mcapinha

mcapinha

Active Member
For sometime now I've been trying to solve the problem with the Cruise Control not engaging in my DSE. This problem was intermittent, then got solved when I refurbed the alternator and then it presented itself again, with the CC never working since then. Being intermittent lead me to believe that it's a bad wire somewhere.

Sticking it on diagnostics it shows that the BECM is seeing the correct signal for the clutch pedal (it changes state when the pedal is depressed) but that the ECM is always seeing a LOW signal.

Looking at the CC electric diagram on RAVE, I understand that the clutch signal to the BECM is HIGH when the pedal is not pressed and LOW when the pedal is pressed. But now, for my troubleshooting, I need to know how this signal is passed between the BECM and the ECM, ie, is it also LOW for pedal not pressed and HIGH for pedal pressed, or maybe it's the other way around :confused:

Pic for clarity:
2z3qw5h.png



For now, I'm going to check the wire between C626 (BECM) and C572 (ECM), first for continuity and then to see if it is grounded (as I think this would cause a LOW signal on the ECM).

Comments, help ? :)
 
mcapinha said:
For sometime now I've been trying to solve the problem with the Cruise Control not engaging in my DSE. This problem was intermittent, then got solved when I refurbed the alternator and then it presented itself again, with the CC never working since then. Being intermittent lead me to believe that it's a bad wire somewhere.

Sticking it on diagnostics it shows that the BECM is seeing the correct signal for the clutch pedal (it changes state when the pedal is depressed) but that the ECM is always seeing a LOW signal.

Looking at the CC electric diagram on RAVE, I understand that the clutch signal to the BECM is HIGH when the pedal is not pressed and LOW when the pedal is pressed. But now, for my troubleshooting, I need to know how this signal is passed between the BECM and the ECM, ie, is it also LOW for pedal not pressed and HIGH for pedal pressed, or maybe it's the other way around :confused:

Pic for clarity:
2z3qw5h.png



For now, I'm going to check the wire between C626 (BECM) and C572 (ECM), first for continuity and then to see if it is grounded (as I think this would cause a LOW signal on the ECM).

Comments, help ? :)
Click to expand...

Get diagnostics and see if BECM lists cruise control as fitted. If it is not selected as fitted in BECM it will not work.
 
wammers said:
Get diagnostics and see if BECM lists cruise control as fitted. If it is not selected as fitted in BECM it will not work.
Click to expand...

Yup, it's activated on the BECM. Checking with the Nanocom also reads that everything else is OK: main button works as expected and both buttons on the steering wheel also read OK. The brake pedal switches are also behaving correctly. The only thing that seems off is the ECM not reporting a change when the clutch pedal is depressed. If the ECM expects a LOW signal when the pedal is pressed (like the BECM), having it constantly LOW will prevent the CC from engaging (as pressing the clutch will disengage the CC).
 
mcapinha said:
Yup, it's activated on the BECM. Checking with the Nanocom also reads that everything else is OK: main button works as expected and both buttons on the steering wheel also read OK. The brake pedal switches are also behaving correctly. The only thing that seems off is the ECM not reporting a change when the clutch pedal is depressed. If the ECM expects a LOW signal when the pedal is pressed (like the BECM), having it constantly LOW will prevent the CC from engaging (as pressing the clutch will disengage the CC).
Click to expand...

Ok that's one idea out the window then. Maybe a ground problem on ECU. Try a ground to ECU case i seem to recall someone doing that to solve some problem can't remember what.. Also think it is the cruise selector switch that should set the ECU to high to allow cruise function. Clutch only sets it to low and overrides it to disengage. You maybe looking in the wrong place.
 
Last edited:
Think you maybe going about this the wrong way. ECU will never read high (Cruise engaged) whilst car is stationary. Cruise must be switched on by selector switch, then and only then ECU can be switched to high (cruise engaged) by left button on steering wheel, at above around 26/28 MPH. It can then be disengaged by depressing clutch. It can be re engaged, providing the speed has not dropped below 26/28 MPH by pressing resume button. Right hand button on steering wheel. Which will return the speed back to that previously set. If speed has dropped below 26/28 MPH left button must be pressed to re engage above 26/28 MPH. Not sure about manual but there will be a switch somewhere that prevents cruise activation in certain gears, without looking it up i am not sure, but maybe it only works in top gear. That isolator switch could be your problem.
 
