Tach no worky!

[Crazy91GSX]

Hey guys.. lets see if you can point me in the right direction. I have a 90 laser turbo. FWd, and the tach doesn't work. I noticed that I could get it to work for a minute when I washed the engine bay.. But it would only jump around a bit, very unaccurate, then would settle back down to 0. I changed the tacho interface and gauge cluster from a buddy's car, and it didnt fix it. is there anything else that could work??

Adam

 

[styx]

Hey guys.. lets see if you can point me in the right direction. I have a 90 laser turbo. FWd, and the tach doesn't work. I noticed that I could get it to work for a minute when I washed the engine bay.. But it would only jump around a bit, very unaccurate, then would settle back down to 0. I changed the tacho interface and gauge cluster from a buddy's car, and it didnt fix it. is there anything else that could work??

Adam

disconnect the harness from the ignition coil clean the pins spray them off with wd-40 (wd stands for water displacer not lubricant so people stop using it as that use silicone spray thank bored.com for that info) to get the moisture out. see if that works. from the ignition coil the tach signal runs into a low pass RC filter for noise suppression from the impulses of the ignition which will produce noise across almost the entire spectrum of frequencies. check the filter by measureing the pin with a multimeter for resistance i am not sure what the spec but as long as it is under a megaohm even that is high maybe some one can measure theres for you. oh ya clean those pins to wire brush them and stuff. and try to poke at the harness and see if it will work if you apply pressure a certain way on it, if the issue is under the hood and you get some response when it is when sounds like oxidation of the pins, if that doesn't help then we can get into how to actually test for signals and see where the signal is preterminating


the spark plugs are fired in sets of 2 so each time on the 90 style (or gate) it will produce a spike, this can be tested with an led wired up in series with a resistor, or test light to the tach signal pin.

the 91 style uses and open collector nand gate to generat a signal and will only produce a tach signal when both power transistors fire.

thats why you only need a few dollars in parts from radio schak to make a 90 ecu compatible with a 91.

now the 92 style uses an (or not gate) its like the engineers could stick with one. this is why it is so easy to interface the tach signal with any year ecu all you need is just a couple of gates which cost $0.50 each usually maybe a dollar at most.