes the the solinoid on the MFI pump. Above 1500 RPM, or so, there should be 13 volts or so present on the transducer side of the switch. Below, it the voltage should be approaching 0.

You can also check the switch by using the resistance function of the DMM. This is best done with the microswitch disconnected. With the switch pressed, the resistance should approach 0 ohms. With the switch released, it should be large.

Assuming the switch and RPM transducer is working, the next step would be to follow CMA.

Larry

The two wires that connect to the micro switch, does it matter which wire goes to what terminal?
It's not a cma/FI/ignition issue as the car ran pretty damn good without the micro switch/transducer in place.
Wouldn't the transducer control if the car oscillated when the micro switch is pressed when revs are kept above 2500? Something else must be going in the silver box besides it shutting fuel completely?
Thanks for the help.
One other thing, the car doesn't have to be completely warmed up for this to work, does it?

As long as the solenoid is not sticking, it sounds like your idle mixture is not correct. The car should not shut off during the test. It is best to perform the test on a fully warmed engine, however.

Solenoid-you mean the device that is on the pump, right? I have read a bunch of your posts in regards to mfi, so would you say that the transducer is indeed working? I just think it's weird that the second I press the switch it shuts the car off. Granted I started the car in 32 degree weather, let it sit for maybe 5 minutes max before I did this, but it was not warmed up at all. I will pull it, clean the contacts and let the car warm up before I try again. I have had this car for 15 years and have been working on the mfi myself and before I hooked this up, the car idled and ran pretty damn good, but would slightly backfire through the exhaust at times when letting off the gas in gear, thus me wanting to hook the transducer up. What tells the system that the car is at a certain rpm and to either shut gas off and back on, isn't this in the transducer itself which in turn gets a signal somehow from the tach?
Thanks again

Between age, corrosion, vibration, heat cycles and that damnable brown gunk used to secure components on the board, it's a wonder these transducers even try to work. Maybe it's a bit premature to fiddle with MFI tuning just yet?

As hinted at by lwa, there are two circuits that determine at what RPM the switch energizes and de-energizes the MFI rack solenoid. Changes to the resistance of either circuit changes the RPMs at which it does it's magic.

If the car is otherwise running/idling fine, I would check those circuits.

BUT, I think the first step would be to pull the switch, open it up, and clean out the brown gunk. A dental pick works great. Inspect for physical damage. That is how I found the damaged base lead on T4 pictured below.

You can replace components based on Warren Hall's RPM Transducer schematic. This is just a guess but I'd bet you could replace most all the components on the board for $20.

http://forums.pelicanparts.com/uploa...1226964670.jpg

For testing, I made a 15' 4 strand test lead with 1/4" male spade connectors on one end and female on the other. I used some 16 ga. wire I had on the shelf but I think a 4 strand telephone cable would do. That way, I could bench test the unit using the car as a signal generator by simply extending the connections out of the engine compartment.

http://forums.pelicanparts.com/uploa...1267630642.jpg

Then, get the circuits at R5 and R6 back to the correct voltage (8.4v ?) using resistors. Flieger mentioned he used variable resistance pots at R5 and R6. That way, he could vary the resitance with the turn of a screw and change at what RPMs the switch operates. Once set to how you like it, measure the pot resistance and replace with matching resistor.

You are shooting roughly for:
Above 1500 - power OFF
1500-1300 - power ON
below 1300 - power OFF
(as posted by 304065)

http://forums.pelicanparts.com/uploa...1267633332.jpg

The best source of info on this subject: For MFI Gurus...

I pulled a little of the information together here.

Hope this helps! It's not that hard to do. A digital multimeter, test leads, some bits and bobs ordered from Jameco, and a soldering iron is all you need. OK, you also need this forum...;)

It does not matter which terminals on the microswitch the wires are connected.

It sounds likely the RPM transducer is working.

Under normal situations while the car is warming up, with the hand accelerator up, the microswitch will not engage when you release the accelerator pedal. I would let warm up. If on a warm engine activation of the solinoid cause the engine to die, that points to what Flieger says.

Larry

Unless, maybe, the 38 year old timing circuit resistance is out of spec causing the solinoid to remain activated to an RPM too low to support an idle? I don't suggest I'm an MFI guru of any sort but it seems that adjusting the switch would be as attractive an option as turning pump screws.

This is one hell of a post and I thank you for it-great info as I was certain there was something in that box that knew what rpm the motor was at, but I'm not an electrical guru and some things are best left to the experts like Otto, so off it goes to him.
If his fix job does not work, I will post back.
Thanks everyone.

I think it is an idle problem. How are your throttle bodies?

Does the car still die when you test the car at operating temperature?

Although the TB's will need replacing in the near future it's not them or the pump or one of the ten demandments because I just took it off, gave it a light shake and I can hear two or three things rattling around in there.
Thanks for all the help all, off it goes.

You mean you shook the RPM transducer? The rattling could be just some of the vibration-insulation rubber stuff but more likely is an electrical component. I am surprised it would fail on only 1 circuit.

Just opened it up and one of those red X items in the pic above came out (looks to be T3) along with some brown crud. The board seems to be pretty jammed in there as I can slide it out, oh well.