rb20det tach output problems

[Jonh317]

My tach output is starts at 5v at idle then it drops while rpms are increased.This isn't related to nistune hardware, but i am hoping that someone might know what might cause this.

[kenbam]

Tach output would be an increasing frequency signal. Sounds like you are reading it with a multimeter set to volts. Multimeters read very slowly so your voltage reading is going to be basically an average of the voltage signal based on off/on time. Try changing the meter setting to frequency and see what happens.

[Jonh317]

In freqency mode it is going from about 9v and then decreasing with increased rpm, i noticed in the service manual it is supposed to be 1 to 2.4 at 2500rpm. The diagram has the tach signal going to the tach and then splits to go to a v in a circle then to a ground i believe.

[Jonh317]

Does anyone have a diagram of the ecu circuits, i am going to guess that something inside is knocked loose or burned out cause it not to allow a accurate tach signal.

[Matt]

sorry i dont. might be easier to hunt on ebay / wreckers for another ECU to try out

[PL]

Should just be a matter of tracing the track on the board inside the ECU from the tacho pin to whatever device it goes to. It'll be a transistor in "open collector" configuration.

If you're an electronics type guy then you could try to identify the transistor or just grab one from a dead ECU. Another option would be to trace back an unused output (like errr - EGR solenoid) and steal the driver transistor from it.

Depends how much time you have and how patient you are!

PL

[Jonh317]

I just got another one and replaced it and now it is working. My ecu had shorted out recently, due to a random loose wire in the harness, but i fixed that and got a replacement ecu and its working fine. The loose wire is probaby from when they took the motor out of the car since i got a motorset. I didn't try tracking it down in the ecu because i did not know what i should be looking for.