P38A Cruise Control - Just asking for suggestions.

DanClarke · Mar 25, 2020

Cruise Control is the only item "not working" on my P38. Or is it?
Today I had an opportunity to try it on a dual carriageway.
Strangely... If I sat at 50 and activated cruise I could take my foot off the accelerator and it stayed at 50 and c.2,000 RPM. IF cruise wasn't working (after a fashion) surely it would drop in speed and RPM down to tick-over fairly quickly? What it will not do is accelerate when the SET button is held depressed, or stay at any speed other than about 50..55 when under cruise control. dab the brakes and it drops in speed and down towards idle revs. So this suggests to me that the cruise system is operating (to some degree).
Could it be a knackered vaccuum pump that hauls the cruise throttle-cables around?
i.e. enough guts to hold at a certain throttle position but not to go above that or hold below that position.
It is a bit odd. All the vacuum lines are new, the brake switch is new and set properly, the ECU has been swapped out, only really leaves the vacuum pump as far as I can see.

kermit_rr · Mar 25, 2020

That's an odd one! If it can hold 50, you'd expect it to hold 40
It's easy to check it holds a vacuum by pulling the pipe off at the bellows, pressing it in and putting your finger over end, it should hold in place if there's no leak

Datatek · Mar 25, 2020

DanClarke said:
Cruise Control is the only item "not working" on my P38. Or is it?
Today I had an opportunity to try it on a dual carriageway.
Strangely... If I sat at 50 and activated cruise I could take my foot off the accelerator and it stayed at 50 and c.2,000 RPM. IF cruise wasn't working (after a fashion) surely it would drop in speed and RPM down to tick-over fairly quickly? What it will not do is accelerate when the SET button is held depressed, or stay at any speed other than about 50..55 when under cruise control. dab the brakes and it drops in speed and down towards idle revs. So this suggests to me that the cruise system is operating (to some degree).
Could it be a knackered vaccuum pump that hauls the cruise throttle-cables around?
i.e. enough guts to hold at a certain throttle position but not to go above that or hold below that position.
It is a bit odd. All the vacuum lines are new, the brake switch is new and set properly, the ECU has been swapped out, only really leaves the vacuum pump as far as I can see.

There is a minimum speed at which CC operates. On my diesel, I might be wrong, but I seem to remember that it requires repeated presses of the SET button, each press giving a speed increment, long time since I have had to opportunity to use it so I could be wrong.

DanClarke · Mar 25, 2020

Datatek said:
There is a minimum speed at which CC operates. On my diesel, I might be wrong, but I seem to remember that it requires repeated presses of the SET button, each press giving a speed increment, long time since I have had to opportunity to use it so I could be wrong.

Yes it is 30MPH or above, but that's not it. If I am doing 39 and hit cruise and take my foot off, it has no effect and the car drops in RPM/Speed down towards idle. At 50 ..55 it locks in place, but cannot accelerate or go up a hill. Only stays there on the flat.

kermit_rr · Mar 25, 2020

Mine works just over 30mph
Hold a speed, press the button, let it think for a second or 2, then i can release the accelerator. If i do it too quick, it thinks i didn't mean it

kermit_rr · Mar 25, 2020

DanClarke said:
, but cannot accelerate or go up a hill. Only stays there on the flat.

That does sound like a vacuum leak, from what I've read previously

DanClarke · Mar 25, 2020

kermit_rr said:
That does sound like a vacuum leak, from what I've read previously

I will try feeding the pump motor with some angry pixies while it is in neutral on the drive and see what happens to the RPM.

Grrrrrr · Mar 26, 2020

How about the buttons on the steering column stalk?

Although ... Wouldn't it take the speed at which you switch it on? Where's that speed stored? BECM?

DanClarke · Mar 26, 2020

Grrrrrr said:
How about the buttons on the steering column stalk?

Although ... Wouldn't it take the speed at which you switch it on? Where's that speed stored? BECM?

The Cruise cannot be enabled until a number of conditions are met, the first being roadspeed >28.
I have tested all the buttons and been through the troubleshooting guide right to the point where it asks you to spin a roadwheel to get a voltage from the abs sensor.
All is OK to there. It isn't a desperate quest for perfection, but it is the only thing that doesn't work as it should. I will see if I can coax the vacuum pump into operation and see if it can up the engine revs OK.
If it can't, then that should probably be my starting point.

Datatek · Mar 26, 2020

DanClarke said:
The Cruise cannot be enabled until a number of conditions are met, the first being roadspeed >28.
I have tested all the buttons and been through the troubleshooting guide right to the point where it asks you to spin a roadwheel to get a voltage from the abs sensor.
All is OK to there. It isn't a desperate quest for perfection, but it is the only thing that doesn't work as it should. I will see if I can coax the vacuum pump into operation and see if it can up the engine revs OK.
If it can't, then that should probably be my starting point.

Sounds like a plan

P38A Cruise Control - Just asking for suggestions.

DanClarke

Well-Known Member

kermit_rr

Very

Datatek

Well-Known Member

DanClarke

Well-Known Member

kermit_rr

Very

kermit_rr

Very

DanClarke

Well-Known Member

Grrrrrr

Technician, Bodgit & Scarper Ltd

DanClarke

Well-Known Member

Datatek

Well-Known Member

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