Tach swap

[63a16p]

I've installed a 3100 V6 in my Opel Kadett and all is working great except for a few minor issues. I'm using a 7727 ECU. Next project is to graft a tach into the existing speedo cluster. I bought several tachs from the pick-a-part: 88 Firebird V6 (Denso), 88 Toyota 4runner V6 and an 82 Datsun 280ZX. I'm hoping 1 of 3 is working but so far either all are dead or will not work with the tach signal output from the 3100 ign control module (white wire). I did measure the signal from the ICM and it is a steady 2.5V AC at idle and increases according to engine RPM as it should.

Any ideas how I can get one of these tachs to work with the ICM tach signal or is there any wire that I can tap into to provide a tach signal? I actually tried 2 different 88-92 Firebird V6 tachs assuming since they were also GM and from the same year range they would be likely compatible but no such luck. I did check both Firebird tachs with a batt charger and they appear to work when connected to battery charger but not off ICM tach output.

[Guest]

The GM and Nissan tachs will work with your ignition module, though the Nissan tach might act funny after about 4500 RPM...

So you will need to recheck your connections and the signal coming out of your ICM. Do you have access to an Oscilloscope?

[63a16p]

Unfortunately, I don't have an oscilloscope. Based on further research, I think the problem is the 3100 ICM tach signal is a 0-5v square wave but the tach is looking for a 0-12V square wave since the tachs all came from cars with a distributor single coil ignition system that produce a 0-12V square wave. I bought a mid 90s Lumina (I think) gauge cluster thinking it would work because that car a 3100 in it with the same ICU I am using. I planned to remove the tach from the cluster and graft it into the Kadett speedo cluster but the Lumina tach is not stand alone in the cluster so I don't know what circuits fromthe main cluster board would need to kept in the system.

[Guest]

Again, the F-Body and Nissan tachs will work with the tach output from the DIS ICM. With a potentially bit of funkiness at higher RPM with the Nissan tach...

I've used GM and older Nissan tachs with the tach output from the DIS ICM. In fact the Nissan tach in my Datsun is currently being driven from the tach output from DIS ICM. I had to put together a little circuit that is 555 timer based to get rid of the high RPM funkiness. The problem there is that the duty cycle of the signal is too high at higher RPM, so the tach usually sees it as no RPM, and will in most cases drop to zero, other times it will read it as extremely high RPM and bounce off the highest reading a few times. I have driven my original 1985 S10 tach from the tach output on a DIS ICM as well, with no issues, which will have the same electrical design as your F-Body tach.

So if you're not getting a reading on those tachs, you either have defective parts, or you're not connecting them correctly.