Lazy stepper motor?

[Naybr]

My Chim is reluctant to idle properly, but it does get there eventually e.g. I drive up to traffic lights and take my foot off the gas. The engine 'idles' for 30 - 40 seconds at about 1300 RPM and then drops down to a smooth 950 RPM. I've never noticed this happening before.

I've removed the stepper and cleaned it thoroughly inside and out, and also cleaned the electrical connections.

Ideas anybody...?

[Alexdaredevilz]

Mine does that, I just live with it,

[phillpot]

No amount of cleaning would cure mine but a new stepper motor (about £25 Ebay) fixed it.

[Naybr]

I'm still fiddling with this problem ...

A bit more investigation at the weekend and it dawned on me that the Stepper Motor only 'steps in' when the car is stationary. That's when the revs do drop to about 950 RPM, so the stepper is working as it should.

So to re-state the problem, the idle is too high when the throttle is closed, but the car is moving - as in coming up to a road junction. I've had a look at the linkage and cleaned the throttle butterfly to no effect. I'm now wondering about a faulty throttle pot ..

Opinions or ideas please?

[blitzracing]

Its worth checking the throttle pot voltage when the throttle is shut (so it knows its at idle setting). Also unplug the stepper when the engine is warm, and see it the idle is stable when you go for a run, as there are various fueling issues that can make the ECU hold the RPM high.

[Naybr]

Thanks - I'll try both of those. What should the throttle pot voltage be?

[blitzracing]

The limit range with it closed between the red and green wires is .083- .547 volts, as the 14CUX can self calibrate too a certain extent, and you may find that you dont have elongated holes on the throttle pot to allow you to tweak it. It quite permissible to elongate the holes with a needle file if you wish to set it and the general target voltage you should aim for is .3 volts closed and at least 4.7 volts at WOT. If you are significantly out on these voltages the ecu will either not know the throttle is shut on the over run,(too high a voltage) and likewise it not give you full power fuel enrichment if the voltage is below 4.7 volts at WOT.

[jr6yam]

Blitz, I keep getting refused access to your website?
Get a 'forbidden on this server' message (at work and home)
This is a shame as it's a mine of useful information
Any idea what's happening?

John

[TVR Beaver]

One fix here (but not the answer) is to restrict the air flow in the pipe feeding the stepper .. Is will have it fall to 950 (or what ever it's set to) as soon as you come off throttle.
If you've had the ECU plug off and I thing the stepper motor, it can take a few cold starts before it re-set's itself...
I think this is part of the rear wheel lock up protection... I know when mine has done this, if I've disconected the speedo input (so the ECU thinks it's not moveing).. it drops straight away...
There is a box feeding the ECU somewhere converting this moving / not moving signal.. but I'm not sure where it is and if this is it's exact function (may be Mark can tell us)....

The only other way to corrcet this is in the ECU and a trip to seee Mark Adams..

[blitzracing]

jr6yam said:

Blitz, I keep getting refused access to your website?
Get a 'forbidden on this server' message (at work and home)
This is a shame as it's a mine of useful information
Any idea what's happening?

John

OK, Ill go and pay the web site provider what I owe them to keep in on line then. Should be back soon.

[Naybr]

Thanks for advice.

It occurred to me this morning that maybe I hadn't tried all the simple stuff. I've just cleaned & lubricated all the linkages and adjusted some (well quite a lot actually!) of slack out of the throttle cable. This seems to have made a difference. I'll drive the car a bit more over the w/e and see.

Incidentally the slack in the throttle cable is presumably because the stop above the pedal wasn't set. I hadn't even noticed it there before. It's not that I'm heavy right-footed or anything

[Jonny_uk]

Have you checked the speed sensor ? This tells ECU when car is stationary and whether to drop to idle ete, all in the bible

[blitzracing]

TVR Beaver said:

One fix here (but not the answer) is to restrict the air flow in the pipe feeding the stepper .. Is will have it fall to 950 (or what ever it's set to) as soon as you come off throttle.
If you've had the ECU plug off and I thing the stepper motor, it can take a few cold starts before it re-set's itself...
I think this is part of the rear wheel lock up protection... I know when mine has done this, if I've disconected the speedo input (so the ECU thinks it's not moveing).. it drops straight away...
There is a box feeding the ECU somewhere converting this moving / not moving signal.. but I'm not sure where it is and if this is it's exact function (may be Mark can tell us)....

The only other way to corrcet this is in the ECU and a trip to seee Mark Adams..

The speed sensor feeds the variable road speed signal into a box of electronics under the dash,( marked something like speedometer conversion) that in return produces a fixed frequency pulsed signal into the ECU. It only produces this signal once the car is moving, and once the ECU gets this signal it stops the stepper motor from trying to control the engine speed. I cant see how this would control the rear wheel lockup as the road speed is effective fixed as the ECU sees it, so it could not work out if the road speed was too high for any given gear as you changed down. There was a speed limiter on the original Range Rover mapping that a fixed rate pulse would overcome, but its supposed to have been removed in TVR mapping. It certainly has been on the 3.9 catalyst map Ive fitted to various Ginetta's, that where previously limited. I have come across a case where this fixed signal was present all the time, so the stepper was permanently disabled, leading to an fairly random elevated tick over once the car was warm. This is not the same as the symptom of the rev's stepping down as you come to a stop, as the stepper is working, albeit a bit late. The fix of restricting the airflow to the stepper is really masking another fault, as if all the ECU sensor inputs and fueling is correct (and no air leaks) this elevated tick over should not occur.

