Question: tachometer on my 90

[eddie20875]

i was wondering if anyone can help me on this.the tach on my 90 is about 4-500 rpms higher than what it actually is. when i hook up my scanner it reads approx 800 yet the tach on the dash is showing about 1200.

my question is: does the computer read the rpms that the tach is showing or does it read the actual rpms? i ask this as i am having trouble with the iac number being to high at an idle. i have replace the iac and cleaned the throttle body and all the other specs are in perfect range. i installed new plugs and the number dropped some but is still too high. the tach showing the wrong rpm is the only thing that i can see wrong.

hope someone can answer this or may have another idea of something i can do to lower the number. thanks

 

[JAKE]

What's your IAC count number? Are we talking Open or Closed loop?

Jake

 

[JAKE]

By the way, there is a procedure for re-setting the IAC, but it's rarely necessary to use it.

Jake

 

[eddie20875]

closed loop. the number should be anywhere from 20-40 and if you can get it under 20 that would even be better. mine is about 75. according to what i read you should drive the car at approx.40 mph then it should reset itself. other than that the manuals say there is really no other adjustment.

 

[JAKE]

closed loop. the number should be anywhere from 20-40 and if you can get it under 20 that would even be better. mine is about 75. according to what i read you should drive the car at approx.40 mph then it should reset itself. other than that the manuals say there is really no other adjustment.

There is a manual re-setting procedure which may not show up in your FSM. It involves disconnecting and re-connecting the IAC under certain conditions to establish a new zero base.

Another thought is your IAC's pintel may have a little carbon build up requiring it to retract farther to establish the necessary airflow in order to maintain the programmed RPM. The pintel can be removed from the IAC, cleaned, re-installed then re-set.

Another way to lower the count is to slighty open the minimum airflow setting on the throttle body. This would cause the pintel to extend farther, lowering the count.

Jake