So over the weekend the local U-Pull-It had 50% off everything. Picked up a complete 13b(manifold to flex pipe, oil/water lines and injectors on the rail for $100) Also picked up a tach from the same Talon. This all came from a 90 car. I thought the tach was for a 90 but it appears to have been changed out since it doesn't work in my 90 car!
After a little investigation I found that there are atleast 3 different tachs for the Eclipse cluster:
90 NT: 6k redline with 8k limit. Has label "A" on it. Has controller board type 1. motor has 55.6ohm resistance.
91+ NT: 7k redline with 9k limit. Has label "B" on it. Has controller board type 2. motor has 53.3ohm resistance.
91+ Turbo: 7k redline with 9k limit. Has label "C" on it. Has controller board type 2. motor has 53.3ohm resistance.
90NT
91+ NT
91+ Turbo
We all know that only a 90 tach will work on a 90 harness and ECU/tach filter. But from what I found messing with them yesterday I think there is a way to hack the board to work. Going off my table above I found a few similarities.
The two motors are swappable but they respond differently. For my situation I can only use the controller board from the 90 to run at all. If I put the 91+ Turbo tach motor on that controller board it wants to idle around 3k instead of 1200-1500. I know my car has a consistent idle and has been for months so I know 100% that its just the tach is reading too high.
The 90 tach works off a voltage somewhere in the 6v+ range. When my car is at idle it was giving 6.5v. When I try to rev it to 2.2-2.5K (by ear) it changed to 6.7v. So that means that the motor just goes up as the voltage goes up. Makes sense.
I figured that since the motors have different ohm of resistance that might be the only difference. So I took the two tach motors off their board and plugged them up to a 1.57v power source. That somehow made them rev way higher than in the car???
The 90NT Tach read 5150RPM from a 1.57v power source.
The 91+ Turbo Tach read 7650RPM from a 1.57v power source.
I then played around with a few resistors I had and found a really small adjustable resistor. I dialed it to around 22ohm of resistance and it made the 91+ Turbo Tach read just like 90NT Tach (5150RPM from a 1.57v power source). I put that resistor in the car and plugged it all up to see what it does. It did nothing...Read just like it did without the resistor.
I don't know enough about hardware and circuits to figure the problem out exactly but I think I am on the right track. From what I found the motors can be changed out and run off the same controller board. That board just sends a different voltage for the different style (A, B, C) and size (A, B, C) Tach interfaces. If we can adjust the power that is sent to that motor to adjust it to fit the interface (scale it down on the newer tachs) they should work just fine in a 90 car with all the 90 parts.
Anyone who knows a little more about this than me care to lend a hand? I have all the parts and most of the know how
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