Urgent Tachometer help needed

[RCWD21]

I just got done swapping my 87 over to mpfi (was a 91 donor) and need some help with the tach from the TBI setup not working.

 

The MPFI tach works just fine when plugged in.

 

Are the signals different between the TBI and MPFI? I have 2 clusters from TBI models and neither of them will work. Only the original MPFI tach will work. I've seen that the wire that would normally go directly to the tach on the TBI setup goes over to the MPFI ECU and then over to the tach. Its pin 2 and 3 on the ECU and when testing with a multimeter I dont get continuity across those pins.

 

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[Frenchy]

I cants say if the wire changes positions or not between the two but I can say that if the cluster from the 91 fits then I would recommend using that. Why you ask? Well it gives it an updated look and you know it works. As long as the needles aren't broken you should be good. Also for odometer correction you can go into the speedometer (it's able to be removed) and you can turn the numbers on the clock(you have to be careful doing this so you don't damage stuff).

[Slartibartfast]

The service manuals confirm what you've found with the wiring. The '87 manual shows the white wire from the tach going through a 2.2k ohm resistor to the + side of the coil. The '90 manual shows the white wire going to the computer, then a white/green wire going from the computer, through the same 2.2k resistor, again to the + side of the coil. So, yeah, sounds like the TBI system runs the tach right off the coil, while the MPFI runs it off a signal generated (or at least modified in some way) by the computer.

 

If you're married to/stuck with the '87 cluster, you could cut the white wire from the computer to the tach, tape up the end going to the computer, and splice the end going to the tach into the white/green wire. That would give the TBI tach the signal it expects. 

 

Or, if you'd rather modify the cluster than hack up the harness, you may be able to swap just the tach from the MPFI cluster into the TBI cluster. The drawings of the clusters in the two manuals are not quite identical, but they are very close. I would not be surprised if the tachs interchanged mechanically. I would however be a little nervous about swapping the plastic needle from one to the other to make the tach look at home (I'm assuming the '87 is orange and the '91 is white).

 

If the tach itself is the same between the two clusters, but has different control circuitry, and that circuitry is easily separated from the tach, maybe you could just swap that circuitry from one tach to the other and leave the rest alone. If that pans out, you could basically convert the TBI tach to an MPFI tach, and get the job done without hacking up the harness or prying on a 36-year-old plastic tach needle.

 

Or, yeah, if you don't mind the look of the '91 cluster, save yourself some screwing around and just huck that in there.

[RCWD21]

The service manuals confirm what you've found with the wiring. The '87 manual shows the white wire from the tach going through a 2.2k ohm resistor to the + side of the coil. The '90 manual shows the white wire going to the computer, then a white/green wire going from the computer, through the same 2.2k resistor, again to the + side of the coil. So, yeah, sounds like the TBI system runs the tach right off the coil, while the MPFI runs it off a signal generated (or at least modified in some way) by the computer.
 
If you're married to/stuck with the '87 cluster, you could cut the white wire from the computer to the tach, tape up the end going to the computer, and splice the end going to the tach into the white/green wire. That would give the TBI tach the signal it expects. 
 
Or, if you'd rather modify the cluster than hack up the harness, you may be able to swap just the tach from the MPFI cluster into the TBI cluster. The drawings of the clusters in the two manuals are not quite identical, but they are very close. I would not be surprised if the tachs interchanged mechanically. I would however be a little nervous about swapping the plastic needle from one to the other to make the tach look at home (I'm assuming the '87 is orange and the '91 is white).
 
If the tach itself is the same between the two clusters, but has different control circuitry, and that circuitry is easily separated from the tach, maybe you could just swap that circuitry from one tach to the other and leave the rest alone. If that pans out, you could basically convert the TBI tach to an MPFI tach, and get the job done without hacking up the harness or prying on a 36-year-old plastic tach needle.
 
Or, yeah, if you don't mind the look of the '91 cluster, save yourself some screwing around and just huck that in there.

Well, I've already converted the 91s dash harness to feed the 87 cluster (I really like it and the 91 cluster was really sun faded and the speedo needle us crispy curly).

I've made up a little breakout harness to feed the TBI tach directly off the cluster and BOTH my TBI tachs dont respond sadly.

Do you think they're dead from powering them via the MPFI wiring coming from the ECU? Everything else in the cluster works and I can even plug in the 91 tach and it works.

Unfortunately the tachs themselves are completely different design wise being that one is back lit and the other is the crappy front lit needle. Also the spacing and center hole for the face plate is slightly different.

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