7K Tach Conversion (High Windshield)
Posted: Fri Dec 15, 2023 12:57 pm
My stock tach on my 70 1600 car had been inoperable ever since I acquired the car back in the early 80's. I have never diagnosed the exact issue (bad cable or bad tach), but after seeing multiple posts on the forum about tach conversions done either professionally (Jon Frampton at Global Instruments) or other members (spyder, iloveredmeat, akara and others), I decided to try a conversion from mechanical to electrical.
Other users have converted their 8K tachs, but the 1600 models have a 7K tach. I started by sourcing a 7K Autometer model 1398 (3-1/8" diameter in-dash) but ran into trouble. Mounting the stock face to the armature was pretty straight forward, tapping the plastic housing to match the mounting screw locations of the stock face.
But the armature was soldered to the circuit board, unlike the model used by others where the circuit board is screwed to the armature leads. My attempt to de-solder the armature from the PCB was disastrous, and rendered the tach inoperable. Crap...
Back to square one, I restarted the whole process by ordering Autometer model 1477, a 7K dash mount 2-1/16" diameter tach. I suspected the PCB would be small enough to fit into the stock housing, and the armature would be the same as the 1398 model.
Mounting the stock face was easy (tap new holes in the plastic armature housing). The sweep range of the Autometer tach is a bit broader than the stock face, so I needed a way to calibrate the new assembly. The Autometer has an "air core meter" and sets to zero RPM when 12V is applied. I applied 12V to the unit, attached the pointer at "zero" (having to crimp the stock pointer shaft just a smidge), then using a tach signal generator I found from Zip Products I adjusted the Autometer tach using a potentiometer on the PCB. Seeing that pot, I suspected it was for adjusting the tach. An email to Autometer tech support confirmed that the POT was indeed there for "minor adjustments" (they also made their disclaimer statement that my experiment with their tach voided the warranty - ok sure).
I used the Autometer's threaded standoffs to assemble the unit into the stock housing. Drilled two new holes in the housing and used two nuts as backing nuts to space the face correctly in the housing.
The Autometer's PCB fit extremely tight against the stock housing's rubber turn signal grommet deforming it's shape a bit...
...so much so that after assembly I had to reach in with a small tool and adjust the grommet to ensure the green lenses were illuminated when a lamp is inserted. One more check post assembly.
I also got a new plastic lense cut (same thickness and diameter) from Tap Plastics that I'll put in before final installation.
What I am especially pleased with is that everything fits into the stock housing - no extra cables running to external electronics.
Hope this post helps others with a late model 7K tach who want to make this conversion.
-jt
Other users have converted their 8K tachs, but the 1600 models have a 7K tach. I started by sourcing a 7K Autometer model 1398 (3-1/8" diameter in-dash) but ran into trouble. Mounting the stock face to the armature was pretty straight forward, tapping the plastic housing to match the mounting screw locations of the stock face.
But the armature was soldered to the circuit board, unlike the model used by others where the circuit board is screwed to the armature leads. My attempt to de-solder the armature from the PCB was disastrous, and rendered the tach inoperable. Crap...
Back to square one, I restarted the whole process by ordering Autometer model 1477, a 7K dash mount 2-1/16" diameter tach. I suspected the PCB would be small enough to fit into the stock housing, and the armature would be the same as the 1398 model.
Mounting the stock face was easy (tap new holes in the plastic armature housing). The sweep range of the Autometer tach is a bit broader than the stock face, so I needed a way to calibrate the new assembly. The Autometer has an "air core meter" and sets to zero RPM when 12V is applied. I applied 12V to the unit, attached the pointer at "zero" (having to crimp the stock pointer shaft just a smidge), then using a tach signal generator I found from Zip Products I adjusted the Autometer tach using a potentiometer on the PCB. Seeing that pot, I suspected it was for adjusting the tach. An email to Autometer tech support confirmed that the POT was indeed there for "minor adjustments" (they also made their disclaimer statement that my experiment with their tach voided the warranty - ok sure).
I used the Autometer's threaded standoffs to assemble the unit into the stock housing. Drilled two new holes in the housing and used two nuts as backing nuts to space the face correctly in the housing.
The Autometer's PCB fit extremely tight against the stock housing's rubber turn signal grommet deforming it's shape a bit...
...so much so that after assembly I had to reach in with a small tool and adjust the grommet to ensure the green lenses were illuminated when a lamp is inserted. One more check post assembly.
I also got a new plastic lense cut (same thickness and diameter) from Tap Plastics that I'll put in before final installation.
What I am especially pleased with is that everything fits into the stock housing - no extra cables running to external electronics.
Hope this post helps others with a late model 7K tach who want to make this conversion.
-jt