Confusing Tachometer Functionality

[Jason84NA2T]

This is still bugging me, so here it goes:

When I bought my 87T I noted it had a MSD 6A ignition on it (the one I just recently took off and installed on my 84NA2T). Now, the strange thing was that the stock tachometer in that car worked with the MSD, and so I figured he had a tach adapter on it. Come to find out when I was ripping it all out that there was no tach adapter.

So there I stood, a little confused as to why the tach was working without the adapter.

BTW: Taris Blue can vouch for the fact that the tach worked as he was there when I bought the car; it's not just my imagination.

I installed the box on my 84, and low and behold the tach didn't work (as it should... not work).

So now it has been bugging me. Why did the tach work in the 87 with the MSD when the tach signal is supposed to not be compatible?

[Stinky]

Have you looked over the fsm to confirm the 87 and 84 are wired the same? I want to say the tach gets its signal from the ecu. Maybe this isnt the case in the older models?

[OR-Zman]

Stinky may have a point. Does the 87 get its tach feed directly from the coil and run through a 20k ohm resistor like the 84? I don't have an 87 FSM to check that out on, but either the source or the resistor could be the difference.

Also, don't later year turbo Zs have an electronic speed sensor unit. Where as early models don't? Perhaps RPM related....shot in the dark there. :?

[Taris Blue]

Actually if you remember correctly it worked fine on that 87T up to about 3000-3500 and then it just plain fell off and started bouncing around...also dave told us that the reason it did that was because he had not put a tach adapter on it.

[Jason84NA2T]

Taris Blue wrote: Actually if you remember correctly it worked fine on that 87T up to about 3000-3500 and then it just plain fell off and started bouncing around...also dave told us that the reason it did that was because he had not put a tach adapter on it.

The intermittant issue pretty somewhat went away after I got the car back home, so that may have just been a "sticky" tach because the car was sitting so long. Still, if it were wired like the 84-86 cars it would not have worked at all so there must be a difference of some kind like stinky said... I'll have to pull up the FSM's and look.

I've never heard of the tach working without the adapter for people with later cars with the MSD... dbruce, if you see this, I thought you had the adapter on your 88. Does your tach not work without it?

[dbruce]

My tach adapter install

I went from a Jacobs to MSD. I did not need the diode for either install.
If I had the diode in, the car would not run at all.
I may not need the adapter at all. I can't remember trying to run the car without it since I installed the MSD.
When I get my car back I'll wire the adapter out and see what happens. I think mine may need it for the dgi dash tach (87T GLL).

[Ken in AZ]

My 88NA had the same box and the tack worked till about 5500rpm...that is when things got bouncy...but it wasn't bad at all till I got the turbo upgrade

I just added an rpm switch and linked that to a red led in the dash as a shift light for 6800rpm

never needed a tach adapter