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idle stepper

skipc

NAXJA Forum User
I've had a persistent high idle, and very high starting rev, on my 89 XJ. Decided to look at it today. Cleaned the stepper pintle and it's seat. Looked fine. No change.

Disconnected various vac lines, and noticed that each disco would up the idle (as it should), but when reconnected, it would stay down, until driven or restarted.

So, it seems the stepper is doing _something_... So, while it was down at about 800 rpm in N, I disconnected the stepper. Now it stays at a proper idle at start and all conditions (not tested cold starting yet).

Does anyone have a diagram or specs for the stepper so I can test it without the exerciser? It's just a regular stepper, but what's the pinout or voltage rating?

Any ideas on what might be going on otherwise?

Thanks,
Skip
 
skipc said:
I've had a persistent high idle, and very high starting rev, on my 89 XJ. Decided to look at it today. Cleaned the stepper pintle and it's seat. Looked fine. No change.

Disconnected various vac lines, and noticed that each disco would up the idle (as it should), but when reconnected, it would stay down, until driven or restarted.

So, it seems the stepper is doing _something_... So, while it was down at about 800 rpm in N, I disconnected the stepper. Now it stays at a proper idle at start and all conditions (not tested cold starting yet).

Does anyone have a diagram or specs for the stepper so I can test it without the exerciser? It's just a regular stepper, but what's the pinout or voltage rating?

Any ideas on what might be going on otherwise?

Thanks,
Skip

Bi-polar stepper motor. Do a search and you will find plenty about it since it's a common component for modern EFI engines. I bought one of these - http://www.electrokits.com/electronic-kits/motor-control/81.htm I don't intend to wire into my XJ but I'm gonna keep it in the XJ as an emergency fix when the idle goes crazy. I can connect it and bring the idle back down and/or qualify if the stepper motor is working properly.
 
On our Renix-era (up to 1990) 4.0 engines, a high idle is a common sign of a TPS that is either dying or out of adjustment. My 1990 is on its third since new. The idle stepper motor is fine- it is just doing what it is supposed to, but the computer is sending it wrong instructions due to faulty TPS signal. Replace the TPS, and adjust to spec (or have a mechanic with the right connectors for Renix do it on his scope- more accurate).
 
Thanks guys. I tested the TPS with an ohmeter and it was within range and smooth, but will revisit it. What mfgs are good ones and which are not for the TPS?

On the stepper, that controller uses 5-30v. Is ours a 12v motor? Do you have a wiring diagram for the motor connections? I was thinking of doing something like that and manually controlling the idle. So far, even cold, works great. Although it's a bit unnerving to have it so low (vs before), since it has the CPS stall/no-start problem once in a while :repair:. At worst the idle dips just a bit on hard steers, like a carb model would. :doh:

Thanks.
 
4xBob said:
On our Renix-era (up to 1990) 4.0 engines, a high idle is a common sign of a TPS that is either dying or out of adjustment. My 1990 is on its third since new. The idle stepper motor is fine- it is just doing what it is supposed to, but the computer is sending it wrong instructions due to faulty TPS signal. Replace the TPS, and adjust to spec (or have a mechanic with the right connectors for Renix do it on his scope- more accurate).

I agree in part but my ECU seems to have an intermittent driver circuit that sometimes prevents the IAC from closing. 'Been through a couple different TPS potentiometers properly adjusted and qualified wiring harness. I have a good understanding of adjusting the TPS component. All it takes is for one of the IAC signals failing to reach the IAC and it will not move from it's previously "parked" shutdown position left by the latch relay.

skipc said:
Thanks guys. I tested the TPS with an ohmeter and it was within range and smooth, but will revisit it. What mfgs are good ones and which are not for the TPS?

On the stepper, that controller uses 5-30v. Is ours a 12v motor? Do you have a wiring diagram for the motor connections? I was thinking of doing something like that and manually controlling the idle. So far, even cold, works great. Although it's a bit unnerving to have it so low (vs before), since it has the CPS stall/no-start problem once in a while :repair:. At worst the idle dips just a bit on hard steers, like a carb model would. :doh:

Thanks.

There are 2 pairs of motor coils which are paired together on each end of the connector and this can be verified by measuring about 50 ohms between the motor coils. To identify the magnetic polarity of each winding I tested an old IAC with a couple different connection combinations (there's not very many) until I found the polarity of each coil pair that worked with the driver circuit. I also used roughly 5 volts which is what I guessed the Renix uses for it's stepper motor driver output. However, it would probably be fine with anything between 5-12 Vdc.

