73.5 tach issue

[chauser]

Hi All

73.5 911
When I converted from points to crane optical pickup the tach stopped working.
I've seen many of the threads here, however I'm puzzled by the ~ intermittent functioning of the tach.

With tach connected directly to white wire from crane unit:

At idle - blip throttle nothing. However, under load, and accelerating ~ 2-3k I could see needle begin to bounce, and if I kept accelerating the tach read accurately up to 4-5/6 k, but as soon as I maintained an rpm, tach dropped to 0.
Car is running fine, however, the behavior of dropping to 0 under even load I don't understand.

Suggestions?

Car has Crane Hi6 CD unit. I get no tach response when tach connected to green tach wire from Hi6 unit.

Charles

[rennworks]

might want to look into the tach adapter from crane.
warren

[304065]

The white wire from the crane unit is the equivalent of the points output. It's a square wave, but I don't know what voltage. The tach requires an 11 volt square wave.

Crane recommends you use an 8920 (same as MSD part number) tach adapter which is basically a transistor buffer stage. Some use a MOSFET for this:

How I use Crane Optical Trig with Bosch CDI (3-pin)

It specifically says do NOT use an 8910, this is basically a doorbell transformer in a plastic case. One leg is connected to +12v, the other to the white wire (ground) and when the "points open" there is about a 200 volt whack that goes up the line to trigger the old-style (SWB) tach.

[chauser]

Thanks for reply, here is some info I left off the original post:
Car has Crane Hi6 CDI

I had previously been in contact w/ Crane re: the problem,and the response I got from them was to use the MSD 8910, not the 8920:

No, sir. It’s a 12v square wave output with a ¼ amp maximum load. It can only be correctly measured with an oscilloscope. You will need the MSD 8910 to trigger the tachometer if it does not work on the green wire (tach output).

Thanks, Robert

I have also tried using TachAdapt, connected to either the green output from the Hi6, or the white wire from the optical unit - neither worked.
Bob from TachAdapt has been great and has offered to take a look at tach to see if a problem may lie there.



Charles

[304065]

What are all the numbers on the back of your tach, and what are the labels on the white plastic terminals? A good clear photo will help.

[chauser]

Sorry for delay replying.
Tach details are shown below. Good News - the problem turned out to be blown diodes. tach is reinstalled and working fine.

Many thanks to Jaime Novak who helped diagnose the problem, and Bob Ashlock (TachAdapt) for identifying the bad diodes and replacing, and all who chimed in. !!!