P38A Cruise Control - Armchair theorists required for ideas and help.

MY2000 4.6 Thor powered Rangie.
Hello folks, I am hoping to get some useful suggestions on "next steps" to diagnose/fix/improve my Cruise control.
What's wrong with it? - It only "Sort-of" works.
What do you mean "Sort-of works"? - Pootling along at 50-55 I can get it to engage and hold that speed, but it will not accelerate when pressing the SET button permanently (which it is supposed to do). If I brake it releases the accelerator pedal immediately. Unless I then give it a bit of help (foot pressure) it is unable to accelerate back up to the set speed on its own via the "RES" button.
If you encounter a hill while "cruising" at this magic 50-55mph the speed drops off and it is unable to maintain the set speed.

So, all the key functions seem to exist, but it has "no guts" to accelerate the car on the button or recover from a dab on the brakes with "RESume" or sit under Cruise control at higher speeds.

Its almost as if the Vacuum actuator cannot do the whole job it can only just hold the engine revs at a level suitable for 50-55mph against the return spring mechanism. Duff Actuator vacuum pump? Leaking bellows? Anyone had a similar issue?
I changed all the perished black rubber vacuum lines for nice new Silicone ones already.

Your thoughts and wise words would be much appreciated.

TIA Dan.

Did you also change the small vacuum lines inside the pump housing ? Cable might need adjusting - see RAVE.

pwood999 said:

Did you also change the small vacuum lines inside the pump housing ? Cable might need adjusting - see RAVE.

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Small vacuum lines? OMG there's more.

Take the lid off. There's small pipes (prob 3 or 4mm not sure) between the pump & release relay.

I checked the cable adjustment so I guess I need to go look at the internals of the pump unit and any piping within it.
Thanks!

Check the bellows aren't leaking too shouldn't be, but who knows

if you connect 12V & Ground carefully to the 3-pin connector (with plug removed), you can operate the pump & relay manually & check the bellows hold fully in. If the relay rubber pad is worn it can & does leak, which you can prove by pressing it down manually while the pump is running.

another thing. The Thor vehicles have a diagnostic connection, so might be useful to hook up Nanocom & see what values are in there. These will affect how the cruise controls acceleration, etc. and are different for 4.0 & 4.6 and P38 or Disco applications.

Maybe it forgot the coding, or has been replaced & not programmed correctly ?

I don't have a Thor, but others could check & compare.

This something that I need to check on mine as I too find mind swims a little but seems better than your as using set does accelerate it just seems to swim a bit and it's already had vacuum lines replaced, should check what nanocom says aswell in case that gives any hints.

pwood999 said:

another thing. The Thor vehicles have a diagnostic connection, so might be useful to hook up Nanocom & see what values are in there. These will affect how the cruise controls acceleration, etc. and are different for 4.0 & 4.6 and P38 or Disco applications.

Maybe it forgot the coding, or has been replaced & not programmed correctly ?

I don't have a Thor, but others could check & compare.

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Thanks for this timely reminder. I will drag my test probe out of the cupboard and see if I can get the bellows to suck & hold and stick the Nanocom on it and see what it says.

Khemitude said:

This something that I need to check on mine as I too find mind swims a little but seems better than your as using set does accelerate it just seems to swim a bit and it's already had vacuum lines replaced, should check what nanocom says aswell in case that gives any hints.

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By swimming do you mean it hovers either side of a set speed? I assumed it was normal for old tech using cables rather than electronics. Possibly needs some fine adjustment on the cable?

The simple way to test the bellows, pull the vacuum hose off, push the below all the way in and ouyt your finger over the end of the vacuum port.
If the bellows holds then it's good

kermit_rr said:

By swimming do you mean it hovers either side of a set speed? I assumed it was normal for old tech using cables rather than electronics. Possibly needs some fine adjustment on the cable?

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Yeah pretty much, I would assume being a vacuum line to the cable it would essentially be able to hold exactly but maybe I'm wrong. I've already tried adjusting both the cruise admittedly only with the old mk1 eyeballs but as I do a lot of wood working fine measurements I'm pretty good at eyeballing so tried everywhere from the RAVE recommended range and seems to be the same, I also adjust the throttle cable too as that did need a little adjustment.

Will do a bellow check tomorrow

Khemitude said:

Yeah pretty much, I would assume being a vacuum line to the cable it would essentially be able to hold exactly but maybe I'm wrong. I've already tried adjusting both the cruise admittedly only with the old mk1 eyeballs but as I do a lot of wood working fine measurements I'm pretty good at eyeballing so tried everywhere from the RAVE recommended range and seems to be the same, I also adjust the throttle cable too as that did need a little adjustment.

Will do a bellow check tomorrow

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Be good to know how cruise works on other p38 v8s.
I think if you had leaking bellows, it would fault

Ok, so with everything inactive no key in etc. I pulled the actuator all the way in and held my finger over the vacuum pipe spigot on the bellows housing, and it did not leak and return back. So that's OK.
I pulled the 3-wire connector of and stuck power across the pump and it fires up and pulls the actuator in. So that's OK.
I started her up and did the same again and the revs just went through the roof.
So it seems all the business end attached to the engine works.
I have already changed the DC power regulator for the Cruise control and the Hella ECU. So why she no worky for real?
When I press the set button she does not increase speed as I travel and the RESume doesn't get her to pick up again from 30 to accelerate back up to 50/55 (the only speed it seems to be happy with). Most odd.

I will have to go back through the Trouble Shooting guide I guess.

DanClarke said:

Ok, so with everything inactive no key in etc. I pulled the actuator all the way in and held my finger over the vacuum pipe spigot on the bellows housing, and it did not leak and return back. So that's OK.
I pulled the 3-wire connector of and stuck power across the pump and it fires up and pulls the actuator in. So that's OK.
I started her up and did the same again and the revs just went through the roof.
So it seems all the business end attached to the engine works.
I have already changed the DC power regulator for the Cruise control and the Hella ECU. So why she no worky for real?
When I press the set button she does not increase speed as I travel and the RESume doesn't get her to pick up again from 30 to accelerate back up to 50/55 (the only speed it seems to be happy with). Most odd.

I will have to go back through the Trouble Shooting guide I guess.

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The buttons work through the rotary coupler, could it be the circuit for resume is dead?

Datatek said:

The buttons work through the rotary coupler, could it be the circuit for resume is dead?

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Might be both are dodgy. When you are above c.28MPH holding SET is meant to increase speed until you release the SET button (it doesn't), and if you are set for say 70MPH and you brake to slow down, pressing RES is meant to return you to the previously SET speed (it doesn't do that either).
Last time I tried the trouble-shooting guise both button tests were fine.

Why not just connect your Nanocom & see what the inputs menu shows ? will at least show what the ECU thinks is happening.
Easier than probing with a voltmeter, although you should probably do the trouble-shooting as well.

Mine is Gems with earlier EDU so no diagnostics for me, but it does set, accelerate & resume properly. Speed doesn't seem to vary at all once it's set.

also check the target speed after using the brake & then pressing resume. Maybe the ECU forgets the target ?

kermit_rr said:

Be good to know how cruise works on other p38 v8s.
I think if you had leaking bellows, it would fault

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I find the cruise control on my 2001 Vogue is functional but pretty crude compared to the cruise control on my 2006 Jaguar S-type. The cruise control on the Jag allows me to tap +/- buttons to increase/decrease speed & keeps to the set speed much better than the P38 which can hover around by a few mph either side of the set speed.