E39 Overspeeds and kicks out of cruise

[Hambone]

Another speed issue that drives me crazy... If you "sneak up" on the limit (H0129 = 110 Kmh / 69 Mph), it works, shows "Speed Limiter" on the dash, no problem. If you floor it from a dig, it still says "Speed Limiter", but strolls right on by. If you set the cruise at 68 Mph, it works until a downhill stretch, says "Speed Limiter" on the dash and kicks off the cruise. Normally, the cruise isn't an issue, but this gets driven long distances, and unfortunately, the speed is required to be governed. What have I missed here? Why the difference in operation?????

[GMPX]

If you "sneak up" on the limit (H0129 = 110 Kmh / 69 Mph), it works, shows "Speed Limiter" on the dash, no problem. If you floor it from a dig, it still says "Speed Limiter", but strolls right on by.

I have no idea, that makes no sense at all, a speed limit is a speed limit, shouldn't matter where the throttle is, in fact it seems that it would work exactly the opposite how you would expect.

If you set the cruise at 68 Mph, it works until a downhill stretch, says "Speed Limiter" on the dash and kicks off the cruise. Normally, the cruise isn't an issue, but this gets driven long distances, and unfortunately, the speed is required to be governed. What have I missed here? Why the difference in operation?????

I am sure there is a tolerance in there that when your speed under control of the Cruise gets close to the speed limiter it will kick it out.
Eg, if your Speed Limiter is set to 70 MPH the Cruise Control might work up to say 67 MPH then the ECM will decide there is not enough 'give' between Cruise speed and Limiter and disable the Cruise Control.

[Hambone]

I have no idea, that makes no sense at all, a speed limit is a speed limit, shouldn't matter where the throttle is, in fact it seems that it would work exactly the opposite how you would expect.

That's what's so irritating about this. I have several different PCM's getting limiters, some work great, some do this. WHY??? Is there some other related or override table I don't know about?

I am sure there is a tolerance in there that when your speed under control of the Cruise gets close to the speed limiter it will kick it out.
Eg, if your Speed Limiter is set to 70 MPH the Cruise Control might work up to say 67 MPH then the ECM will decide there is not enough 'give' between Cruise speed and Limiter and disable the Cruise Control.

Setting the cruise so close to the limit is for the long distance drivers who have to put up with this limiter (mandated) even if the actual posted limit is higher in other states. It sux, but they may have to live with a max of 67 / 68. Just trying to eke out that last Mph for them.

[Hambone]

No solution for this? Have I found a limitation of this software??

[Hambone]

Well after many complaints, I was forced to return everything to stock. I guess this guy has more pull than the rest, so he got what he wanted. There are still other E39's to do, and I don't want this issue again. Is there something else to try? Perhaps a table I don't have access to?

[joecar]

Cruise control will disengage when it gets close to the limiter since there is not enough "room" for it... it's just how it works.


Why does your customer require the limiter set so low at 67 MPH, that is not safe (can't keep up with traffic, slow moving obstacle on freeway, can't get out of harm's way)...?

[swingtan]

I think there are some misconceptions here.

The cruise control will disengage when certain vehicle conditions are met.

• Activation of the brake
• Activation of the clutch (manual vehicle)
• Inability of the ECM to command sufficient torque to maintain set speed.
• Other factors such as torque management activation (wheel spin, traction control, over speed......)

This last point is an important one, especially if you mess with the stock speed limiter settings. When you go into "speed limiting" mode, the ECM enters torque management and disables anything that may try to alter the ECM's delivered torque. So because you have set the speed limiter so low, it's going to abort the cruise control to ensure the speed limiter setting is obeyed.

As for the speed limiter seemingly not responding when at open throttle, this likely to be due to torque management as well. The E39 will adjust the amount of available torque depending on the engine RPM and throttle position. Nailing it off the line will see the speed limiter setting reached in a different gear and at different RPM's than when lightly cruising up to the set limit. Given the ECM also self checks on what it's doing vs/ what is happening, it's likely that it can't control the engine torque sufficiently to arrest the vehicle speed, so it gives up after setting the dash alert.

Simon.

[Hambone]

Why does your customer require the limiter set so low at 67 MPH, that is not safe (can't keep up with traffic, slow moving obstacle on freeway, can't get out of harm's way)...?

That's up to Corporate, if you can figure out their logic, my hat's off to you. Coming from a motorcycle and pilot background, speed is a critical tool I would hate to lose. It must look good on some balance sheet for fuel or insurance i guess.

[Hambone]

This last point is an important one, especially if you mess with the stock speed limiter settings. When you go into "speed limiting" mode, the ECM enters torque management and disables anything that may try to alter the ECM's delivered torque. So because you have set the speed limiter so low, it's going to abort the cruise control to ensure the speed limiter setting is obeyed.

As for the speed limiter seemingly not responding when at open throttle, this likely to be due to torque management as well. The E39 will adjust the amount of available torque depending on the engine RPM and throttle position. Nailing it off the line will see the speed limiter setting reached in a different gear and at different RPM's than when lightly cruising up to the set limit. Given the ECM also self checks on what it's doing vs/ what is happening, it's likely that it can't control the engine torque sufficiently to arrest the vehicle speed, so it gives up after setting the dash alert.

Simon.

Good point, I didn't think about the torque limiter. I thought it was strictly a speed condition. Would you happen to know what the "headroom" is on the cruise though? If it goes down a big hill and picks up say, 7 mph with a closed throttle, will it kick off? At least that will give me a window to operate in while I look into this some more.

Why would the PCM "give up" though? Seems every other time there's a fault or something odd, it goes into failsafe or something. How can I tell if that's what going on here?

[GMPX]

Off th top of my head I am sure there is some PID's in the scantool that report Cruise status, more importantly why it decided to disengage, might be worth logging those to get some idea of what the ECM is thinking.