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Cm870 VGT issue - marek4792 - 10-28-2018
So after replacing my CAC, turbo rebuilt and getting my truck mechanically squared out (Thank you Rawze and Mr. Hagg) I soon found out when ever I would try to sustain normal boost (25-37lbs) that the turbo would chuff rather violently. For some odd reason when commanding the turbo from 0-30% it would stay open but above 30% the turbo goes to 100% closed. The actuator arm starts to extend about 18 psi and maxes out about 60 psi but the circuit is pushing 120 psi and lower but still well above 60 psi. The air pressure does adjust when commanding the position which makes us think the ECM is doing it's job, but not low enough to retract the actuator rod until commanded below 30% which just returns the actuator to 0%. Is the turbo control valve circuit supposed to be regulated from the factory or is the turbo control valve suppose to regulate the pressure down to 60 psi and below? According to testing the minimum and max pressure of the actuator rod and its travel makes me and Rawze say yes. Another obversation that was made that the arm from the crosshaft seems to be a bit stiff possibly from the rebuild but still moves from end to end with little effort. Does this sound like a turbo control valve not doing its job properly? Did anyone have the same issue as I?
RE: Cm870 VGT issue - Unilevers - 10-28-2018
the control valve regulates the pressure down to 0-60ish psi. I believe max travel is assured at 60PSI.
i had one do this to me and it turned out the ECM was bad, just a thought.
RE: Cm870 VGT issue - marek4792 - 10-28-2018
So if 120 psi is getting to the actuator rod itself it almost sounds like I have a bad control valve then not sure if it applies to the older style control valve too? Rawze and I checked the voltage going to it as well and it did go up and down when commanding the turbo in 10% increments possibly ruling out the ECM and wiring?
RE: Cm870 VGT issue - Rawze - 10-28-2018
Here are some observations while he was here ...
He has a very old style of actuator system. All it consists of is a pwm (pulse width modulated / analog valve control signal) output from the ECM with zero feedback for position or pressure of any kind. It is unlike a lot of CM870's out there, and is an early version.
Something is wonky with it for sure. Even when we manually sent 0 - 60 psi air pressure to it with an external regulator connected right to the actuator piston, it did not move smoothly, but rather jerky. It would hang, then jump by 30-40% at a time or more as the pressure was changed slowly, indicating that perhaps the control arm was too stiff.
The PWM signal looked noisy due to the large coils of the actuator control on the o-scope, but it changed consistently whatever you commanded the thing to anywhere from 0% to 100%. The output pressure was form 0 psi to 120 psi at 100% consistently as well. - We tried 2 different actuators and control valves with the same results.
I monitored all the data in the ECU and it is commanding the turbo exactly as it is directed to without any derate, etc. - When it asks for 20-35% or so, all seems well and it somewhat moves close to these positions. Above this though, it start asking for 40% or 50%, and the air pressure matches this, but the thing jumps all the way closed instead of moving smoothly. when it asks for 40% or so, engine sees boost pressure low, demands more from the turbo, but by the time the arm overcomes the stiffness, it is already demanding max closing and it over-boosts/spikes like mad, chuffs, then has to back it off again. - I.E.> Wonky as all hell until it settles out.
- Something is very wrong with it somewhere. I have never messed with a CM870 myself, so I have no reference on how the turbo arm is supposed to move when applying air to it manually. Nor do I know how stiff it should feel when moving it by hand. I also don't know what typical pressure should result in what position.
-- He has apparently had this problem for a while, and that explains why he has blown out several exhaust manifold gaskets, etc. over this. I measured more than 90+ psi in the exhaust manifold when it would spike. It is also a very likely cause of why his CAC blew apart a couple times in the past.
While he was here, we pressurized his engine and found/fixed fixed the CAC, stopped a massive boost leak between his air compressor into his crank case, replaced all the exhaust manifold gaskets and polished it all up so it would hold. He also had YTS replace the guts of his turbo, but how stiff the control arm was when he got it back is in question at this point. It feels like the wiper ring (anti-carbon buildup ring) he used is perhaps too tight?. Maybe even if it is supposed to have one installed on that model at all?.
it could also be the ECM, or perhaps both?
