Running a P38 with axles in the air. Cruise control check.

P38 2.5DSE

Is this a bad idea due to the viscous coupling or does it merely lock and run them both with no problem at all?

I want to lift both axles and run the vehicle in the shop to fault find the cruise control.

I have measured the set and reset button in the steering wheel.
Measuring to ground I get 12ohms. Which seems a little strange when it looks like the buttons merely ground a signal to the inverter module according to the wiring diagram.
Since both buttons have the same resistance maybe the common ground in the rotary coupler has gone bad?

No harm running the car with all wheels off the ground, it will have no effect on the VCU, it works on speed difference between axles. Don't try with just one axle lifted
The rotary couplers do fail.

And it only works above a certain speed any ways so would think it a bad idea but what do I know

biketeacherdave said:

And it only works above a certain speed any ways so would think it a bad idea but what do I know

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I agree it's a bad idea, simple electrical trouble shooting would do the job.

Well I did mention lifting both axles right?

Been testing a bit now.
If I apply ground to pin no 5 on the inverter module which means the SET button, sure enough the speed starts to increase. But as soon as I remove ground it drops down to idle almost instantly.

If I try to RES the earlier chosen speed by doing the same to pin no 6 I cannot tell it tries to go back to the speed I but only for a split second.

But now that I try the same with the buttons on the steering wheel it behaves the same.
So rotary coupler might be fine after all.

Decided to measure voltage to engine ECU on pin no 4.
With cruise control switch off: 4.2V
With switch on: 3.4V
Pressing SET button 0.8V
Releasing SET button: 3.4V

For a moment it held the RPM/speed but I haven’t been able to replicate it.

biketeacherdave said:

And it only works above a certain speed any ways so would think it a bad idea but what do I know

Click to expand...

Yes it works from about 60km/h maybe?
Then how is it a bad idea to have the car standing still in the air with wheels running to simulate the car running on the road?

Datatek said:

I agree it's a bad idea, simple electrical trouble shooting would do the job.

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Simple electrical troubleshooting? What do you think I’m doing? This is the only way to troubleshoot in my opinion. But do tell how you how you would have done it.

First thing also to check is the brake pedal switch, it needs to be set correctly as per the instructions in Rave, as it is a two pole switch and effects the cruise control.

brianp38dse said:

First thing also to check is the brake pedal switch, it needs to be set correctly as per the instructions in Rave, as it is a two pole switch and effects the cruise control.

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Forgot to mention I tested it. While increasing speed with SET I touched the brake pedal and it shut down the speed increase as normal.

Worth testing switch as its easy to do and rules it out, Push brake pedal down, pull out plunger then let pedal up to set switch.

Oh there is a calibration to be done on that switch. Didn’t realize it.

Well on my drive home I tested the cruise control and lo and behold, it works. It hasn’t worked for months and now it does.

I did try this on my ramp and the traction control started grabbing wheels and freaked me out!!
It was literally ticking over and in drive...