Cruise control not working is normally the rubber pipe that goes to the cruise control bellows. The pipe being split (just behind the air suspension control box) in the engine bay. Ultra simple fix.
 
stopover said:
Cruise control not working is normally the rubber pipe that goes to the cruise control bellows. The pipe being split (just behind the air suspension control box) in the engine bay. Ultra simple fix.
Click to expand...

I Don't think it tells you that in diagnostics :p:)
 
Success!!!

After two hours, sore fingertips and bruised knuckles, my cruise control is working again!

I removed both connectors from the ECM and BECM and checked pins 20 and 26 for continuity. As I was expecting, there was none. So I traced the wiring to a plug that is connected to a black box on the right hand side of the engine. Tried to remove the plug with no success but while I was at it the blue/brown wire that I was chasing came loose, revealing no spade connector on its end. Checked the plug side with the multimeter and it showed continuity to the ECM side. Checked the loose cable and it showed continuity to the BECM side. By this point I was already happy knowing that I wouldn't need to trace the entire cable for a break. I managed to push the loose cable further into the protecting rubber that goes into the plug and I got continuity from the BECM to the ECM. I'm very happy at this point. So, fired her up, grabbed the nanocom and, behold, the ECM is reporting the clutch as HIGH!

Took her for a spin and the CC is working just as expected :)

Regarding the comment about the CC only working in 4th and 5th, that is incorrect. The CC engages at least in 2nd (just did it on mine). Also, the ECM doesn't now in what shift the gearbox is.

Looking back at this problem, that I've been chasing intermittently for some time now, 2 things threw me off my track: the CC troubleshooting in RAVE doesn't mention this wire anywhere. Just checked it again before starting tonights job and there's no reference to this connection. The other thing was that I assumed that the information known to the ECM, about the clutch state, was the same as the BECM's. Still don't know how I missed this in the ETM.

As a side note, it feels like I now have better response at low rpms. I have a steep incline just after a narrow curve before getting to my house and most of the time it would feel like the engine was about to stall and it doesn't seem to be doing that now. Maybe the wrong clutch pedal info was messing something up.

Happy P38 ownership day for me!
 
Last edited:
mcapinha said:
Success!!!

After two hours, sore fingertips and bruised knuckles, my cruise control is working again!

I removed both connectors from the ECM and BECM and checked pins 20 and 26 for continuity. As I was expecting, there was none. So I traced the wiring to a plug that is connected to a black box on the right hand side of the engine. Tried to remove the plug with no success but while I was at it the blue/brown wire that I was chasing came loose, revealing no spade connector on its end. Checked the plug side with the multimeter and it showed continuity to the ECM side. Checked the loose cable and it showed continuity to the BECM side. By this point I was already happy knowing that I wouldn't need to trace the entire cable for a break. I managed to push the loose cable further into the protecting rubber that goes into the plug and I got continuity from the BECM to the ECM. I'm very happy at this point. So, fired her up, grabbed the nanocom and, behold, the ECM is reporting the clutch as HIGH!

Took her for a spin and the CC is working just as expected :)

Regarding the comment about the CC only working in 4th and 5th, that is incorrect. The CC engages at least in 2nd (just did it on mine). Also, the ECM doesn't now in what shift the gearbox is.

Looking back at this problem, that I've been chasing intermittently for some time now, 2 things threw me off my track: the CC troubleshooting in RAVE doesn't mention this wire anywhere. Just checked it again before starting tonights job and there's no reference to this connection. The other thing was that I assumed that the information known to the ECM, about the clutch state, was the same as the BECM's. Still don't know how I missed this in the ETM.

As a side note, it feels like I now have better response at low rpms. I have a steep incline just after a narrow curve before getting to my house and most of the time it would feel like the engine was about to stall and it doesn't seem to be doing that now. Maybe the wrong clutch pedal info was messing something up.

Happy P38 ownership day for me!
Click to expand...

Well done. ;);)
 
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