RE: Cm870 VGT issue - Chamberpains - 10-29-2018
This must be the "high mount" control module. Thats an early simpler version. I haven't worked with any of those yet. If it was a "Low Mount" Control Module I'd be very leary of it operating correctly. I had mine a few years back that was all over the place and very intermittent with sending the wrong pressures to the actuator. Random low boost and 3rd stage jake being very weak or spiking violently. They are not well designed and can throw pressures all over the place at random. I've worked with 2 guys that bought brand new Holset modules and still had a bad one right out of the box but they are far better than any reman or Ebay garbage. But that doesn't seem to be where your problem is. It all sounds like it's in the actuator. I'd call Cummins directly (not a Cummins Authorized Dealership) and see if they have some differences in actuators between the earlier models.
Also, this may sound simple but I've used a drop of oil in my actuator to ensure it moves smoothly. Obviously these systems are a bit clunky compared to an electric controlled system so I believe they need all the help they can get to operate smoothly.
You've eliminated this as a possibility, but, I recently had an actuator that would only leak at 40% - 65%ish commanded. So you couldn't just simply run the actuator open close test to find this sucker leaking. I had to pull the dog box and drive it to hear the air pissing out in those commanded percentages. I didn't have an adjustable pressure regulator at the time to run it thru the complete operating pressure. I do now though.
Also I don't know if this helps, but thru watching mine, I've noticed that while driving, the commanded percentage generally stays within 3 different values. 30, 55, and 70% mine generally only falls a few percentage point either way off those 3 values during operation.
Quote: when it asks for 40% or so, engine sees boost pressure low, demands more from the turbo, but by the time the arm overcomes the stiffness, it is already demanding max closing and it over-boosts/spikes like mad, chuffs, then has to back it off again. - I.E.> Wonky as all hell until it settles out.
This is why I tell everyone with these systems that any little kink, pinch, minute leak, in line obstruction (do not use teflon tape at any of the connections to avoid in line obstructions) any problem at all with them and they over boost or get really erratic boost pressures. You have to go over all the piping with a fine tooth comb and ensure that it has absolute unrestricted commanded air pressure.
Rawze, if you happened to record the ECM commanded percentage with its correlating control module signal voltage and then finally what the final actuator control arm measured movement is, and posting that, it would be a HUGE help to any one diagnosing or narrowing down problems with this circuit.
RE: Cm870 VGT issue - marek4792 - 11-06-2018
So unfortunately this will the last update as the truck got wrecked pretty badly few days ago and probably will be written off. Anyways before that I ended replacing the control valve and shut off valve which seemed to control the spike mostly with the external regulator removed, but would still close the turbo too much at times and seemingly whenever it felt like it choking itself out and rolling coal so to speak. Idling the turbo would be fully open, halfway, or closed like it couldn't decide what position it wanted to be in.
I was made aware that the orientation of the center housing to the turbine section was the most likely reason for the actuator arm's stiff movement due to the vg ring fins dragging against the turbine section where they go into. Which I hadn't have a chance to try due to heading out and not wanting to risk breaking a clamp on short notice or midtrip. Unfortunately I won't ever find out if it was a ECM issue or a reorientation of the center section of the turbo was needed.
RE: Cm870 VGT issue - Chamberpains - 11-06-2018
Im wondering, did you guys never pull the actuator off and move the VG ring manually?
Also you may want to venture down the number crunching of buying the salvage back from insurance. That engine has a good bit of value if it wasn't damaged in the wreck.
RE: Cm870 VGT issue - Rawze - 11-06-2018
(11-06-2018 )Chamberpains Wrote: Im wondering, did you guys never pull the actuator off and move the VG ring manually?
I did that while it was here. Did not have a reference when I did so, but later got a chance to check another one.
The actuator arm was about 2 - 3 x more stiff on his than one that I tried several days after he left. I talked to YTS turbo about it and he said that he has seen that before. He said that some of the older turbos have a fixed ring in the exhaust housing for the VG blades to go into instead of a floating one. His Marek's was this older style. What happens with them sometimes is that when installing one of that type, the exhaust housing - to- main housing joint can get tweaked a tiny bit, making the VG blades drag against the slots they protrude through in the exhaust housing. He said it is not unusual to have to loosen the v-band clamp that holds the exhaust housing and move the main housing about 1/8" back and forth to find the place where it frees it up and centers it.
That is the most likely reason why it was stiff and would jump instead of operate smoothly. It just needed a bit of tweaking to free it up after it was put back on the truck, as it was this older style of turbo housing.