Tach way off

[mjb765]

Just got my cluster back from Instrument Specialties and they said they checked the tach and it's all good. I installed in the car and it is 4-500 RPM off. Tried the tach lead from my Rev-N-Nator box and then the tach doesn't work at all. Tried using a new wire and it's the same thing. Looking for ideas or is it the tach itself?

[Chryco Psycho]

all the tach does is read / count the # of times the coil fires so it would seem that it is the  tach that is wrong assuming all the connections are tight & clean

[blown motor]

I have exactly the same thing. My tach was off when I bought the car five years ago. Three years ago I put in a Rev-N-Nator so I don't believe the Rev has anything to do with it. Sent my gauges to Mr Heaterbox this winter for therapy and they were suppose to calibrate the tach but it's still off just as much as before. 

[mjb765]

I have exactly the same thing. My tach was off when I bought the car five years ago. Three years ago I put in a Rev-N-Nator so I don't believe the Rev has anything to do with it. Sent my gaugews to Mr Heaterbox this winter for therapy and they were suppose to calibrate the tach but it's still off just as much as before. 

Instrument specialties said they calibrated my tach also.....guess it's going back next winter.

[blown motor]

I have exactly the same thing. My tach was off when I bought the car five years ago. Three years ago I put in a Rev-N-Nator so I don't believe the Rev has anything to do with it. Sent my gaugews to Mr Heaterbox this winter for therapy and they were suppose to calibrate the tach but it's still off just as much as before. 

Instrument specialties said they calibrated my tach also.....guess it's going back next winter.

I've considered sending it somewhere else next winter. I talked to Performance Car Restorations about it on Friday. They couldn't assure me of making the tach accurate. His words were "really, I'd just be taking your money".
Unless I'm certain that someone can fix it accurately I'm not willing to rip my dash apart again.

[JH27N0B]

Back in the day, I installed an Autometer tach on my steering column in front of the factory tach on my rally dash.  The factory tach was fairly accurate as I recall, but was sluggish.  You could really see the difference when you'd rev the engine, you could see the needle moved much faster on the autometer than on the factory tach.
I had my instrument cluster gone through for my restoration.  When we got everything installed and the engine started up, it is apparent that the factory tach is now reading close to 2X the actual RPMs.  I don't know what to do now, I sure am not going to put the Autometer back in my restored car!

[mjb765]

At least I am not the only one.......I guess the next step is just put a tach on it and see how far off it is at cruising RPM. Right now the needle reads at the second small line at idle which is about 300 RPM I guess....maybe it gets more accurate the higher it goes.

[Chryco Psycho]

what you need to remember is the factory tach is a dinosaur  electronics have advanced a lot since 70 even with updated electronics installed the gauge is an antique .
The rebuild kit offers a calibration tool so you tweak the rheostat so it reads accuratly , this is a major pain as the gauge has to be out of the dash to access the adjustment so rather than send it back you can adjust it yourself  I guess you could tweak he rheostat without the tool & see how close you get by trial & error or we could maybe do a group buy , use it & send it to the next guy that needs it ? 

[Chryco Psycho]

http://rt-eng.com/rte/images/8/8e/RealTimeEngTach_MoparActionApril20005.pdf
I guess RTE is out of business now 

this may help also https://www.youtube.com/watch?v=c00gzYNcLyA

[blown motor]

Good info Neil. Thanks. 

[HP_Cuda]

They look like they are still in business, maybe you know something more:
http://rt-eng.com/rte/index.php/RTE_67-74_Tach_kit

This would be a bummer because I have bought a couple of items from them.

http://rt-eng.com/rte/images/8/8e/RealTimeEngTach_MoparActionApril20005.pdf
I guess RTE is out of business now 

[Chryco Psycho]

at the bottom of their main page it states they sold to Dashworx in 2011 & Dashworx closed in 2012

[erik70rt]

Bummer.  Is anyone making these circuit boards now?

[HP_Cuda]

Well thats a bummer.

[61K T/A]

Here's a shot in the dark. MSD makes a tach adaptor. Basically it's a signal amplifier. Maybe Pertronix makes one too.  Maybe there is a signal issue here rather that a calibration issue. You might be battling a weak or floating input signal. Other than using an oscilloscope to check amplitudes I dont know any other way to accuratly check the input signal to the tach. Also you can see if it's a clean signal or not ie. amplitude  differentiating or attenuating signal patterns. If the signal is dirty you can back track it with in the circuit to the source. This was one of our trouble shooting techiques we  did in my main frame gate chasing days.