OK, before all of you go telling me to search, try reading my entire post before you say anything. My tach needle worked fine for the past few years, but has recently began bouncing at random. It seems to like being accurate around idle, but when the motor is rev'd it likes to either bounce or drop down to zero. Now hang on, I have a PFC, and both the datalogit and commander read out perfectly, so I know the sensor is working, and the signal is reaching the PFC. I have already swapped out a known working tach and it seemed to work fine for a few minutes before it began acting up again. I also have already re-soldered all the connections on the back of the original tach as well as replaced the capacitors on the board, and still am having the same symptoms. Now, I have a few questions.

Does anyone have any other ideas of what might be causing the problem? I was thinking possibly a loose connection/corroded wire between the PFC and the cluster, but wanted to make sure.

Do any of you know where the tach signal pins into the cluster, or if it would be possible to run a new wire from the PFC to the tach or the cluster directly? I thought I read somewhere that the tach gets its signal from the speedo computer, but want to be sure.

And finally, If what I remember reading about the tach/speedo computer is correct, would a bad speedo computer cause any other symptoms I might have missed. (The speedo, odometer, and trip meter work perfectly)

Thanks guys/gals, Clint

 

check that your crank sensor wheel is not hitting the sensors. this happened to me and my PFC read normal but my tach would jump all over at anything other than idle. just look for rub marks on the sensors.

 

I know you said you tried replacing the tach with a working one, but did you try using a known working board?

 

I will check that today.

Sorry, I meant a known working cluster.

 

I have the same problem, I done an engine rebuild and the tacho was working fine before.

I'll check the crank sensors today.

 

I would look toward the CAS, like 85GSL said, or something around the ignition pickup. If that's wompy it'll make the tach move, too.

 

which one of the cas sensors runs the tacho on the FD?

I have a had a quick look today and both look fine, it could be the wiring.

 

Ok, I figured out what the problem was in my situation, but it might not help the rest of you. A few months ago I installed my Viper alarm and remote start, this required splicing the alarm into the tach signal wire coming out of the ECU (The Yellow and Blue one, pin 2B) As shown in the picture.



I disconnected this wire and noticed the tach started working normally again, so I thought the signal to the cluster might be weak due to the alarm being tapped in. I need this signal to the remote start because it kills the engine if the engine is brought above idle for more than 2 seconds. Now that I figured out the problem, here's the solution I came up with.

I had a few different resistors laying around my shop, so I tried a few different ones until I found one that allowed me to connect the signal wire to the alarm and still have the tach work correctly (I believe it was a 1 ohm, but I'm not positive). I soldered that in between the ECU signal and the alarm. I remote started it and brought the rpms up slowly, but it didn't cut out around 1500 like it normally does. I reran the tach signal learn on the remote start system (which only took about 2 minutes), tried again, and it worked perfectly!

Like I said, this might not help many of you, since I haven't heard of many 7's with remote start, but everything seems to be working fine now.

 

I will check this wire tomorrow, I looked at the sensors on my crank and they look fine.