Tach goes down instead of up ?

[Big Red 211]

On my 90 TSI I just swapped the ECU with one from a 93 TSI cause a cap burnt on the 90 ECU. After I installed the 93 ecu the tach will go down when I'm accelerating then it levels off when I take my foot off the gas. Before with original 90 ecu the tach would jump around (about 100-200rpm) at any engine speed. It did that all the time and I'm guessing that's why the 90 ecu burnt up.
The car runs fine. The tach just isn't working right. Also I did switch the 2 wires at the ecu terminals.

Anyway, if anyone knows why my tach is going the wrong way please lmk, I don't want to burn up another ECU.

Thanks

 

[sjgYFZ200]

isnt it true that very early ECU's cant be used with later ones?

 

[i1nk83]

you are going to have to go and find either a 91+ tach/cluster or swap back in a 90 ecu

 

[Big Red 211]

Damn, I never heard about that before. I guess I'll have to get my old one fixed.

 

[jott5555]

another thing to check that can mess with your tach is a small "noise filter" it was only on 90's its on the drivers side of the intake manifold. the connection is very close to where the knock sensor connects. its a small 1in x .5in silver box with i beleive 2 wires comming out of it..

now from satan this little trinket is over 50 dollars i beleive. if you know anyone in the area that has a 90 dsm bum his or hers plug it in and see if you still have a problem.