[TVR Beaver]

blitzracing said:

TVR Beaver said:

One fix here (but not the answer) is to restrict the air flow in the pipe feeding the stepper .. Is will have it fall to 950 (or what ever it's set to) as soon as you come off throttle.
If you've had the ECU plug off and I thing the stepper motor, it can take a few cold starts before it re-set's itself...
I think this is part of the rear wheel lock up protection... I know when mine has done this, if I've disconected the speedo input (so the ECU thinks it's not moveing).. it drops straight away...
There is a box feeding the ECU somewhere converting this moving / not moving signal.. but I'm not sure where it is and if this is it's exact function (may be Mark can tell us)....

The only other way to corrcet this is in the ECU and a trip to seee Mark Adams..

The speed sensor feeds the variable road speed signal into a box of electronics under the dash,( marked something like speedometer conversion) that in return produces a fixed frequency pulsed signal into the ECU. It only produces this signal once the car is moving, and once the ECU gets this signal it stops the stepper motor from trying to control the engine speed. I cant see how this would control the rear wheel lockup as the road speed is effective fixed as the ECU sees it, so it could not work out if the road speed was too high for any given gear as you changed down. There was a speed limiter on the original Range Rover mapping that a fixed rate pulse would overcome, but its supposed to have been removed in TVR mapping. It certainly has been on the 3.9 catalyst map Ive fitted to various Ginetta's, that where previously limited. I have come across a case where this fixed signal was present all the time, so the stepper was permanently disabled, leading to an fairly random elevated tick over once the car was warm. This is not the same as the symptom of the rev's stepping down as you come to a stop, as the stepper is working, albeit a bit late. The fix of restricting the airflow to the stepper is really masking another fault, as if all the ECU sensor inputs and fueling is correct (and no air leaks) this elevated tick over should not occur.

Indeed.. That's why I said it's not the answer... But the singal to the ECU... I thought the box of tricks feeding the input came into play so it didn't drop straight to 900 rpm and in theory causing the rear wheels to skid if say you were powering totaly off in a wet bend?.. But as you say.. the real reason is to trigger the stepper in to action a zero speed.. (does it work in reverse?)... or may be it has 2 functions .. both of the above?... (so the stepper only works when the car's totaly still??)
But how does this go wrong (will read the Bible).. I assume it can stop giving the output or continualy give an output?
But your right... all things working okay you should not have this issue anyway...

Edited by TVR Beaver on Friday 12th November 15:53

[blitzracing]

Umm... interesting idea about the anti skid, So the stepper would have to wind itself out as you moved above idle to lift the minimum tick over to say 1200 rpm, and then wind it back down as soon as the car stopped, and I must admit Ive not physically checked for stepper signals when the car is moving, Ive only taken the info from the Lucas manuals for the speed sensor, and done stationary tests. I don't really think the system is that smart, and Ive only ever seen it step down slowly when something else was wrong but I'm quite happy to be proved wrong. I always assumed the reason why the tick over became higher was simply the engine was warmer with the stepper still positioned for a cold idle so it then corrected it for the new temperature. I guess an easy test would be to simply drive the car, then depress the clutch and lift the throttle but leave the car rolling, and see if the idle always holds at say 1200 rpm. You would need to stop between a couple of tests to check the idle was say 800 rpm between tests. Any takers?

[TVR Beaver]

blitzracing said:

Umm... interesting idea about the anti skid, So the stepper would have to wind itself out as you moved above idle to lift the minimum tick over to say 1200 rpm, and then wind it back down as soon as the car stopped, and I must admit Ive not physically checked for stepper signals when the car is moving, Ive only taken the info from the Lucas manuals for the speed sensor, and done stationary tests. I don't really think the system is that smart, and Ive only ever seen it step down slowly when something else was wrong but I'm quite happy to be proved wrong. I always assumed the reason why the tick over became higher was simply the engine was warmer with the stepper still positioned for a cold idle so it then corrected it for the new temperature. I guess an easy test would be to simply drive the car, then depress the clutch and lift the throttle but leave the car rolling, and see if the idle always holds at say 1200 rpm. You would need to stop between a couple of tests to check the idle was say 800 rpm between tests. Any takers?

Yes.. thats what they do... I can be going at 70 mph... put the clutch in and it will drop to 1200 rpm... but it will only drop to 900 when I go below about 3 mph. Although when i'm going up the box and put the clutch in, im sure it goes to 900 straight away? Mmmm... not sure myself now?...
Wouldn't the steper screw out to hold it at 1200 and adjust for tickover... (but still be out).. I assume it then goes in when you are going?.. As you say.. may be this is a bit too complex..

[blitzracing]

It all falls apart at the seams however with a fixed speed pulse that the TVR uses, as the ECU has no way of knowing if you are increasing or decreasing in speed, so why would it hold the idle in different states? I can believe it might simply say we are moving, so dont let it drop below 1200rpm........

[TVR Beaver]

Yes.. but there are other inputs that show you are accelerating.. it going into open loop, Lambda feedback, RPM etc... I'm not talking a slight speed increase.. more like putting your foot down a bit...
Or may be mine is just running a bit rich..
you know the post I put up the other day about adjusting the MAF volts.. well before it didn't do this ..
then it did it a lot (as i'd taken the ECU out).. and now it does it a bit as it's re-set itself after a number of miles....
who knows ? (MA does).. but will the rest of us ever find out

[blitzracing]

Ill ask MA on this one, hes always helpfull, and it will always help the overall understanding.

[TVR Beaver]