Source for used RPM Transducer

Kevin G. · 05-24-2002, 12:46 PM

Hi, all

Do any of you with older MFI cars have a source for a good, used rpm transducer? I tried ressurecting mine by replacing the rusted out transistors, but no joy. I understand new units are available, but don't particularly want to part with that much $$$$.

Thanks -

Early_S_Man · 05-24-2002, 01:09 PM

Kevin,

Did you replace D1 with a transistor or with a Zener Diode around 8-9 Volts rating?

Ron,K · 05-24-2002, 01:42 PM

Kevin,

I'm having the same problems. I replaced 3 transistors with NTE 123a per several tech articles on the board, but no joy. Warren has a schematic for a tester that I built that will help with diagnosis. I finally ordered a used switch from Parts Heaven. I'm a little bit leary of buying this part used, but they guaranteed that if it didn't work, they would return it.

I haven't given up on the old unit, but I want to get the car running. If I can get it fixed, I'll have a spare.

Warren, you mentioned in your reply that D1 should be a Zener Diode but in your schematic you indicate that it should be a transistor. Am I missing something(VERY possible).

Ron

Kevin G. · 05-24-2002, 03:48 PM

Warren:

I haven't a clue what a Zener Diode is

I had one good transistor left, which I removed and used for the 'critical' one listed in the instructions. All of the other transistors were replaced with new equivalents as recommended.

Don't know how to test the unit, but the microswitch and solinoid check out ok, so I either blew it on the rebuild or maybe there's something in the wiring that I missed.

Thanks for any advice you can provide!

Early_S_Man · 11:52 AM

Kevin,

A Zener diode is a Voltage regulator with precise characteristics when installed reverse-biased in a circuit.

In my unit a transistor was installed at the D1 loacation, but others that have emailed me have had a small glass 400 Milliwatt Zener diode mounted with the colored band Cathode end going into the 'E' hole and the other end going into the 'B' hole on the board. When a diode is installed, nothing goes in the 'C' stenciled hole location, which is electrically isolated from the rest of the board! Just to reiterate ... on my schematic, the Zener Anode end is grounded at the 'B' terminal and the Cathode end (with the band) goes to the 150 Ohm 2 Watt resistor connected to the 'E' terminal!

It isn't too unusual to find the emitter-base junction of a silicon transistor used as a Zener diode, but since an exact replacement for that Siemens transistor can't be readily found, it is much easier to use an 8.2 Volt Zener diode like a 1N756A that is still available from many sources.

Ron,

Did you get my 'calibrator' circuit built? If you connect the output leads from the contacts between ground and the Black/Violet wire pulled from the distributor ... with the ignition on, the tach should read 1200 rpm or 2400 rpm, and the presence of the +12 Volts at the Microswitch should tell you if the Speed Switch is working.

Ron,K · 05-25-2002, 12:57 PM

Warren,

Just tried the calibrator on the speed switch. No voltage on the microswitch, and no movement of the tach. The tach works when the engine is running, but not with the calibrator. Tach is from a '69 so I think it get it's signal from the points. I'm going to recheck my circuit board to make sure I didn't miswire anything and try again. Now I'm still not sure I've got a bad speed switch or a bad calibrator. I AM GOING TO FIGURE THIS OUT!!!!!!

Later

Ron