The problem car is my '94 Si hatchback with JDM GSR engine and USDM P72 ECU. Both are stock.

Earlier this year, I pulled my trans to rebuild it. Like a boob, I forgot to unplug the VSS and managed to rip the wires out of the harness plug. I was able to repair it and everything seemed fine. Except for my tach.

My tach has been reading wrong ever since. Shortly after my mishap, the tach would stop between 7000 to 8300 rpms at the rev limiter. For a while, I thought the actual limiter was moving and preventing the engine from revving higher. It wasn't a big enough deal for me to pursue so I let it go until I was done for the season. Since then, redline is consistently showing 7500 rpms (sometimes 7200).

I went to Gingerman Raceway and ran four 20 minute sessions. On the way home, I noticed that my rpms were really low for the seed i was driving. I shifted down and ran to the rev limit and it stopped at 6000 rpms. The engine was definitely revving higher than that.

Here's what I've done:

Researching the wiring diagrams I found that other than ignition power, there is no correlation between the tach signal and the VSS. I unraveled my harness and I found no broken, cut or shorted wires.

I used a GPS app to make sure my speed was reading right and it was.

I used a Fluke meter to check RPMS and it showed that I was getting 8300 rpms while the tach showed 7500 rpms.

You would think it was the tach. No. I bought a used OEM tach and it showed the same rpms.

After I tried that, I tried running a wire directly from the ICM tach output on the distributor to the tach and ran a separate power feed directly from the battery (easy to do with the spare tach). Same problem, even read fewer rpms.

The next part in line is the ICM. I went out and bought a new ICM and I still have the exact same problem.

Great thing about buying a part that I didn't need is that I had a spare ICM that I needed to revitalize an old dead distributor.

I threw my now good spare distributor in and I still have the same problem.

One other thing I need to note, this is a bone stock P72 ECU. VTEC changeover is 4400 rpms and IAB changeover is 5750 rpms. Right now, the changeover for vtec is indicated at 4100 rpms and the IAB changeover at about 5300 rpms. I really believe the ECU is happy with it's distributor crank sensors and it's doing what it's supposed to do.

Oh, I also swapped in a spare VSS with no changes. I am also getting good voltage from my alternator.

I don't think I missed anything so that's all it in a nutshell. The only other parts I can think to change out are the engine harness or an ECU of which I do not have spares. I need to try and find a timing gun with tach to verify engine speed.

I've never been so stumped by a problem. Any help would be greatly appreciated.

Thanks,
Alex