P38A Cruise control question ( wheel speed issue )

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AndyF07

Active Member
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134
Location
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Hi,

I've searched the forums for this question but can't find an answer anywhere. Hopefully the vast knowledge of the forum can help.

The CC on my 4.0 2002 P38 doesn't work. I have tracked the issue down to the BECM wheel / road speed output on pin 20 which goes to the CC ECU. I believe it should have a 5v reference when stationary and a modulated waveform when moving?? It currently reads a steady 0.4v so I am assuming it is kaput. All other electrical and mechanical elements of the CC are good.

Now the CC is a nice to have rather than a must have as I don't do a lot of road miles with her so am not going to the risk and hassle of changing the BECM as it works perfectly in every other way.

I was wondering if I could get the CC working by tapping into the speed signal from somewhere else , eg. straight from the ABS module or one of the other output pins for the speed on the BECM? Does anyone know if the signals are the same and this is feasible or even sensible?

Any advice greatly appreciated as always :)

Andy
 
Or you could do an ohms test on a suspect sensor compared to another. If both have signals with interference when the wheel is turned then it could be else where.
If a suspect wheel has too low or too higher ohms reading an doesn't respond to the wheel rotation then it could need tapping in gently to meet the ring inside the hub and it will push it out to the correct airgap. If still no reading on a dvm when rotated the sensor could well be dead 🤔
Have you got any abs faults?
 
pwood999 said:
Check the ABS for faults first. If I remeber correctly, the ABS speed output goes to BECM, and then to Cruise & other modules. RAVE will tell you which wires.
Click to expand...
Mark Piercy said:
Or you could do an ohms test on a suspect sensor compared to another. If both have signals with interference when the wheel is turned then it could be else where.
If a suspect wheel has too low or too higher ohms reading an doesn't respond to the wheel rotation then it could need tapping in gently to meet the ring inside the hub and it will push it out to the correct airgap. If still no reading on a dvm when rotated the sensor could well be dead 🤔
Have you got any abs faults?
Click to expand...

Thanks guys. I will chuck a scope on the abs module later when home. I think it's fine as I don't have any ABS faults and it seems to work as best I can tell. If I remember the wiring diagram off the top of my head it feeds the speedo amongst other things which is fine. Best to be certain it's ok though and if not I can get after the sensors.
 
AndyF07 said:
Hi,

I've searched the forums for this question but can't find an answer anywhere. Hopefully the vast knowledge of the forum can help.

The CC on my 4.0 2002 P38 doesn't work. I have tracked the issue down to the BECM wheel / road speed output on pin 20 which goes to the CC ECU. I believe it should have a 5v reference when stationary and a modulated waveform when moving?? It currently reads a steady 0.4v so I am assuming it is kaput. All other electrical and mechanical elements of the CC are good.

Now the CC is a nice to have rather than a must have as I don't do a lot of road miles with her so am not going to the risk and hassle of changing the BECM as it works perfectly in every other way.

I was wondering if I could get the CC working by tapping into the speed signal from somewhere else , eg. straight from the ABS module or one of the other output pins for the speed on the BECM? Does anyone know if the signals are the same and this is feasible or even sensible?

Any advice greatly appreciated as always :)

Andy
Click to expand...

Most likely a pin in a connector not making good contact.
 
The Nanocom guide for Wabco-D only shows one connection from ABS to BECM on C1584, pin-3. I'm guessing this is the speed signal ?

1722327684799.png
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1722327484908.png


1722327464489.png


1722327746619.png
 
pwood999 said:
The Nanocom guide for Wabco-D only shows one connection from ABS to BECM on C1584, pin-3. I'm guessing this is the speed signal ?

View attachment 322656View attachment 322655

View attachment 322654

View attachment 322653

View attachment 322657
Click to expand...

For some reason I thought it was 4000 pulses per mile. Maybe they doubled up between C and D? Or maybe I just misremembered.

I'm guessing there's a connector behind the A-post for it? Famous place for bad connections.

The EAS needs a speed signal too. Wonder what that shows?
 
Thank you everyone so far.

The plot thickens. A scope on the wheel speed feed to the CC ECU shows a pulsed waveform between 0-12v after. a quick drive which looks fine so I believe that was a wild goose chase. All of the other inputs to the ECU look to be fine also.

The only tests from RAVE I have not completed are those for the Engine ECU which shows connector CO411 (55 pin). Looking at my engine ecu I can't see a similar connector. I wondered if this was from the GEMS engine? If so is there a pinout for a the THOR ECU and a guide on what to test for?

