rpm transducer... see anything wrong?

[derek murray]

i've checked the transducer and the microswitch as described in "check, measure and adjust". the microswitch proves to be working but there is no signal from the transducer. can anyone see anything on the attached picture that could cause a non-working transducer.

[jkeyzer]

Looks ok from this angle. There may still be rusted transistor legs that are causing broken connections - only visible from the side and with some nudging on the suspect components.

Also make sure the transducer is actually getting power. You can probe the connector to do this. Also make sure all the fuses in your engine compartment fuse block are there, I believe the RPM transducer gets power from one of them.

[Early_S_Man]

Derek,

As Jeff indicated, nothing apparent from your pic! The 'usual suspects' are D1 and T4. Please, also note that the Speed Switch terminal numbers are embossed inside the plastic cap adjacent to the male terminals. The numbers are small, and the plastic cap my need to be cleaned to find them!

I recommend building a 10-12 foot four-conductor extension cable with 1/4" Faston male connectos on one end, and female connectors on the other end ... so you can make some checks on the circuit board on a workbench while hooked up to the engine. 18 ga ribbon cable for making trailer light connections will work fine. The same extension cable can be used on the CDI-unit, either on a workbench, or inside the cockpit when troubleshooting!

For the following tests, I suggest a DMM and a TTL/CMOS logic probe such as the one Radio Shack sells for $19.95. All tests should be done with the logic probe set to the CMOS position, and the power leads for the logic probe connected to the power leads #2 and #3 of the Speed Switch.

When connected and open on a bench for testing, check the following with engine running just above 1600 rpm:

1. Low resistance ground from Terminal #3 -- Brown wire -- to chassis ground ... less than 0.5 Ohms!

2. Steady +14 Volt power (11.0 Volts to 14.2 Volts) between Terminal #2 and #3 on the circuit board.

3. Regulated +7.5 Volt to +9.0 Volt bus Voltage at 'north' end of large resistor R12 -- nominal 8 Volts.

4. Pulse activity at base lead of transistor T1 -- silkscreened 'B' on component side of board. This can be checked with a DMM with Hz input function, oscilloscope, or CMOS logic probe.

5. Pulse activity at collector lead of T1 ... silkscreened 'C' on component side of board.

6. Pulse activity at the base lead of transistor T2.

7. Pulse activity at collector lead of T2.

8. Steady high logic level at base lead of transistor T4.

9. Logic low with CMOS logic probe at collector lead of transistor T4.

10. +12 Volt to +14 Volt output at output terminal #1.

[Early_S_Man]

Here is the pinout ... looking at the Speed Switch terminals, as you hold it in your hand:



And, just for clarification ... the diagram below which I have posted in several related threads ... shows the pinout of the Speed Switch/Speed Relay/RPM Transducer socket on the regulator/relay/fuse/CDI panel:

[derek murray]

thanks... if i can get my head around this (i.e., electrically challenged person) its off to radio shack i go.

if i do find something wrong... what then... can these things be fixed easliy?

[boyo]

Not really! The zinger is that replacing the transistor usally isn't enough. Those two resistors up on leads are hand selected and hand soldered based on the variances of the transistor and capacitor. It will affect at what RPM the circuit engages.

Mine was a rattle can of components when removed; I am considering either re-manufacturing a new set of boards, or redesigning the circuit using more modern components (mostly ICs). But that's a long term plan...

-Boyo

[derek murray]

hmmmm... maybe i'll have to live with the backfiring... or as warren once called it the "audible pleasures of mfi"... lol

... this is starting to get really fun

[Cornpanzer]

Dont want to appear like an ambulance chaser here, but I have a brother-in-law who is an old-school 70s era electonics major who repaired my transducer on my 70S. He has an occiliscope and pulse generator which makes diagnosis a snap for someone smarter than me =) I can assure you that driving your S is much more fun without the embarrasing pops and cracks. He is willing to repair anyones transducer even to the point of complete reconstruction for far less than a new one would cost you. P.M. me if you would like details. Once again, thanks to Warren for all the info in helping me track down the problems with my fuel system.

[Early_S_Man]

Derek,

Actually, quite a few Pelicanheads have successfully repaired their MFI Speed Switches in the past year or so ... though, sometimes the assistance of a relative was required! 'Companzer' and 'jeff m' were the most recent victorious Pelicanheads!

Yes, it is usually a simple repair ... just a replacement of the 8.2 Volt Zener diode D1, or the transistors T1 - T4. In your case, as in many of the Speed Switches, D1 -- the Zener Diode function is being performed by the emitter-base junction of a transistor, but a replacement would be a regular 400 mW or 1 Watt Zener diode. The cathode band of the replacement diode will go into the hole stenciled 'E' at the D1 location.

If you aren't familiar with soldering on delicate printed circuit boards, I recommend that a relative, friend, or technician do the repair work, as the US$283 Bosch Speed Switch is probably not the unit to be 'learning' on ...

The transistors used by Bosch are impossible to find in North America, and any NOS parts in Europe probably have rust problems on the gold-plated iron leads ... so more modern replacement JEDEC devices are probably a better choice, anyway. The closest part to T4 is probably a 2N2102, though a 2N2222 will work for all four transistors, T1 - T4.

Although bad solder joints alone are sometimes the only 'current' problem with the Speed Switches, it would not be a bad idea to replace the Zener diode D1, and all four transistors T1 - T4, as vibration combined with the rust problem on all of the transistor leads ... makes them a failure just waiting to happen. As outlined in last year's "MFI Warning" thread ... applying hot glue or 'Plumber's Goop' to the transistors to stabilize them from vibrating is a good idea, too!

There also have been problems with loose contacts in the socket/connector for the Speed Switch, so cleaning with vinegar and rinsing off with distilled water and isopropyl alcohol, followed by lightly crimping the sides of the female 1/4" Faston terminals ... but, don't close the gap entirely! After the repair and testing is finished, apply silicone grease to the socket connector terminals and the Speed Switch terminals.

Good luck!

[Porsche_monkey]

If you know what is wrong but can't find the parts, or if you are afraid to try and fix it send me a private message, I can probably do it for you (without the border hassle)

[Superman]

This place rocks!

[derek murray]

superman you are right, this place rocks.

i would definately not try to fix the transducer myself (my three year old still cries at the mention of the toy flashlight i tried to fix).

i would like to work through the diagnosis a bit so i can learn more about the workings of the car, but i will definately take up the help of someone like cornpanzer or PBH to fix it.

thanks again guys.

[Mark McClure]

Come on tech heads..........Its not plugged in!!!!!!!! Simple mistake that we have all done?

I have to admit this is strange.....electronic analysis over the net. Must be like virtual child delivery.......no mess and you can have a beer whilst busy at work!