7K Tach Conversion (High Windshield)

[70-1600]

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.

Stock Tach.jpeg

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.

Large Compared.jpeg

Large Attached.jpeg

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...

Armature Large.jpeg

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.

Model 1477.jpeg

Small Compared.jpeg

Sweep Range.jpeg

Small Attached.jpeg

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).

Adjustment Pot.jpeg

Calibrating.jpeg

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.

Ready to Assemble.jpeg

Assembled.jpeg

The Autometer's PCB fit extremely tight against the stock housing's rubber turn signal grommet deforming it's shape a bit...

Mech Tach.jpeg

...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.

Post Assy Check.jpeg

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

[rwmann]

100Hz driver signal = 3000 indicated RPM?

[70-1600]

100Hz driver signal = 3000 indicated RPM?

This specific signal generator is marketed to work with Zip's tach conversion kit for 1952-1982 Corvettes. It can be used for both stock V8 and LS-V8 engines. The instructions read: "This signal generator is designed to work for both standard V8 and LS V8 engines, but the frequency will represent two different RPM measurements; it is important to know which one you are calibrating. For a standard V8 engine, the signal generator Frequency is multiplied by 60 and divided by 4. Example: (100*60)/4 = 1500 RPMs. The LS engine RPM is doubled, the correct formula is (100*60)/2 = 3000 RPMs. It is important to use the correct formula for your respective engine type." So for a 4 cylinder engine, 100 x 60/2 = 3000 rpm. The Autometer tach can be used on 8, 6 or 4 cylinder engines. I verified this connecting the tach to the coil (-) and comparing the reading to my timing light (with tach readout).

-jt

[drieseck]

Damn nice work jt, A bit beyond my pay-grade but the pictures, vendors and description make it look feasible for my thumbs, many thanks. I appreciate your pursuit even after the original unit stumble, valuable insight.

[rwmann]

Any indication of the sampling and display rates? Lags? Thanks!

[70-1600]

Any indication of the sampling and display rates? Lags? Thanks!

I'll check on response/lags by connecting it up to the coil and revving the engine a bit and post a video. Here are a few more snaps of other settings (2.5K, 4K, 4.5K, 5K, 6K).

2.5K.jpeg

4K.jpeg

4.5K.jpeg

5K.jpeg

6K.jpeg

-jt

[70-1600]

Damn nice work jt, A bit beyond my pay-grade but the pictures, vendors and description make it look feasible for my thumbs, many thanks. I appreciate your pursuit even after the original unit stumble, valuable insight.

Thanks! This forum is a huge asset to Roadster owners in updating and keeping our rides running. Glad to contribute to that ongoing endeavor.

-jt

[rwmann]

Nice!

[JT68]

Awesome work here.. This will be a great add for a future project we are working on. Well done!

[70-1600]

As promised, modified tach connected up. No voltage, voltage applied, start up, rev up, shut down.

No Power.jpeg

Votage On.jpeg

https://youtube.com/shorts/9acFObWJDMU?feature=share

https://youtube.com/shorts/Oys8tsMsVbU?feature=share

https://youtube.com/shorts/hzpPVPJ6JQc?feature=share

-jt

[rwmann]

Nice tach response! No bounce, looks to be minimal damping.

[funkaholik]

This is awesome. I love how it all fits in the stock housing, unlike the 8K tach modification. Great job figuring it out and posting the part numbers needed. I also have a '70 1600 and I'm just northeast of you in Concord.

[spl310]

This is awesome. This would be great for the V8 project.

[Gregs672000]

Wow very cool. I wonder if something like this could be done for a speedo. I was very fortunate to get parts to fix my early 160mph speedo, but it still needs work as it reads way high past 50mph, and like I said parts are hard to find... took several months and a very good deal given (much appreciated Tim).

[drieseck]

jt, thanks for these very important------finish up post(s) and results, very helpful. Re-using the OEM body and face---simply first class and professional. Most realize, once a tough mod is complete it is nice to move-on, but this post-op truly helps others, much appreciated. If possible, maybe a tip/caution/technique(s)/pics showing how-to-start? There has been much struggle and vague opinions on how to remove the crimped-with-sealer housing, then actually re-fitting. This critical procedure, jig or insight kept Mr. Frampton in everyone's debt. Hopefully he's now simply enjoying his car (which is fantastic) or retired, so we're back on our own. Your help is appreciated. Dave