RPM transducer success story

[jkeyzer]

Thanks to the wealth of information on this BBS, I was able to restore the non-operational RPM transducer on my 72 911T w/MFI to perfect working order. In hopes of getting better gas mileage (11-12 mpg on average in the city right now for whatever reason) I tested the transducer and found that it wasn't doing much of anything. Well it turns out this was two issues:

1. I took the transducer out and a transistor fell out of the case. Two more were hanging by one or two leads. I have never seen transistors rust before! I found that the NTE123A was a suitable replacement for the BCY58, although a similar NPN in a can might have worked also. The hfe (current gain or beta) of the NTE parts is lower than the BCY58-IX's that I replaced (the IX represents the sorted grade of device, X being the highest in roman numerals), but that doesn't seem to have hurt anything. Fortunately the voltage reference device was fine and I didn't have to replace it!

I swapped in the new devices, cleaned up the board, and conformally coated it with some MG Chemicals Silicone conformal coating (tough stuff!). I put blobs of RTV silicone near the transistors and capacitors as a shock absorber. This thing should be pretty bulletproof now.

2. My mechanic removed a fuse from the engine compartment for whatever reason when it was in the shop once. I just left it out thinking it didn't do anything. Turns out this is the fuse for both the RPM transducer and I believe the cold start solenoid. I put a fuse into that spot (#2) and now I have power to the transducer, and better cold starts!

With those things in order, tapping the microswitch with the engine revved causes the RPMs to dip and rise periodically as they should.

It's alive!!!

We'll see how MPG is affected. It can only get better...


PS. The pdf datasheets for the BCY58 (and probably the other larger BJT) can be downloaded from ST Microelectronics' website. (www.st.com)