measuring tach signal

[Alan L]

Can I measure the signal to my tach (on the Bl/Vi wire) with my multi meter. Will it register an AC signal?
My tach is dropping out when the revs drop and engine hot. Sometimes it stays at zero until I blip the throttle, then it picks up again. Trying to isolate the source of the problem.
Once the revs are up again, it seems to pick up.
As I understand it , it gets the signal from the CDI box, via the green trigger wire on the dizzy. Assuming the car is running right, ignition wise, this would seem to let the green wire bit off the hook.
So it must be in the CDI box or the tach. Switched tach and same issue.
Need to be able to measure what the tach is supposed to be receiving.
Only seems to happen when car is hot.
Thanks
Alan

[cmonref]

The 911SC and Carrere 3.2 tachs receive a 12v pulse for each ignition pulse -- every time a sparkplug fires. The tach measures the frequency that the pulses are received and turns that into the reading on the tach. You need an oscilloscope to read that signal -- a multimeter won't do it.

[Alan L]

Can I measure it at the tach with my multi meter? Any idea what form of signal it is when it gets to the terminal on back of tach?
Thanks
Alan

[cmonref]

Alan L: see the post BEFORE yours.

[Alan L]

Thanks Brian. believe it or not, the last sentence of your post got chopped off with what ended up in my inbox. Now the reply is clear. So that makes it hard to sort what is happening with the tach signal. My early DWUR had a small external module that hooked into the tach wire and converted to DC signal - for calibrating the rpm signal to DWUR. That would be very useful now.
Thanks

[Dr J]

There are multimeters that measure frequency. A few years ago I purchased one through omega.com model hhm596c. I have used it to measure tach frequency. In our vehicles, 1000 rpm is 50 Hz. It also does other stuff like reading large currents or a thermocouple input.

If you use an ac meter you will read a voltage, but its not going to help you in determining the rpm.

[Alan L]

Excellent, thanks Dr J. I will beg or borrow one.
I managed to hook in a wire to the tach and tried it on AC and DC - like you say - not much help. It gave a signal - but made no sense vs rpm.
Regards
Alan

[Alan L]

OK, purchased one of these meters (not expensive), and wired the signal terminal from the tach, so I can hook the meter in whenever. I am getting 150 hz at 1000 rpm, if I am reading it right, and 300 at 2000rpm. So at least I can see if the signal is getting to the tach. When the tach next died I had the meter with me, and read 150 Hz at idle - ie correct signal coming to tach. So, you might conclude the tach is faulty - but remember, I have now the same problem with 2 tachs. Now the weird bit. When I let the clutch out to move off in gear, the signal picks up on the tach. If I just let the clutch in and out in neutral, no change. It seems to need the car rolling, to get something to change in the tach?
suggestions?
Alan

[Alan L]

Finally found the problem - intermittent voltage regulator.
Cooked a battery last race day - literally the plastic case melted to the body of the car. Smelt something a bit odd, so pulled off track and checked the car. Found battery case too hot to touch. Checked voltage output when things cooled down. 14V So figured that was not the problem. Then it did it again later in the day. The tach problem has been getting worse - dropping out more than in.
Checked voltage output again today. 14V. Blipped throttle and checked for a period of time. Then it climbed to 18V. Everytime the voltage jumped around, so did the tach. So finally solved two problems in one hit.
Hopefully this might help someone else in future.
Regards
Alan

[AndrewCologne]

Bringing this thread up.

Today I measured the TD signal at the tach of my '82 SC and also here at 1000 rpm it shows 150hz instead of 50hz at the DMM.

Was there a change of the frequency within the years?
Cause I remember 3 pulses per rev which results in 50hz at 1000rpm.

[AndrewCologne]

No one an idea?
Thx in advance