For your stalling condition you should be aware that Renix will hunt for idle and the throttle body butterfly should be adjusted for a base hot idle of around 600 RPM with the IAC fully closed. Of course, the TPS will need to be re-adjusted to account for the new idle throttle position.

CPS issues? I spliced the connector out of circuit and haven't had CPS trouble for many years. 260k + miles on my '89 - the head's never been off yet.
 
I thought of spllicing around the CPS connector, but tried Ox-Gard in it (carefully) last time and so far so good. We'll see though...

I'm about 750 rpm idle in Park, and a little lower in D now. Any reason why I can't just leave it this way? I would assume that if I ever got in a situation that needed it changed (sub zero starting perhaps?) I could reconnect it and reset it later, right?

ALSO, do you know if the outputs on the ECU to the stepper are open collector or ones that I could just parallel a manual switch arrangement to adjust idle without blowing the driver?

Thanks again.
 
skipc said:
I thought of spllicing around the CPS connector, but tried Ox-Gard in it (carefully) last time and so far so good. We'll see though...

I'm about 750 rpm idle in Park, and a little lower in D now. Any reason why I can't just leave it this way? I would assume that if I ever got in a situation that needed it changed (sub zero starting perhaps?) I could reconnect it and reset it later, right?

ALSO, do you know if the outputs on the ECU to the stepper are open collector or ones that I could just parallel a manual switch arrangement to adjust idle without blowing the driver?

Thanks again.

Hey bud, cold weather isn't gonna matter much to idle speed once engine is warmed-up. The only need for a working IAC is cold start, air-conditioning and SMOG test. I've driven mine for long periods of time with it disconnected. However, the wife doesn't understand or care about cold-start and keeping it running until warm. So, I had to figure out a method that even she could correct a run-away idle speed.

That's where the driver board came into play. I powered mine with 6 AA batteries, stuffed into a project box, sourced a female connector from the junkyard and now it's in the XJ tool bag. If/when idle goes high, unplug the OEM IAC connector and connect the home made "excersizer" to re-adjust the idle. If that doesn't work, you know the IAC is hosed. Leave long enough wiring and you can operate the box from the driver seat for off-road idle speed adjustments.

I never tried it in-parallel with the ECU/IAC driver output but it would probably work.
 
XJXJ said:
The only need for a working IAC is cold start, air-conditioning and SMOG test.

Interesting - SMOG TEST. I know about the cold start, so far so good. I help it a bit, but no big deal.

What about the TEST? Is it just a visual that it's still connected, or does it actually DO something? The reason I went this way is FOR the test. I figured a 1400rpm idle would fail, so I adjusted it down and left it.

It's usually a visual inspection, turn off, turn on, anal probe (car, not me) 2000 rpm for a minute, then idle. What could go wrong?? I think we have a dyno test now, not just the 2000 rpm in Park, but haven't ever been through that yet.
 
skipc said:
Interesting - SMOG TEST. I know about the cold start, so far so good. I help it a bit, but no big deal.

What about the TEST? Is it just a visual that it's still connected, or does it actually DO something? The reason I went this way is FOR the test. I figured a 1400rpm idle would fail, so I adjusted it down and left it.

It's usually a visual inspection, turn off, turn on, anal probe (car, not me) 2000 rpm for a minute, then idle. What could go wrong?? I think we have a dyno test now, not just the 2000 rpm in Park, but haven't ever been through that yet.

Aside from the visual inspection, as I understand it - idle mixture is a large part of many SMOG tests. If the IAC is disabled, the ECU will not be able to dial-in a very precise mixture at idle. I assume you/we will probably fail that part of the test with IAC disabled.

In my case, the ECU/IAC works correctly 98% of the time so I just leave it OEM and let it do it's normal thing for daily driving and SMOG. The 2% when there are problems (only upon startup in my case) I simply connect the "excersizer" and bring idle back under control. Even my wife can handle that so it's a small annoyance for driving an old XJ. She's also been "trained" how to do NSS bypass for the starter solenoid when that little problem comes around.

I like the way the engine starts with IAC disabled. It doesn't ramp-up very fast/high and I think that is probably "slightly" better for the engine bearings until some oil cushion is established. Very minor, but something to think about for really weird stuff...
 
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