1985 911 Tachometer - how do I test with multimeter?

[bmoody33]

Hi guys,
I own a 1985 Porsche 911 (non turbo) cabriolet with 94K miles.

My tachometer has gone dead. Previously it was erratic for about a month then just died completely. My car runs smooth and fine.

I've read through I think all the tachometer threads on here but many of them are over my head.

So, my question to all of you is how do I simply test the signal coming from the DME control unit to make sure the signal is good? I've pulled out the tachometer from the dash and have disconnected all the wires. On the multi-wire connector I've connected the:
black lead from my multi-meter tester to the black wire on the connector (bottom right in the picture with the I mark on the back of the connector.

and the red wire from my multi-meter tester to the black/violet wire on the connector. Bottom middle in the picture with the II mark on the back of the connector.

I have my multi-meter set to 2000 DVA (not sure if that's the right setting or not?) I'm clueless when it comes to the proper settings.

When I turn the ignition on without starting the car my multi-meter reads -033 and when I start the engine it fluctuates between -124 and -152 at idle. Pressing the gas pedal a bit it steadily rises to - 200.

So am I doing this right?
Do I have my multi-tester leads connected to the right wires on the connector?
Do I have my multi-tester set to the right setting?
Does my test indicate that my tachometer is bad?

Here's a picture of the tachometer connector.


and I'm not quite sure what is meant by testing the tachometer with a course file? What connections on the back of the tachometer would I connect to do that and how? I'm a bit confused by other threads on that test.

Thanks in advance Bruce.

[A horse with no name]

Hi...I can you help a wee bit...The black with a purple stripe wire is the tachs signal for sure...

Your tach receives a 12 v pulse for each ignition pulse -- every time a spark plug 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.

These car are notorious for having bad grounds..They have a tach sensor but when it fails or is on the way out your car will tend to run as erratic as the tach so it 'appears' not to be that as yours runs good ....The easiest way to find out if it is the tach would be to swap it out with someone--No Pcar guys in your area?

Rather than drive yourself crazy with it - As it would do to me- I suggest that you drop by an automotive electrical shop as they will check it for nothing for you... Then you would know for sure which way to go....It very easily could be a broken or loose wire including a bad connection and or it could be in the main box.
If you want to pull the large connector off the box to check the pins, this is how you would check to see if the RPM sensor is faulty

RESISTANCE: Put your multimeter on olms.

RPM Sensor, Pins 8 & 27, should read between 600 - 1,600 Ohms.
TDC Sensor, Pins 25 & 26, should read between 600 - 1,600 Ohms.

AC Voltage:
Change your meter to AC VOLTAGE - Your polarity does not matter on this test- Either wire on your probe is OK

RPM Sensor, Pins 8 & 27,- Crank the motor over and it should read over 1V.-
TDC Sensor, Pins 25 & 26, - Cranking motor it should read under 1V.

Sorry, but that's about as much as I can tell you about tach's...Maybe another pelican will chime in who really knows a lot more than I do.

Lets us know how you go...


.

[Dr J]

Some of the better multimeters have other functions such as the measurement of the frequency of the signal. If yours has that then that is what you need to use. Otherwise you would need an oscilloscope.

[bmoody33]

Thanks very much for the replies and help

There isn't an electrical shop that can bench test the tachometer that near me and I don't have anyone near me that I know who would let me try using their tach so I decided to send the tach to Palo Alto Speedometer as they only charge $35 to test it and will apply that amount to my bill if the tach needs repair.

I'll post the outcome when I find out more next week.