Tachs reading low, in dash and on SAFC

[Black91TSiAWD]

In my 90 the tach is off, in the cluster and on the safc, they are both at least 1000 rpm's lower than what they should be. But it kinda varies, it'll say its idling at 500-520rpm's and at 65 on the highway Im only 26-2700 rpms or something, which should be more like 3500. So it almost seems like its not as off at idle as it is at cruising speed. Kinda boggles me.

Anybody have any ideas as to why this is happening?

 

[Black91TSiAWD]

anyone?

 

[spyderturbo007]

Do the tach and the SAFC match? If so, how do know that they are 1000rpm off from where they should be?

 

[Black91TSiAWD]

The safc and tach do not match.

This is my 5th daily driver dsm so I know where the rpm's should be at all times.


Cruising at 65-70 my tach will read ~2500 SAFC will read ~1500. And in reality the motor is really at ~3500

 

[94cyclone]

Here are afew ideas to maybe get you started.

1. Decide which one you want to fix first (the tach or the safc) until you get one fixed ignore the other.

2. Your SAFC is tapped into a wire off your ecu. Cut the wire and attach it directly to the safc to rule out the possibility of a short. (I think this will fix your safc)

3. Your ECU may not be from a 90?

4. Do you have a logger? If so, what does it read? If not, it's your 5th DSM, it's time to buy one anyways!

Hope this helps!