Overboost causing cruise control to stop working

[bobsimes]

I have a 75hp 1.4 tdi 2002

for the last few months i get an intermitent underboost code ( negative charge pressure) thrown on vag-com which causes the car to go into limp mode until the engine is turned off and back on again.

this limp mode seems to happen on full power on an incline eg joining a motorway maybe once or twice a month.

i suspect a leak in one of the hoses and at worst the intercooler (which was last changed about 4 years ago) which i am going to try and locate this weekend.

however over the last couple of days i have lost cruise control, i have made all the usual checks and ruled out the brake switch and bulbs ect. vag com shows the brake and clutch pedal switches working fine however for some reason i cannot get group 66 to show cruise switches, it says error.

i cleared the fault code re the underboost and cruise worked again until joining the motorway, again i lost cruise but the turbo did not go into limp???
cruise will not return until i log onto vag-com and reset all fault codes of which there is normaly only the one overboost code.

has anyone else had this problem re loss of cruise after going into limp mode or can offer any tips.

i am also thinking it may be n75 valve, what should it normally sound like at idle or is there any way for the home mechanic to check it is working correctly??

 

[john-wingnut]

The n75 ticks away merrily if its working.

If that was the cause though you would have a constant fault.

Yours is intermittent so more likely a leak or cracked intercooler

John

Ps you can also test it by doing an Output test when in the engine control module via vag com

 

[Skipton01]

The reason you can't get group 66 to display cruise is that diesels are catered for with groups 6 & 22!!

Take a look at these and see what you can see.

Cheers,

Mike

 

[bobsimes]

cruise

Thanks Skipton,

I had forgotten about block 22,

Turns out after investigation the cruise control stalk has a ribbon cable, this had worn through (through repeated left/right movement of the stalk during turning, this showed on block 22 as a missing "1", i rubbed the cable down to remove some plastic coating over the copper ribbon, soldered some solder to bridge the broken gap on the copper ribbon and wrapped in a small amount of insulation tape before re-assembling, and all is now well re the cruise.

Still not found the intermitent fault re boost though.

I checked the boost using vag-com n75 test and it was .870 bar on the n75 valve which didnt fluctuate, i believe this should be about 1 bar and 1.25 bar fluctuating under test conditions,any ideas re this, mine is the 75hp turbo not variable vane so could point to a faulty n75??.

All pipes look fine and nothing obvious loose, intercooler looks fine no oil leaking from joints all was bone dry.

95% of the time it is ok and i dont get limp mode, it always seems to happen under heavy acceleration up hill at higher speeds ie a motorway slip road.


simon

 

[john-wingnut]

How did you actual do your n75 test??

The one I was on about is the output tests under the engine control module.
It will rev the car to 2000rpm and cycle the various outputs on the engine.
If you do it with engine off, it will just cycle the valve and you will hear it tick on and off.

What you need to do is plot the n75, alongside the actual MAP and specified MAP boost reading on a run with vag com attached.

Get a nice long road and floor it through the gears.

Elsawin sates to use the reading in third gear with WOT.

When you get home see that the actual MAP readings pretty much follows the specified MAP reading on the graph and that the n75 plot varies accordingly.

Then you will know if your n75 is dodgy, but like I say, when mine was busted it didnt work full stop.

John