Old-school tachometer with “modern” engine?

Last summer I bought a Corvair IP for $10 because it was from a manual trans car (no shifter cutout in the panel) and because it has a cool old-school SW Stage 3 tach neatly installed in the original clock location.

question: will the Tach output from the LT1 ECU (obd1 if that matters) play nice with the old-school tach?

Probably. I'm running the old MG tach off the signal from an LS1 PCM. They're just pulses.

Internet says there's a tach sending unit required to operate these things normally.  Hopefully one of the folks here has direct experience with these things.  Hadn't heard of a tach sending unit before.

In reply to pres589 (djronnebaum) :

Those early sending units were used to replace cable type tachometer drives, if you were switching over to an electronic tach.  The sending units were just a simple hall effect pickup, if I remember correctly. It's probably been a decade plus since I've looked at one. I want to say some of those early kits could also be hooked up to the alternator, and then thru some math a series of dipswitches on the back could be programmed according to how many pulses per revolution the alt or tach sender would put out at a given RPM. 

As far as your setup, if it's a standard style electric tach with either a rotary switch or set of 8 (maybe 10, I can't remember) dipswitches on the back you should be able to make it work. 

Offhand, on the optispark garbage  I want to say the ignition module harness has a tach filter on it with 3 wires. I think one to coil, one to cluster from filter and one to ground.  The wire to tap into is the one between the cluster and filter, NOT the one that runs to the coil from the filter.  At least that's what I seem to remember on that LT1 setup I did in the '29 A, but even that's been like 8 years now. 

In reply to GaryC83 :

Your assessment of the Opti tach filter sounds right. I have a wire coming out of the OE 94 corvette engine harness that is labeled "tach signal to cluster" (labeled by me), so I'll roll with it. There's a "CAL POT" sticker covering a hole on the back of the tach, so there's probably some 4-6-8 adjustability beneath.

In reply to AngryCorvair (Forum Supporter) :

Sounds just like I remember.  If you can't figure out the correct series of switches to make it work, the guys at Stewart Warner are awesome to deal with, they should be able to get you a copy of the manual for that if you can find the model # on it, to throw the correct sequence of switches, or position if a rotary, if it's indeed more complicated than just selecting the correct number of cylinders and needs some oddball sequence entered.  I doubt it will be more complicated than setting it up as an 8 cylinder though, truth be told. 

If the tach won't run off the ECU tach output, you probably need a high voltage tach adapter. 

81cpcamaro said:

If the tach won't run off the ECU tach output, you probably need a high voltage tach adapter. 

Yep a lot of tachs from the mid-90s and earlier expect a high voltage kickback from the ignition coil, nicknamed a spike signal. Newer tachs expect a low-voltage square wave signal and modern ECUs put those out. You can convert a square wave signal to a spike signal with a device made from a relay, my AE92 has one of these allowing the MS3X to drive the stock tach:
 

With an Optispark, if the factory tach wire is a low voltage signal and won't drive the tach, you can wire it straight to the coil unless you're using an aftermarket CDI or distributorless conversion.

MadScientistMatt said:

With an Optispark, if the factory tach wire is a low voltage signal and won't drive the tach, you can wire it straight to the coil unless you're using an aftermarket CDI or distributorless conversion.

nothing aftermarket, just an OE Opti and an OE coil.  thanks!

GM tends to be pragmatic.  For pre-CAN clusters, the cluster expects to see what a distributor would put out, because they didn't want to reinvent the wheel.  So the pre-CAN PCMs put out a 4 pulses per rev signal.

In reply to Pete. (l33t FS) :

does "4 pulses per rev" mean the tach will think it's looking at a 4 cylinder engine?

In reply to AngryCorvair (Forum Supporter) :

V8.  Two revolutions per eight ignition events.

I forget if this is a tunable setting or not.  Not that it matters if you can change the tach to suit the output.

It's a model that's compatible with 6- and 8-cyl, and has a calibration pot in the back. I really dig it, so hoping I can make it work without too much effort or expense.