Tach not working, suspect ICM

[Rettax3]

Okay, so my '90 turbo 'Retta is having some serious problems with the instruments since I swapped-in a 3400 block, '99 GA GT PCM (to run the engine ONLY, as the car is, and originally was, a 5-speed, V-6), and ('88? '89? '90?) Cavalier Z-24 Muncie 5-speed (to replace the original Muncie 282 that died due to a gear-oil leak and lack of attention -my fault). Neither the speedometer nor the tach work. The speedo doesn't work because the Cavalier Z-24 used a different VSS that is incompatible with our cars' speedometers, I am resolving this by putting in a Cavalier digital instruments package, but this still uses an analog tach, which IS compatible with our cars' ICM. For most of this year, I was using a Fiero speedometer (also compatible with the Cavalier's VSS) that I grafted into a spare Beretta instrument cluster. But the tach still isn't working, and I have tested three different tachs now...

The ICM is hooked up to the '99 GA GT's PCM, leaving my 'Retta's ICM harness unplugged. I have this taped-off to keep out the elements. The white tach wire (there are two white wires leading from the ICM, I am using the correct one for tach) has a male pin connector after only a short run of wire, and I have this plugged into the car's original ICM connector. I even tried hooking this lead directly to two different tachs (one Cavy, one Beretta) in case there is a wiring problem in the car itself, but with no results. Additionally, when I first built this engine (3400 with an LG5 3.1 MPFI top-end), I had it in the car for a short while (removed due to a bad crank), and I don't remember having a tach problem then (although I may have -my eye was more on the virtually non-existent oil-pressure caused by the bad crank and rear main bearing, and it was several years ago). But, I was also running the car's original MPFI ECM and remote-mounted ICM...

So, I am thinking that there is a problem with the ICM. I think I have a spare ICM handy to try swapping it out, but I wanted to run it by the board to see if anyone has had an ICM fail, just on the tach output? The car still runs fine. Any helpful input on this?

Quick stats:
-1990 3.1 V-6 5-speed.
-Tach is not working.
-Total of three tachs tried, two of them directly hooked off of the ICM.
-ICM is from '99 GA GT, which is the same as the car's original 3.1 ICM.
-Car runs fine.

[spacecadetz26]

i always thought you had to grab the crank signal off the ecm.

[3X00-Modified]

Tach should come from pin C on the ICM large connector. Feed that directly to the dash, or go to pin C on the ICM Plug on the stock harness if your leaving from the 99 GA harness and don't want to run a full wire.

[Rettax3]

Thanks Jon, I had back-shelved this car for a while, since I got my yellow Indy to work on, mostly because of the speedometer issue -I don't have a functional Z-24 digital instrument cluster anymore, mine fried-out. Since you responded, I decided to spend a little time swapping the ICM out today, and found that the connector (like the one you have pictured) wasn't making proper contact to the ICM pin -that is fixed now, and I tested it. Unfortunately, the tach won't work anyway, since I've put in an early nineties S-15 Blazer digital cluster -everything but the tach and speedo are working fine, and I have some plans on fixing both of those issues. The tach will run straight off of a coil lead, like the aftermarket tach I have in my 3800 II SC GTU, since the S-15 uses a distributor ignition, but the calibration will be off by 1/3rd until I run a diode trio from the coils so they can all ping the tach without shorting each other. The speedometer is more complicated, and a little off topic here, but once it is working, I'll try to write up a thread on it, unless I can get my hands on another good Z-24 cluster first...

[3X00-Modified]

The white wire feed should resemble the same type of feed from a distributor ignition... I used to run a 5" Autometer tach (set at 6cylinder) off that wire back in the day when I was much younger and much stupider... Are you sure the S15 dash doesn't get the tach reading from the ECM like the Beretta digi dash's do.

[Rettax3]

I've built a few cars that originally used a distributor ignition, and ended up with a coil-pack system like our Berettas, and I've never gotten one of the tachs to work. I built a late nineties 2.8 Firebird with a Series 1 3800 (ran great, virtually no CO or NOX emissions either), I have a Fiero GT 2.8 with a '97 OBD2 3100, and even the aftermarket tach in my GTU that is supposed to work with a distributor-type ignition system won't function off of the 3800II ICM, and needs to be driven off of the 12V primary coil switching supply on the coil-pack.

I did hook up the jumper-wire running from the turbo's ICM to the car's original ICM 'C' connector (which in 1990 was a black/white wire to the tach instead of a second white wire) yesterday, and the dash still has no RPM readings -which could still be a wiring problem or an issue with the digital S-15 dash, but fits with past experiences as just being incompatible.

I suspect that your old Autometer tach was more versatile (and certainly better quality) than my GTU's add-in tach, and may have had a built-in signal amplifier that the others (even factory) do not.

Another possibility is to try running a tach signal from the G/A PCM, pin number C2-8, and see if that works, but I suspect the same results. I will jury-rig a fake distributor onto this engine before I buy an $80 Dakota Digital module for it though...