Rovergauge and odd stepper motor behaviour

[taylormj4]

Had some rough idle and high idle problems so plugged in Rovergauge.
No fault codes, all sensor inputs seemed correct.

Pulled the stepper motor and overhauled it (as I hadn't done this for years). I used Rovergauge to eject the stepper motor plunger, cleaned and greased it and refitted the unit to the plenum.

Drive to work was fine but on way home, idle goes to 2500rpm on start up and stays there all the way home. This burnt the clutch a bit and having to brake quite hard down hills to stop it running away !

Hooked up Rovergauge and started the engine. Revs are at 2500rpm but Rovergauge showing everything OK. So I checked the plenum butterfly and hoses into the plenum for air leaks, all OK.

Pulled the stepper motor and found the lunger fully retracted, then observed something interesting: the stepper motor plunger was fully retracted so with it still connected to its wiring but out of the plenum I used Rovergauge to operate the stepper. Sending open and close (255 step) signals using Rovergauge had no effect on the stepper position (it buzzed but did not move). However, the indication on Rovergauge showed it moving from fully open to fully close.

So two things/questions:
1. I had thought that the stepper motor fed back its position to the ECU and this is what Rovergauge was reading but it appears that the ECU is telling Rovergauge where it thinks the stepper is but it may not actually be in that position at all. Can anyone confirm that or advise what the ECU is reading ?

2. The stepper appeared to be stuck fully retracted. After some light manipulation and use of Rovergauge to step the plunger out, it moved out. I have relubricated it and reassembled the stepper and it now appears to work correctly. However, it seemed to work correctly before so maybe I'm looking at a replacement. Anyone found they can get stuck open before ?

Thanks.

[Hoover.]

you are probably better off posting this in the Griffith forum where Stevesprint and Blitzracing do mots of their posting / lurking.... as these two seem to be the ones doing most of the experimenting and understanding of the Roverguage stuff

[Colin RedGriff]

You're right. There isn't any feedback from the stepper motor to the ECU to tell it what position it is in. The Stepper motor resets to a base position when you turn off then engine (you should hear it buzz. The ECU sends the instructions to open and close the stepper but it assumes the stepper motor has done as it is told. This can cause problems obviously if the stepper motor is not operating properly.

[taylormj4]

Colin RedGriff said:

You're right. There isn't any feedback from the stepper motor to the ECU to tell it what position it is in. The Stepper motor resets to a base position when you turn off then engine (you should hear it buzz. The ECU sends the instructions to open and close the stepper but it assumes the stepper motor has done as it is told. This can cause problems obviously if the stepper motor is not operating properly.

Thanks.
Everything was fine again last night after I stripped and regreased the stepper motor. Started up this morning and 2500rpm again. It seems the stepper is getting stuck in the fully open position, which is where it parks when you turn the engine off I think. Oddly though, if I remove the stepper and use Rovergauge I can close the stepper. Maybe removing it is enough to free it up again.

So I have now manually (Rovergauge) set the position of the stepper to get a good idle and then disconnected it. So I have at least a temporary fix.

A couple more questions that have come to mind:

A) How does the ECU get the engine speed (rpm) information - reason for asking is I noticed that when I went open loop (above circa 1800rpm) and then opened the throttle, I could get normal engine braking, which suggested that the stepper had closed. So maybe the stepper is working but the ECU isn't instructing it correctly ?

B) What needs to happen for the stepper motor fault code to come up. It doesn't even come on if you disconnect the stepper.

[QBee]

Have been reading this thread thinking "I must have misunderstood".
I always thought that the only function of the stepper motor was to step in when the idle rpm drops and stop the car from stalling.

I associate a very high idle with an air leak anywhere between the AFM and the engine, or an incorrectly adjusted throttle pot, or even muck in the flutterby jamming it open, or a sticking throttle cable.

[davep]

Steve Heath thoroughly explains idle and stepper motor control in his ECUMate manual:

http://ecumate.com/docs/Ecumate%20inst.pdf

See pages 37 onwards.

The high idle and stepper control code is currently a topic of interest in the 14CUX thread on the Griffith forum.

[taylormj4]

QBee said:

Have been reading this thread thinking "I must have misunderstood".
I always thought that the only function of the stepper motor was to step in when the idle rpm drops and stop the car from stalling.

I associate a very high idle with an air leak anywhere between the AFM and the engine, or an incorrectly adjusted throttle pot, or even muck in the flutterby jamming it open, or a sticking throttle cable.

Hi QBee, that is the main function of the stepper motor but if it sticks full open, it allows a lot of air to bypass the throttle so even with the throttle fully shut, the engine can reach 2500rpm.

I believe I am correct that there is also another function of the stepper, which is to raise the idle when the car is moving so that it doesn't drop as low between gear changes, preventing a tail-out due to engine-braking torque application to the rear wheels.

I went through exactly the same thought process as you described, checked the throttle was closing when pedal released, looked for air leaks in the elbow and all the vent pipes to the plenum and then checked and cleaned the butterfly and plenum entry. I removed the stepper initially as I thought it may have cracked/failed and was letting in air.

