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TPS/ISC Reset (87-89)


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If your car has an erratic idle and the RPM's either drop or raise up when you are just idling at a stop you need to first address this by Resetting the TPS/ISC. All you need is a $15 multi-meter. Take it one step at a time.


Mr. HP says, "This is from Mike C. I wrote a procedure too, but this is a little easier to follow."


1. Before warming up the engine, pry out the rubber end of the TPS connector so you can stick a voltmeter in there later. Do the same on the ISC motor position sensor (MPS) connector. The ISC connector should be the rear-most 3 wire connectors on the throttle body: yellow+blue, yellow+black, and yellow+red wires on the wiring harness side and red, green, and black wires on the end going into MPS on the throttle body.  Pry out the MPS end of this connector - it should be the end pointing upwards.  The TPS is the black cylinder at the bottom/front of the throttle body.


2. Loosen the accelerator cable. Get lots of free-play in there since the idle climbs as things warm up and this cable tightens too much if not enough slack is present.


3. Warm up the engine. Then shut it back off.  The engine must be fully warmed up.


4. Turn the key to ON but do not crank/start. Leave it there for at least 15 seconds. You should hear the ISC motor "zip" in as it commands a drop from fast idle to normal idle. If you want, stick a voltmeter on the motor position sensor (not the TPS) Black wire (-) and Green wire (+) and look for 0.9 volts after the motor zips back to the low idle position.  If you don't get 0.9 volts, there is a problem with the wiring harness between the MPS and the ECU.  Some cars seem to settle at 0.93 to 0.95 volts; that's close enough.  Turn the key off, wait a few seconds, and then turn it back on.  The ISC will "zip" again and after 15 seconds you should be back at the 0.9volt position.  Do this several times and see how consistent the voltage reading is; if it varies more than a tenth of a volt your ISC/MPS is either dying or your throttle linkage is binding.  Or your car isn't warmed up... if the ECU thinks the engine is cool it holds the idle higher and the MPS voltage is correspondingly higher.


5. With the ISC/MPS in the 0.9 volt position, remove the ignition key. Then unplug the ISC servo motor electrical connector. This locks the ISC at the position the ECU thinks will give a perfect idle speed. Run a jumper wire from harness Yellow+Red wire to ground to simulate a closed Idle Position Switch to the ECU.


6. Fire it up and look for 850 to 950 RPM. If it's not there, crank the ISC screw until it is there. This screw is the hex-headed one visible on the linkages if you look from the passenger side of the car.  A 7mm socket should fit it.  Make the adjustments in the tightening direction (i.e. increasing the RPMs); if you need to lower the RPMs go well below the correct idle speed and then creep back up.  This compensates for lash (slop) in the screw.


7. Now crank the old-fashioned idle screw at the base of the throttle body, the Fixed SAS, until the RPMs increase (touch point). Back off one full turn from the initial touch. This is the mechanical min-idle speed backup in case the ECU can't control the idle speed for some reason.


8. Shut down the engine, then turn the ignition key back to ON. Measure the TPS voltage from the Green+White leads (+) to Green+Black and look for the famous 0.50 volts. Move the TPS potentiometer as needed to adjust this. Get within +/-0.02 volts.


9. Turn the key off. Tighten the accelerator cable but leave a little slack, restore the rubber end caps to the connectors, and plug everything back in. Then unplug the battery for 30 seconds to clear any ECU errors.


Note: during step 4, watch the throttle linkages as the ISC does the "zip" movement. If you notice oscillations, there is binding in your throttle linkage and the ECU can't control the ISC well enough to do the job.


modified April 28th, 2002 by Mike C.  I changed the wire colors in step 1 to reflect the end of the connector easier to reach.  And a few other clarifications were done.

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  • 4 months later...

Well I'm a simple man so I get intimidated by all those words.  I think the following might be from Mike C as well, maybe not, might be from Tony Waldrup, but it's a bit condensed and it's the one I use.


The following procedure must be followed to the letter for best results.

01 - Engine must be at operating temp.

02 - After engine is warmed-up, turn it off.

03 - Turn key to the ignition on position.

04 - Connect digital voltmeter to #2 and #3 terminals on TPS sensor.

05 - Turn TPS all the way counterclockwise and snug bolts.

06 - Disconnect ISC motor connector next to firewall and wait 10 seconds.

07 - Turn TPS clockwise slowly to set voltage.

08 - Set voltage to 0.48 - 0.52 (optimal = 0.50).

09 - Tighten TPS bolts without changing setting achieved in step #08.

10 - Depress accelerator fully - meter will show up to 5.0V. This value is directly affected by the setting obtained in step #08.

11 - Loosen accelerator cable.

12 - Start engine.

13 - Set idle speed to 750 RPM's.

14 - Shut off engine.

15 - Tighten accelerator cable - 1mm free play.

16 - Plug ISC motor connector.

17 - Turn ignition to the on position for 10 seconds.

18 - Shut ignition off.

19 - Start engine.

20 - Idle will go up slightly (100 - 150 RPM's - this is normal).

21 - ISC and TPS are now set.

Note: Be aware that the ignition is turned on in step #03 and is kept "on", until step #14.

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  • 6 months later...

I just got done doing the above procedure... while my car idles better, i was curious of somthing..on step 4 KO you mentioned that if I stick the voltmeter to the green and black on the mps I should get 0.9volts... well I checked it twice and I got .4 volts... can you elaborate more on what I should check and what to replace if I need to.... The problem I am having is the car dosnt have the power it should especially uphills... could this be the cause?


Any help would be very much appreciated as I have been tryiong to figure this out for a year now and so far no luck....


thanks again..

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  • 10 months later...

About step 4:


My starion hit  .88 volts... That is very close to the .9 that it needed...


If you get a lower or higher voltage (not sure if up or down volt) you may not have the engine warmed up all the way or you have a temp sensor problem.

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  • 1 year later...

I have had the worst luck with ISC's........... Mine was throwing a code 6,  the ISC didnt "zip" it sorta went "dut... dut... dut... dut... dut...."  


I bought one from Ebay that failed completely. My brother sent me one he found that didnt work either.    I am going to pull them apart and see what i can see.


So..... I Put the original one back in and though it isnt right either.... after warming up the car idles fine... as long as i dont sit for more than 5 minutes ..... then it starts  surgeing progressivly worse and will eventually stall.......  But fires right up again and the cycle starts over.


Oh and it throws a 1 & 6 until it is warmed up then just a 1 while idling...


While cruising i get no codes.

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