Tach won't go past 4k

[Tsi_User]

I have searched on the forums and can't seem to find someone with the same tach. problem I am having.

Months ago my stock tach stopped going past 4k (this is both while stationary and while moving). At the time I tried out an after market tach tapped into the RPM wire off of the ECU harness and it would rev all the way up to redline. Now i've read that it could be a problem with the Ignition Module, but why would an aftermarket one read fine? Also, if I remember right, when I still had my data logger the RPM readout seemed to be normal. Could it just be the actual motor in the gauge cluster itself, or should I still look into the Ignition Module?

 

[redeclipse7782]

Yes the gauge can go bad, as long as it doesnt sound like its hitting the rev limited when it gets to 4k then its the gauge.

 

[Tsi_User]

as long as it doesnt sound like its hitting the rev limited

Sorry what does that mean?

 

[bling5tatus]

The rev limit... that would be as far as the ecu allows your car to rev... normally your rpms will "bounce" off the rev limiter.

 

[Tsi_User]

The rev limit... that would be as far as the ecu allows your car to rev... normally your rpms will "bounce" off the rev limiter.

Ah okay I gotcha. Nope it's definately not hitting the rev limiter. So do you guys suggest I try a different Ignition Module or or try replacing the tach? I have a tach for a NT but would this work at all in a Turbo gauge cluster?

 

[Tsi_User]

I took a look at the gauge cluster, it seems to have the same mouting points but I don't want to mess anything up by putting it on anyone have suggestions?

 

[redeclipse7782]

Give it a shot, yours is already broken, it cant get any worse.

 

[Tsi_User]

Give it a shot, yours is already broken, it cant get any worse.

I don't want to break anything else though, that and if the tach is reading but at the wrong RPM I wouldn't want to jack up the motor..

 

[luv2rallye]

At the time I tried out an after market tach tapped into the RPM wire off of the ECU harness and it would rev all the way up to redline. Now i've read that it could be a problem with the Ignition Module, but why would an aftermarket one read fine?

Which "RPM" wire (ECU pin) are you talking about? On the '91, the stock tach gets its signal from pin 4 (white wire) of the PT power transistor (ignition) module which also goes to ECU pin 109 (to tell the ECU to keep the fuel pump going). Inside the PT module it "NOR's" the 2 coil signals together to output on pin 4. So the PT module may indeed be flakey in this internal NOR circuit (while the ignition may be just fine). Connect an after market tach to this wire to see.

Added on 10/04/07: I should point out that although the NOR circuit could fail strangely (eg. it doesn't pass 1 of the 2 signals), this is extremely rare. If it fails it is much much more likely to not put out a signal at all. So your stock tach guage is far far more likely the problem (especially since your engine is getting proper spark). As I said you can connect an after market tach to this wire to see if the signal there is correct.

 

[Tsi_User]

Which "RPM" wire (ECU pin) are you talking about? On the '91, the stock tach gets its signal from pin 4 (white wire) of the PT power transistor (ignition) module which also goes to ECU pin 109 (to tell the ECU to keep the fuel pump going). Inside the PT module it "NOR's" the 2 coil signals together to output on pin 4. So the PT module may indeed be flakey in this internal NOR circuit (while the ignition may be just fine). Connect an after market tach to this wire to see.

Added on 10/04/07: I should point out that although the NOR circuit could fail strangely (eg. it doesn't pass 1 of the 2 signals), this is extremely rare. If it fails it is much much more likely to not put out a signal at all. So your stock tach guage is far far more likely the problem (especially since your engine is getting proper spark). As I said you can connect an after market tach to this wire to see if the signal there is correct.

When I tapped the after market tach it was i believe a green wire right out of the ECU harness, that's all I can remember. I guess I assumed this was the wire that runs from the ECU directly to the gauge cluster, and that's why I thought it was the tach working inproperly instead of anything else. Where exactly are you suggesting I tap the after market tach? Sorry your post was really helpful, i'm just a little noobish when it comes to electrical problems

 

[luv2rallye]

Where exactly are you suggesting I tap the after market tach?

Pin 4 (white wire) of the PT power transistor (ignition) module which should be near the coil (it has an 8 pin connector).