[taylormj4]

davep said:

Steve Heath thoroughly explains idle and stepper motor control in his ECUMate manual:

http://ecumate.com/docs/Ecumate%20inst.pdf

See pages 37 onwards.

The high idle and stepper control code is currently a topic of interest in the 14CUX thread on the Griffith forum.

Thanks Dave,
Iid link this over to the Griffith thread if I knew how lol !

[ukdj]

taylormj4 said:

How does the ECU get the engine speed (rpm) information - reason for asking is I noticed that when I went open loop (above circa 1800rpm) and then opened the throttle, I could get normal engine braking, which suggested that the stepper had closed. So maybe the stepper is working but the ECU isn't instructing it correctly ?

The output of the speedo transducer (mounted on the diff) is fed into one of TVR's magic black boxes where it is crudely interpreted and fed to the ecu. The output of the black box provides a basic feed that tells the ecu the car is moving once it goes over a nominal speed <10mph from memory

[davep]

ukdj said:

The output of the speedo transducer (mounted on the diff) is fed into one of TVR's magic black boxes where it is crudely interpreted and fed to the ecu. The output of the black box provides a basic feed that tells the ecu the car is moving once it goes over a nominal speed <10mph from memory

The signal from the black box should look like this graph produced from data logged from a test run conducted at various speeds on a Chimaera using RoverGauge:



The black box signal is discussed here, page 27 about half way down:

http://www.pistonheads.com/gassing/topic.asp?h=8&a...

Edited by davep on Wednesday 10th June 09:56

[blitzracing]

Its perfectly possible that the stepper is getting jammed in the fully open position, but there is an easy fix- just pop the stepper shaft out with Rovergauge by putting in 255 steps closed, but make sure you get ready to catch the shaft as it pops out as its spring loaded. Then put some small washers on the shaft to stop the cone pulling back so far - from memory you need 3-4 mm worth of packing. This will limit the peak airflow and stop the high idle and it should work normally the rest of the time. Problem is the stepper has a steel components inside the motor that corrode over time so it can start sticking, even without any carbon on the shaft. Realistically you dont need an idle that ever goes above about 1200 rpm, so the loss of a few mm stepper motion is of no loss.

Edited by blitzracing on Wednesday 10th June 12:24

Edited by blitzracing on Wednesday 10th June 12:29

[taylormj4]

blitzracing said:

Its perfectly possible that the stepper is getting jammed in the fully open position, but there is an easy fix- just pop the stepper shaft out with Rovergauge by putting in 255 steps closed, but make sure you get ready to catch the shaft as it pops out as its spring loaded. Then put some small washers on the shaft to stop the cone pulling back so far - from memory you need 3-4 mm worth of packing. This will limit the peak airflow and stop the high idle and it should work normally the rest of the time. Problem is the stepper has a steel components inside the motor that corrode over time so it can start sticking, even without any carbon on the shaft. Realistically you dont need an idle that ever goes above about 1200 rpm, so the loss of a few mm stepper motion is of no loss.

Edited by blitzracing on Wednesday 10th June 12:24

Edited by blitzracing on Wednesday 10th June 12:29

Hi Mark,
Thanks for coming on the thread. That's an awesome fix - why the heck didn't I think of that !
Have already ejected the stepper shaft out with Rovergauge (so much more civilised than the old noisy technique!) and carefully cleaned and lubricated it but to no avail.

I have been driving since with the stepper manually set to give a warm-engine idle of around 850rpm and then unplugged. The car is so much mre driveable - over-run is really smooth with no shunting. I'm loathe to reconnect the stepper but I will probably need it in the colder months.

What signal does the ECU send to the stepper to move it ? Is it a series of pulses ?

Thanks.

[blitzracing]

Exactly that- it burst fires a series of pulses, then waits for the idle to change. If its wrong a fires another set, then waits. This system is fine as long as the engine speed drops as expected, but fails if the idle changes too fast for other reasons, so the idle becomes unstable.

[spikep]

Did the modification with the washers to stop the plunger retracting full on Friday. Appears to be working a treat as the idle has not gone above 1,200 rpm.

Now got to see if it help at the other end when the engine is hot with cutting out at junctions. Was thinking of a way of stopping the plunger going fully out, like a small weld lump on the side of the pointer.

Thanks for the top tip.

I

[QBee]

I had this issue two years ago, immediately after de-catting the car. Idled fine when cold, could not hold any idle when hot. Tried everything suggested, eventually Rob @ V8D worked out that the stepper had become temperature sensitive. Changed the stepper for another used original TVR one - stepper never missed a beat since.

One tip - if the engine is dying like that, just make sure the electrical plug is pushed well home. I loaned my fully wrking stepper to a friend at a track day on Friday to prove that his stepper was not responsible for his 2750 rpm idle. Proved the point. Put it back in my car. Car started but woudn't idle. Took the plug off, then put it back on but pushed it in more firmly - got my idle back.