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Idle Speed Steeper Motor 1990 Renix XJ

Black1990jeep

NAXJA Forum User
Location
california
On my 1990 Renix 6 cyl. XJ, I suspect an intermittent fault in the Idle Speed Steeper Motor....

Sometimes on start up the idle races at 2000 to 2500 rpm, sometimes it drops back down after seconds to minute. Sometime I turn off engine and restart and it maybe ok, or race a bit then slow down to normal. Most of the time there is NO problem however.

How does one bench test the idle stepper motor?
Does idle stepper motor operate by variable voltage, or pulses, or?
How does one test the signal going to the idle stepper motor?


I assume the ecu inputs that control the steeper motor are...
a.) Engine rpm.
b.) AC clutch on or off.
c.) Engine coolant temperature.

Is that correct?

Thanks!
 
If you try to bench test the stepper motor you are likely to launch the piston and spring.
The best you can do is to clean out the stepper motor well (noun), clean the stepper motor piston and rod then let a drop of thin oil slide down the piston rod into the motor.
The stepper is a tiny motor with a tiny worm gear drive. It is a bi-directional motor, forward and reverse. Pulling or pushing on the piston can damage the gears.
Sometimes a low voltage situation causes the stepper trouble, most times it is sticky gunk.
 
I just changed one out on my '90. Cost is about $60 IIRC. Turned out my issue was that a couple of intake bolts had backed off just a bit allowing air in. Sometimes it would fast idle at startup, other times it would fast idle after running for a while. As the intake heated up it would warp just a little bit, sometimes closing the gap and other times widening it. 9/16" wrench and about 1/2 an hour fixed it right up.


HTH
Todd
 
A more common item that causes this problem has to do with the stepper motor actuation but it is the ASD shutdown relay.

The reason for this is that the normal setting for the stepper motor is different if the engine is warm vs. cold.
When you turn off the key, the relay keeps the ECU powered for a couple of seconds, allowing it to send out the
series of pulses needed to reset the idle to where it needs to be for startup. If it fails, the air mixture is not set for
starting up and the idle is altered. After the engine runs for a little bit, the ECU looks at the RPMs and determines
if it is too high or too low and sends the pulses to the stepper to open or close the orifice via the pintle position
and that should get the idle RPMs back to where they should.

Just as a point of information. A stepper motor is not a regular DC motor where you apply power and it spins. It
is way more complicated. A series of pulses on 2 or more lines and the order in which they occur causes the motor
to move one step in one direction or the other. That is why the ECU computer has to drive the motor.
 
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I just changed one out on my '90. Cost is about $60 IIRC. Turned out my issue was that a couple of intake bolts had backed off just a bit allowing air in. Sometimes it would fast idle at startup, other times it would fast idle after running for a while. As the intake heated up it would warp just a little bit, sometimes closing the gap and other times widening it. 9/16" wrench and about 1/2 an hour fixed it right up.


HTH
Todd

Good point Todd.

Another thing is a bad sensor ground circuit.

www.cruiser54.com Tip 5.
 
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