90 Ecu in a 91 - Tach problems?

[llauer76]

I'm trying to solve my tach issues, and the further I dig the more confusing this all seems. I see that most of the time people are putting the 91+ ecu's into 90 cars. Unfortunately mine is opposite, and I don't believe you can just reverse the process and profit. So here's where I'm at.

I have a 90 EPROM (yes a 90 EPROM)
I have a 91 automatic cluster, the car was at some point in its life manual swapped. Would this cause anything?
91 coils and no noise filter. Although I don't think it should matter since the cluster is correct?
I have no misfires or anything that would lead me to believe my PTU is bad.

Can anyone make sense of this?

 

[luv2rallye]

I'm confused, is it now an A/T or manual? And it once was the opposite? What do you mean by cluster? Did the tach ever work or when did it stop working?

 

[llauer76]

Sorry, that was a little vague. The car was automatic from factory, and is now manual, but the original gauge cluster is (automatic) is still in the car. Since I've owned the car the tach has not worked.

I think I've narrowed it down to being either the 90 ecu throwing something awry, the fact that it is an auto gauge cluster (but that should have nothing to do with it, since that would just be a TCM module?), or a bad PTU?

 

[luv2rallye]

The ECU has nothing to do with the tach. The 91 tach gets it's signal from pin 4 of the PTU (which also goes to pin 63 of the TCU if A/T, so that wire better not be grounded or connected to anything since you no longer have an A/T). [The 90 tach got it's signal from a tacho interface noise filter which got it's signal from pin 3 of the ignition coil.]

So I would check continuity from PTU pin 4 to the tach (also make sure there's no continuity to ground). The tach signal also goes to ECU pin 109.

Not that it should matter for your problem but note that ECU pins 6 and 14 have their signals swapped between the 90 and the 91-94 ECU's.

 

[llauer76]

I will take a multimeter to the PTU later this week. Hopefully it reveals something simple like a frayed or grounded wire. My question now would be this: can the PTU be bad without truly being 'bad'?
If that were the case I would think I should be seeing a whole group of ignition problems?

 

[luv2rallye]

Yes, the PTU tach output circuitry can be bad where the coil drivers work fine.

 

[steve]

As Gary has mentioned, there is no inherent problem with the Tach when using a 90 ECU in a 91+ car. You just swap pins 6 and 14 to address the change in location of the IPS and MAF Reset signals between the ECU's. It's the 91+ ECUs in a 90 car that have a Tach Issue.

Let us know how the testing goes.