Tachometer woes...

John-knee · 07-31-2011, 07:07 PM

I've searched around and found plenty of information regarding tachs, but I seem to have a problem not covered...

I have an '84 911.

The tach was working poorly a few weeks ago, then intermittently jumped around dramatically. Sometimes it'll function fine for a few minutes.

Now my tach pins to red line the minute the ignition is turned on. Upon starting the car, in comes off redline and when revved, it reaches 5k. I looks like the tach is operating in reverse, albeit inaccurately.

Battery voltage is fine, hovering around 14volts.

Is it worth taking the thing apart?

Any help would be great!

Thanks

masterdave · masterdave's Garage

check all the inputs to the tach, im not near my car for the next few days but i will try and measure the signal to see what you should be getting when it is revved. I would advise if you can find out what it is, check the input signal first, make sure it is not a sensor or wire issues. If you are good there then begin to look at the tach. Depending on what you have at your disposal if you can generate the signal, once you find out what it should be, bench test the tach. According to this thread How to test a 1972 911 Tachometer ? it is an 11V square wave, this has some more schematic info Tach troubleshooting... at a roadblock and need suggestions To actually generate a square wave on a bench you will need either a signal generator, which can be costly, if you live near westchester NY i have one you are welcome to borrow, or we can even throw your tach in my car and see if it works. You can generate a square wave with a few different circuits and a simple DC supply but again that may be more time money and effort than just having it checked out or rebuilt by someone. To test for the square wave i would use an Oscilloscope, again i have one you can borrow if you live in the area.

In terms of taking it apart, I have never done it so i cant help you there but i do know that there are a great deal of places that seem to be based on rebuilding dash board electronics. There must be a reason so many people need a service like that. If I had to take an educated guess, I would say it may be easier and cheaper to have it fixed my some one (if it is the meter its self) Than try and take care of the problem your self. It is at hear nothing more than either a Volt or Amperage meter, but if the winds in the meter shorted out or some thing of that nature it can be very hard to fix by hand.

Hope this helps
Dave

wwest · wwest's Garage

Measure the voltage within the engine compartment, a high resistance front to back, positive or negative(ground) could mean the alternator output is above 16 volts in order to charge the battery to 13.5 volts. Also worry about the voltage spiking due to the regulator's use of PWM to modulate the alternator rotor current flow.

John-knee · 08-01-2011, 08:28 PM

Masterdave,

Thanks so much for the input. Ultimately, the easiest solution right off the bat is just swapping it in another car and see what happens!

Unfortunately, I don't live too close to you--I'm actually in Ottawa, Canada. However I do drive to NYC often enough, and will in fact be driving down end of this week...

John-knee · 08-01-2011, 08:30 PM

wwest:

The tach pins even before the motor is running. As soon as the ignition is turned on, it goes from 0 and sweeps to past red line. Alternator isn't even a factor at this point I don't think.

wwest · wwest's Garage

Quote:

Originally Posted by John-knee

wwest:

The tach pins even before the motor is running. As soon as the ignition is turned on, it goes from 0 and sweeps to past red line. Alternator isn't even a factor at this point I don't think.

The CDI, Capacitor Discharge Ignition, begins charging "THE" capacitor the instant the ignition is switched on. Next time turn on the headlights and blowers as you switch on the ignition. The CDI can fail in "this" mode due to past battery overchanging, 12 volt source going too high.

Even after I replaced the regulator/alternator/battery the CDI would still fail in the manner you describe once it was heat soaked to the engine area temperature. I could spray it with cool spray and it would start and run again for the next XX miles.

So the CDI went off for a rebuild, successful rebuild.

Oh, try the cool spray method on teh CDI case and see if the problem abates.

304065 · 08-02-2011, 04:23 AM

Quote:

Originally Posted by wwest

The CDI, Capacitor Discharge Ignition, begins charging "THE" capacitor the instant the ignition is switched on. Next time turn on the headlights and blowers as you switch on the ignition. The CDI can fail in "this" mode due to past battery overchanging, 12 volt source going too high.

Even after I replaced the regulator/alternator/battery the CDI would still fail in the manner you describe once it was heat soaked to the engine area temperature. I could spray it with cool spray and it would start and run again for the next XX miles.

So the CDI went off for a rebuild, successful rebuild.

Oh, try the cool spray method on teh CDI case and see if the problem abates.

The OP has an 84 Carrera- no CDI, he has a motronic control unit that drives the tach.

Rather than making the investment in test equipment, I would send it to North Hollywood speedometer for testing and repair. They do incredible work at reasonable prices.

dshepp806 · dshepp806's Garage

Yeap,..I'd suspect the metering side, at this point (assuming that alt/reg is good-2-go).

NHS: good work.

Doyle

masterdave · masterdave's Garage

Yeah i would say send it out at this point. I have a 924s also which has the motronic unit as well, the tach was disconnected when i got the car and spend a while splicing the harness to make it work right. In the end of the day, unless the computer is bad, it sounds like a meter fail. The computers can break my 924s had a bad ECU when i got it as well.

I will say a re built meter is nice. I bought my 911sc with a rebuilt speedo, not only did it work well but they cleaned it up real nice, may have even repainted the numbers in it, i dont know but i can tell you it looks great, real great, if a gauge is not working a rebuild is with out a doubt worth it. They will not only make it work but replace most if not all components with new ones if need be.

Depending on when you are in the NY area I may be able to help you out if you bring the tach. If you are going to be around on friday check the "calling all brooklynites" thread, there is going to be a porsche meet up in Brooklyn on friday. Ill be there and will be glad to swap out my tach.

regards
dave

John-knee · 08-08-2011, 03:24 PM

Just an update (and an aside):

The starter circuit is also malfunctioning (and has been for awhile). I ordered a new ignition switch to correct the problem.

I just drove the car from Ottawa to NYC (~8 hours) and then to the Outer banks (~8.5 hours)...and the tach worked perfectly and accurately. Something must be shorting the thing--maybe related to the mess in the ignition switch?

Either way I remember reading about a tach "signal generator" in some 911's...does the 84 have that as well? It rained most of the drive--wondering if that has an influence on some wiring. Already checked all grounds. Wiring diagram that I read says tach signal is direct from ECU. Curious to know if ECU has an input from a signal generator...

djbrand1 · 01-29-2012, 05:42 PM

My tach in my 84 started doing this as well. With the engine off and key on run, the tach will twitch a bit go up to 3k go back to zero etc. so the jumpiness isn't related to the alternator regulator since the symptoms exist without the motor running. Am I correct in this logic?