74 Rallye tach adjustment questions

74 Rallye tach adjustment questions - 05/02/11 03:31 AM

My tach needle doesn't start to move until my engine rpm is 1500-2000ish. I have it out, apart and see that it has one phillips head adjustment screw. Anyone know which way to turn it and ballpark how much?

Posted By: ademon

Re: 74 Rallye tach adjustment questions - 05/02/11 06:34 AM

I hooked up a good aftermarket tach and tried to get it as close to the aftermarket tach as posible concentrating on getting the idle to about 3K close. Still your original will probably not move up as fast and drop down slower and be off at least a couple hundred rpms at around 3K. Good luck

Posted By: denfireguy

Re: 74 Rallye tach adjustment questions - 05/02/11 02:27 PM

Quote:

My tach needle doesn't start to move until my engine rpm is 1500-2000ish. I have it out, apart and see that it has one phillips head adjustment screw. Anyone know which way to turn it and ballpark how much?

I have searched for years on information repairing and calibrating the factory tachometer. I have found no schematics and no information on calibration. I know that they were never very accurate even when they are new. They only had two transistors in a rudimentary amplifier that put out more current with more and more closures on the input. Todays technology is much better.
You might consider changing the innards to a Dashworx electronic version. If you send it to him for the install, he will calibrate it for you for an extra fee.
The state of the art method of calibration is to use a function generator to simulate the open and closing of the points or the reluctor triggering the control module. The frequency of the generator is set to 4000 Hz for 1000RPM (8 cylinder engine)indicated on the tach. Calibrating it to another tach works but is not as accurate. It is like setting your watch to the city hall clock. It is fine if it is accurate but it depends on someone else's accuracy in setting it and how long ago it was set. A stuck clock is right twice a day.
Craig

Posted By: ademon

Re: 74 Rallye tach adjustment questions - 05/02/11 08:00 PM

Quote:

Quote:

My tach needle doesn't start to move until my engine rpm is 1500-2000ish. I have it out, apart and see that it has one phillips head adjustment screw. Anyone know which way to turn it and ballpark how much?

I have searched for years on information repairing and calibrating the factory tachometer. I have found no schematics and no information on calibration. I know that they were never very accurate even when they are new. They only had two transistors in a rudimentary amplifier that put out more current with more and more closures on the input. Todays technology is much better.
You might consider changing the innards to a Dashworx electronic version. If you send it to him for the install, he will calibrate it for you for an extra fee.
The state of the art method of calibration is to use a function generator to simulate the open and closing of the points or the reluctor triggering the control module. The frequency of the generator is set to 4000 Hz for 1000RPM (8 cylinder engine)indicated on the tach. Calibrating it to another tach works but is not as accurate. It is like setting your watch to the city hall clock. It is fine if it is accurate but it depends on someone else's accuracy in setting it and how long ago it was set. A stuck clock is right twice a day.
Craig

turn the screw any way it will be evident to which way you need to go, if your racing the car don't go by the stock gauge, or even the stock gauge with modern internals, mine is fine for spirited street driving.

Posted By: cougar_cat_25

Re: 74 Rallye tach adjustment questions - 05/03/11 04:10 AM

The adjustment screw has about half a rotation stop to stop. Went full counter clockwise to get the needle into the numbers at idle. I can tell it's tired and inaccurate, but it's fun watch anyway. thanks guys

Posted By: FBryan09

Re: 74 Rallye tach adjustment questions - 05/03/11 01:27 PM

Along these same lines. I have a 71 GTX with a tach that works part time. I just got it and looked under the dash and there is a second wire spliced into the tach wire. Is there any way this should be? It has been restored and it is a new wiring harness.

thanks