i have a 1989 300te that idles low,about constant 575 rpm when warmed and it will stall mostly as one is slowing to a stop.
i have another 89 300te that works good,holds a steady 700-750 rpm and never stalls. so out of the good one i have exchanged the computer behind battery,swaped the ovp,exchanged the idle control valve -,fuel pump relay and all the exchanged parts kept the good one good and didnt change the stalling one at all.
in addition i have checked the throttle idle switch,and the decel switch.
absoutley about to throw in the towel, on the good car the only difference i can see is that the voltage across the idle control valve input starts from about 9-10 vdc when cold and decreases to say 5.8 dc when warmed to 80c and does not change to increase the idle speed when placed in gear or loading down with the a/c. the good one seems to start with a little higher value when cold say 10-12 dc across the idle control valve but it will regulate to a higher value when the tranny is selected to a gear and a/c will also cause the idle valve to be increased,resulting in a good control of 700-750 rpm.
i am not sure how the full control is done but i did check the vacuum and it is 17 in at low rpm and goes up to 20 at fast(about 2000 rpm)
i did test the ezl ignition vac and it held 15 on the mitty mac,
any suggestions would surely be appreciated
larry perkins lou ky

Check the airflow pot.

__________________
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

thanks, i checked the pot and did see one small difference between the stallind one and the good runner,when i am observing the stalling one i see a real fast ol(over load-meaning an open ckt) when i cycle the pot. if i stop anywhere i can never get the ol to read again,just when its in motion.
we use wire wound pots in the scale business and its common to see the open when cycling the wiper.
i suppose i could replace the sucker to prove the point.
i was looking at the values across the idle control valve today and accidently left the vom on an and saw that it had 6vac across the supply. does the n3 controller have a pulsed output to the idle control valve?
thanks larry perkins

The control of the idle valve is a constant frequency pulse with a variable duty cycle. I think the extent of control is from about 30% to about 70%. It you were looking at this with a voltmeter you would see 3.6v to 8.4v if the source was 12v.

The issue of the air flow pot is that the system stays in a steady no control "waiting for idle" mode till the idle switch closes the decel switch closes and the airflow pot output goes below .7v. At that point it should add control to acquire a stable idle at the specified speed.

__________________
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

thanks for the tech info, i made a harness to allow trouble shooting of the signal values of idle control system.
today i will record values from cold to hot and see if i can identify the fault.
do have one question that i cant read and find, on the idle control valve,does the air flow indicator point toward the engine or throttle base? i have 2 89300t3 and found the idle control valve installed in oposite directions.
thanks again
larry perkins lou ky

thanks in advance
larry perkins lou ky