I guess it could be I have been unlucky and the 2nd hand CC ECUI I got off Ebay is faulty too.

Cheers
 
C0411 designation is for the diesel ECU. The Thor cruise wiring is basically the same as GEMS, but Thor also has the additional wire to the diagnostic connector. Nanocom might help ?

I have the later AMR5700 in one of my GEMS P38 via an adapter cable I made. The other P38 has the standard AMR1173.
 

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pwood999 said:
C0411 designation is for the diesel ECU. The Thor cruise wiring is basically the same as GEMS, but Thor also has the additional wire to the diagnostic connector. Nanocom might help ?
Click to expand...
Thanks Pete. I am trying to work out now if the engine computer can disable the cruise control on petrol models as there appears to be no check other than for the diesel in the RAVE fault testing procedure?
 
Read the first section in the RAVE description. The engine signal is only required for manual cars with a clutch. I assume your BECM is set correctly ?

When you went through all the other troubleshooting checks did the bellows engage properly ? If not, chekc the small 3-4mm pipes inside & the relay valve. One of mine has a worn valve & doesn't engage about 10-20% of the time. Few bumps in the road later it engages !!


1722421738431.png
1722421881134.png
 
pwood999 said:
Read the first section in the RAVE description. The engine signal is only required for manual cars with a clutch. I assume your BECM is set correctly ?

When you went through all the other troubleshooting checks did the bellows engage properly ? If not, chekc the small 3-4mm pipes inside & the relay valve. One of mine has a worn valve & doesn't engage about 10-20% of the time. Few bumps in the road later it engages !!


View attachment 322735View attachment 322736
Click to expand...

The 4.0 litre was available as a manual.
 
Nanocom diagnostic would definitely help. All the Inputs and up to 41 fault codes !!

1722423670398.png
1722423683585.png


There's also an option to Enable / Disable cruise in the BECM settings !!

1722423877139.png
 
Thank you everyone for your advice and help, much appreciated :) Pleased to report all working now. Issues were bad ecu and also a dodgy connection in the inverter. A great forum for sharing knowledge and experience.

To anyone reading the thread where they have followed the RAVE diagnostic steps and failed the wheel speed test. I still can't get the multimeter measurement to work as described although with a scope it is fine. To be fair I drove the car slowly rather than turned the wheel by hand but not sure why it would make a difference to the test. Either way worth being mindful of this as I have seen one or two other similar posts.
 
Mine passes all the test in the Electronic trouble-shooting guide and if I manually jumper it up the vacuum pump fires up and the cable gets pulled and releases etc. etc. But CC still does not work fully.
Some of the conditions reported by Nanocom live monitoring are reversed (e.g. > speed to allow unit to be enabled signal is reported completely wrong/reversed by Nanocom. All buttons are seen and are all good but the choice for the 4.6 litre engine is not selectable. I have 3 AMR5700 units all of which do not offer the 4.6 engine as a selection choice.
So CC "works" sluggishly at 50MPH only and resume does not work or Set/Increase. Luckily I do so little driving it isn't a big problem, but it is the only thing on the vehicle that does not work. Annoying, baffling, but not fatal. :D
 
DanClarke said:
Mine passes all the test in the Electronic trouble-shooting guide and if I manually jumper it up the vacuum pump fires up and the cable gets pulled and releases etc. etc. But CC still does not work fully.
Some of the conditions reported by Nanocom live monitoring are reversed (e.g. > speed to allow unit to be enabled signal is reported completely wrong/reversed by Nanocom. All buttons are seen and are all good but the choice for the 4.6 litre engine is not selectable. I have 3 AMR5700 units all of which do not offer the 4.6 engine as a selection choice.
So CC "works" sluggishly at 50MPH only and resume does not work or Set/Increase. Luckily I do so little driving it isn't a big problem, but it is the only thing on the vehicle that does not work. Annoying, baffling, but not fatal. :D
Click to expand...
I unfortunately don't have nano com and my scan tool whilst specific for range rovers wouldn't connect to the cruise control, although I haven't tried since changing out the ecu so might be good now. Like you in real life I wouldn't use it that often but it was the last thing that doesn't work with the exception of the heated seats lol. I reckon it's pretty for a 22 yr old car but very antiquated in operation compared to modern cruise control :) Still just love my P38!
 
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