DTC 34 Stepper Motor Fault

[arveetek]

I've been experiencing a new problem the past two weeks. Seems to have started after spending some time in the woods recently at low speeds with the a/c on. Never overheated on the coolant temp, but I suspect some higher than normal underhood temps due to little airspeed may have caused the electronics to get a bit too warm.

First problem cropped up two weeks ago during that trail ride in the woods after a long day of slow speeds: rough running, smoking, clattering. Sounded like an injector failing. After getting back on the highway, the engine smoothed out and the clatter and smoke went away. I think either the PMD or the IP itself may have gotten a bit too hot. (PMD is remote-mounted on heat sink in front of driver's battery)

Two hours later I stop for food. Upon leaving the restaurant, the engine was hard to start, and then the timing seemed to be all over the place, along with an SES light.

Stopped for fuel, and then everything was fine again. SES light went out.

Next day she was hard to start, and again the timing seemed to roam back and forth, as the clatter would get loud and then soft, and the engine would speed up and down a little. SES light would come on during this.

I pulled codes and a myriad of DTC's (stepper motor, cylinder imbalances, etc). I decided to disconnect the battery cables and clear the codes. After doing so, everything went back to normal and no codes except for DTC 12 which means all clear.

Seemed to run good for the next week or so, other than some mornings she seemed sluggish and hard to start. I figured maybe failing glow plugs, but then suddenly she went to starting great again with no smoke.

Everything was great until this morning, and then she was really hard to start, she smoked and shaked, and she sounded like the timing was severely retarded. Finally got an SES light again. Checked it later and it was DTC 34 - Injection Timing Stepper Motor Fault; no other codes present. Has started fine and driven great twice now since this morning. Seems to be a random issue.

I know what the stepper motor is, where it is, and what it does, but what is the proper procedure for checking it / repairing it? Is it actually an issue with the stepper motor, or is it a problem with the PCM controlling it?

This is on the low mileage DS4 5521 I just installed a few months ago. Pump was reman a couple of years ago, engine showed 100K on odometer when I pulled it. Was running great on the engine I pulled it from until that motor lost compression in cylinder 1.

Any help or advice is greatly appreciated.

Casey

[greatwhite]

Sometimes, a faulty coolant temp switch will send erroneous data to the PCM. This causes timing advance errors when cold, or maybe when hot. Depends on when it fails.

Next time it does it, try unplugging the sensor and see if it clears. Without the temperature modifier, unplugging the sensor forces the PCM to use a fixed table in the calibration.

I'm not saying its your problem, but it doesn't cost you a dime to try and it's a quick/cheap fix if it is the issue.....

[arveetek]

Thanks for the info. I actually have a relay built into the temp switch system that disables the coolant switch until 30 seconds after the engine starts, fooling the computer into thinking that the engine is at maximum cold temps and advancing the timing and engine speed to max when first starting the engine. So in essence, the temp switch is unplugged every time I start the engine.

It happened again this morning: engine barely started, smoking terrible, with hardly any audible diesel rattle; SES light on. Then suddenly it advanced timing to full tilt making a terrible diesel rattle, and then settled into smooth idle. Sounds like the stepper motor is sticking?

Casey

[Kennedy]

I would first remove the relay and operate ECT sensor as normal just to elimnate this as a potential cause.

If no joy:

From there what you want to do is get a scanner on it and look at desired versus actual timing and see which direction it is missing be it under/over/both.

It almost sounds like it is sticking internally and unable to move smothly so it overshoots and can't correct. That or the ECT relay showed it intermittent cold temps and it is doing just what it is supoposed to do.

[arveetek]

Thanks John.

I will disable my ECT relay and go back to the OEM setup and then go from there.

Casey

[john8662]

It really needs to be timed after an injection pump replacement.

[arveetek]

It really needs to be timed after an injection pump replacement.

I did, using a genuine GM Tech II scan tool. Set TDC offset to -1.94.

Casey

[arveetek]

I returned the ECT switch back to factory last night. In the process I found some chafed wires that led to the switch, and then repaired them. She started normally this morning. Keep your fingers crossed that perhaps it was just a problem with the ECT circuit, and the PCM was getting confused on the signals.

Casey

[greatwhite]

Sounds like a very likely cause....

[arveetek]

Well, she's been running just fine for nearly a year....

Same symptoms started up again. What's weird is that the symptoms again occurred after the same scenario: slow speed driving while crawling around in the woods. Also, I had a fresh tank of home-brewed biodiesel.

Symptoms: hard to start when cold. The timing is not advancing. When she finally starts, tons of white smoke and the engine is shaking; no diesel rattle to speak of. 20 seconds later or so, suddenly TONS of diesel rattle (like a cold Powerstroke), and smoke goes away. SES light with stepper motor fail. Driving down the road you can hear the timing going back and forth from retarded to advanced.

I pulled battery cables and reset the PCM. Started normally this morning. Then after work, wouldn't start again. Disconnected coolant temp switch. No change. Finally started with tons of white smoke; then again timing went full advance with lots of diesel rattle. Finally settled down and ran normally.

I'm starting to suspect that the biodiesel is causing an issue. If the optic sensor doesn't read through the fuel correctly, can it confuse the PCM and not control the stepper motor? Or is it possible that the stepper motor is just sticking?

I currently have a full tank of regular diesel and am trying to see if it clears up. I haven't run the bio all winter until now...

Thanks,

Casey

[john8662]

Air in fuel?

I think it's time for a clear hose on the output of the injection pump to see what's going on.

J

[Yukon6.2]

Hi Casey
Your discription sounds just like my tow truck was starting for the longest time.It's parked inside all the time.I figered it was an air leak,but as long as it would start and run it was low on my list of to do's.Anyhow one day i noticed the voltage gauge was reading low,the alt was not charging it was flaky for a while.
Anyhow after i changed the alt,it has started great,all smoke and rattle is gone.And the tach is now steady,it used to fluctuate with blinkers,headlights turned on.
Hope you find an answer
Thomas

[arveetek]

New alternator, new batteries. Fuel system in excellent shape; new fuel lift pump and Racor fuel filter system in past year. If you recall I had the issues with the sock in the fuel tank getting clogged, so every inch of the fuel system has been gone through in the past year.

Casey

[arveetek]

Same scenario this morning: didn't want to start; unplugged coolant temp switch - no change; finally started and ran rough, puffing tons of smoke. Ran for a few minutes with no change; shut down and then restarted - this time it started up with more diesel rattle and smoke went away. Ran normally after that.

I cleared codes yesterday. This morning SES light came back on. Only one code - 34 stepper motor fault.

It's really acting like the stepper motor is stuck after a cold soak.

Casey

[arveetek]

Doing some more research reveals that it is possible for the stepper motor to stick. Looks like usually it's the actual motor that sticks, not the timing advance lever that it controls.

However, I put a healthy dose of 2-stroke oil and Power Service in the tank today. We'll see if that helps any. If not, the stepper motor will probably need replaced.

I have my old DS4 pump still, so I will probably take the motor off of it if I need to.

Casey

[arveetek]

The Tahoe has started normally and ran great the past two days. I'm really leaning towards a sticking timing advance lever. It makes me think that this particular DS4 does not like my home-brewed biodiesel...... After a tank of regular # 2 and some 2 stroke oil, everything seems back to normal.

Casey