Early S M.F.I speed sensor repaired!! (please read if you have MFI)
 

Thanks to Warren for helping me through the diagnostics of my speed sensor box on my MFI controls. I found that the microswitch was working fine, but that my speed sensor wasnt supplying 12v to the switch. Fortunatly, I have a brother-in-law that is an old-school electronics wiz from the 70s. After looking at the circuit board in the speed sensor box he told me that if needed he could very easily make an entire new circuit board for me. He also already had a signal generator needed to bench test the circuit. Fortunatly, the problem turned out to be no more than a fine crack in a solder joint on the relay. I cant tell you how much nicer it is to drive my car w-o all the popping and backfiring :D So, now to my question........is there a market for speed sensor rebuilds. My understanding is that new sensors are over $250.00. If my brother-in-law could offer a evaluation/repair/test service on an exchange basis, would it be worth say.......$95 plus shipping? Seems like a fair price to me if the only other option is to buy a new unit. Whats your guys opinions. He would be happy to repair other units if anyone is interested. Hope I am not offending anyone by offering this service. Thanks, Dave C

I bought a "refurbished" unit from a supplier whom I thought to be reputable. While the car runs better now than with the original speed switch, I don't think it is right. So, yes. I would be willing to pay $95 on an exchange basis.

Also, would the switch have Warren's reinforcements?

yes, it would be no big deal to hot glue the componants. Now,cmon guys...dont tell me that with more than 40 people reading this that only one person actually has an opinion...I know better than that ;) Halm, if you dont mind me asking, what did you pay for the refurbished unit. I wasnt aware that they were avabilable

Sure it's worth it. I'd do it in a second if I thought mine was bad, last time I checked it worked. Now I'll check again.
If he could repair CD boxes you'd have a good business going.
They are not all that complex and fail just as easily as the speed sensor, just tend to leave you stranded more.

To actually have someone knowledgeable, who has actually made the repairs to the circuit board, $95 including Warren's upgrades is not a bad price to pay.

Bought the car in Ca. The PO would not let me take delivery until he had completed some work on the car he felt necessary. He took it to Otto's, who ended up doing EXTENSIVE work on it. I basically spent 2 days at his shop while this work was going on and learned a lot about his operation. Thus, when I figured out the speed switch was shot, I called him. He told me how to test and then shipped me what he called a rebuilt unit for $65. My guess is that it passed his tests, but arrived 2500 miles away not 100%.

Thus, with no competitent MFI wrench within 3 hours, I would be willing to spend another $95 for another speed switch to see if my diagnosis is correct. . . Especially if was reinforced for the long haul.

This board and the dedication of the owners continues to amaze!

I am unsure if my MFI'd Mercedes Coupe circa '69 had this item but I do know many straight six 2.2-2.8 Merc engines that might benefit from this work. You might include the Merc list in this mix?

John

I guess my next trick is to locate a bad unit to repair and have on hand for exchange. Anyone have anything? Otherwise it will need to be a repair and return basis. (which may be the way to go anyway)

What is supposed to be hard about repairing CD boxes?
All there is inside is a simple capacitor charging circuit.
I took mine apart...checked the transformer...replaced the power transistor (with a bigger one)and also replaced a diode (also bigger)...tests fine...runs the engine.
What more is there?
Bob

Exactly, but the majority of people dont have the electrical knowledge that you do. Besides, if they did they wouldnt be "oh so special" like you ;)

Mine is working at this time. If it were to fail, I would try Warren's fix myself first, as I am an (retreaded) electronic engineer from that era anyway.
If mine could not be fixed, then I would consider a rebuilt one for $95 vs 2X that or more for a new one, if one could even be found these days.
I agree with an earlier post regarding Bosch CD systems. Many more of thise still out there, I would think. Might be a good side business for your brother in law there..
Buy up a few broken ones of each type & see if he can make them sing again, good for a start..:cool:

That would be my take on it.
Offer to buy anyones bad speed relay or CD unit...repair it....then give the PO first crack at buying it back.
That way ppl will be more inclined to contact you.
Bob

Well you probably couldn't make a business out of it, but perhaps a little pocket money here and there. I'm sure some of these things are a bit more of a mess than Companzers...and heck alot of people may not even know or care that their rpm transducer is dead. Found mine is dead, and my car runs pretty good (no backfires or anything like that on deceleration).

Brad

Well you probably couldn't make a business out of it, but perhaps a little pocket money here and there. I'm sure some of these things are a bit more of a mess than Companzers...and heck alot of people may not even know or care that their rpm transducer is dead. Found mine is dead, and my car runs pretty good (no backfires or anything like that on deceleration).

Brad

Refresh my memory here, it's been so long since I checked the transducer in my car I forget how to check it. Is it checked by holding the idle at about 1500 and then clicking the on/off idle switch on the left bank stacks? If the transducer is working the idle should modulate from low to high. Is this correct?

Bring the idle above 1500 and close the microswitch. if the system is working, the engine should die. If you do not notice a difference, you need to check and see if the speed sensor is feeding 12v to the microswitch. If it is, then check to see if the microswitch is operating properly

OK thanks. Sounds familiar.

I have been looking for a working speed switch because mine is kaput... I could be your first customer!

I'm ready to buy one....thanks.