Tachometer Problem Need Advice

Adidas · 07-19-2007, 04:06 PM

I have an unusual problem with the Tachometer on a 75 911. The original Tach works but reads low around 800 tp 1000 rpms lower than normal. Thinking that the problem lies in the tach I replaced it with a unit from a 76 911. For a few seconds this tach worked well and accurately but flickered a few times and died with no reading. I've double checked all the connections on the back of this tach and ran an additional ground as well with no luck. The distributor has a Pertronix Ignitor installed so the tach get its feed from the hot side of the coil instead of the points. The original Bosch 3 pin CD works and emmits the high pitched whine when the ignition is on. I reinstalled the orignal tach again and it works but reads low as usual. Any advice appreciated. Thanks Bill

Gogar · 07-19-2007, 04:17 PM

Is a '76 tach designed to work with a 3-pin CDI?

I don't know, I'm just asking.

I DO know that tachs must be matched to their respective 'type' of CDI boxes to work correctly.

nhromyak · nhromyak's Garage

I seem to recall this might be an indication of a faulty alternator regulator. But what do I know??!?!?

Check your voltages at idle and around 2K 2500 rpms.

It should not be above 15 v (that might even be high).

Adidas · 07-20-2007, 07:12 AM

I know that the Tach and the Bosch 3-pin CD unit are compatible. 1974- 1977 Tachs should interchange. Does anyone else have any thoughts on this problem? Thanks in advance. Bill (704)829-0976 or email billywhitesides1@aol.com

Early_S_Man · 07-20-2007, 08:26 AM

All '74 - '77 tachs are designed to get a trigger signal from the points -- same exact signal that triggers the Bosch 3-pin CDI unit, which you, or somebody replaced with a Pertronix Ignitor ... no rewiring of the tach input signal is needed with Pertronix!!!

If you, or somebody else, rewired the tach Black/Violet wire to get a signall from the '+' side of the coil ... you have been feeding the tach a 350 Volt++ signal, instead of the desired + 12 Volt pulse train from the points! No wonder neither tach is working!!!

304065 · 10:14 AM

Yep, Warren's right. The tachometer expects an 11 volt square wave and you've been feeding it 400+V output from the CDI.

Try hooking the tach to pin "C" on the CDI and see if that fixes it. If not, you fried the tach.

Porsche_monkey · 11:25 AM

I would guess they are both dead. Look for a third....or pull them apart and see if you can repair them. I would suspect that a capable electronics person (tv repair guy?) could fix them.

And then I think you need to get a scope or use a meter to verify the signal that you're putting into the tach before you try again.

Adidas · 07-20-2007, 02:16 PM

Thanks for the advice. I rewired the tach wire in the engine bay to the "C" connection on the CD unit. Neither tach worked. With the car at idle speed I checked the voltage coming in to the tach connector on back of the tachometer and got a reading of 7.7 volts which is below the 11 volts that you say I need. Could this be a problem with the CD unit? Any advice? Can one of these tachs be repaired? Thanks again Bill

Early_S_Man · 07-20-2007, 02:23 PM

Bill,

Yes, they can be repaired.

What did you use to measure the 7 Volt signal ... a meter? If so, the reading isn't really meaningful, as you were probably using the AC Voltage scale. An oscilloscope is what is needed to look at the signal, and a meter won't work as a substitute!

Adidas · 07-20-2007, 03:37 PM

Warren: Thanks for being patient with me on this issue. Yes I used a multimeter. Any suggestions on the repair. Thanks Bill

Early_S_Man · 07-20-2007, 07:37 PM

Bill,

All of the Porsche/VDO tachs I have seen were discrete component circuits using two or three tansistors. These are traditional logic circuits from a bygone era. Your local library should have old books [if they haven't thrown them out or sold them off] on transistor circuits ... a really good one is the 1964 GE Transistor Manual.

Two transistor tach circuits are a one-shot, or monostable multivibrator, and the three-transistor circuit adds a single buffer/inverter stage to trigger the one-shot.

To get at the circuit board you have to take the bezel off, carefully, as described in this Pelican tech article:

http://www.pelicanparts.com/techarticles/Mult_gauge_repair/mult_gauge_repair.htm

After you get the tachs apart, take several digital pics, referably in sharp focus and evenly illuminated ... and post them here. Both component side [several angles] and foil side shots of the circuit board will be needed. You [we] need to be able to read the component markings, such as color codes for the resistors, and lettering on all of the capacitors and transistors!

Porsche_monkey · 07-21-2007, 06:55 AM

I would guess that 7.7 is correct, you're measuring a square wave 'average' not the peak, so it's likely right now.