E39 Cruise Control Issues

[R3CON 0]

Hi all,

I have just brought an e39 (a 96 with mechanical CC)

I LOVE it!!

Except I am having a few wee issues. One of the majors is my Cruise Control acts up. Sometimes it works flawlesly (I brought it in Wellington and drove it to Napier with no issues with it at all). Other times I turn it on (On the dash button) fine. get up to the speed I want to set at and push the + button on the steering wheel (as normal) but when I do that the car lurches foreward (its like the car puts the pedal to the floor and accelerated extremely hard to get to the speed) it ofcourse goes well over the speed I set to so then buttons off until it gets to where it thought I wanted it set (then accelerates very hard again). If I press the - button on the wheel it accelerates also. Basiclly it totally cracks the shits then turns itself off (the CC button on the dash no longer works until the car is turned off.

Then as I said before It sometimes works perfectly. Its got me completely confussed and I havnt found very much on the subject online at all.

[M5V8 337]

lurching forward when you turn on the CC is normal,

my car does this when the throttle you have applied is lower than the throttle the car wants to use to get to the set speed because you are using the increase speed button (+) to active the system.

If you are faster than the speed you want to cruise at this doesn't happen.

Or if you use the CC button (set button) to active rather than the + this should help. I think the system is working normally here. All the + button needs is a tap, don't hold it down.

You can set the CC and accelerate using the + button from anywhere over about 50k's I think. I once tried from standstill but it wouldn't engage.

Are you meaning it accelerates hard, then slows, then accelerates hard again all on it's own?

Cause that IS freakin weird. Sticky/failing + button?

sorry can't help with it cutting out all together, this has never happened to me but I would assume it would do this if there was a fault with the system. Better that than doing a Prius.

Edited June 15, 2011 by M5V8

[hybrid 1040]

Have personally had it once or twice where you have a little fight with it and it decides to leave the sandpit with toy in hand.

The way to use cruise is to turn on the cruise control which will give you a green light on the dash by pressing the cruise on button then drive up to the speed you want. then press and hold for a couple of seconds the speed set button. make corrections to your speed with the + or - buttons. obvious stuff I know but sometimes people get it wrong.

The surge is normal as Darren says.

Electrically, there is a signal wire that goes to the distrubution box. This tells the DME when to switch on or off.

Software wise, its coded into every car weather your have a MF steering wheel or not.

At a strech having it scanned will bring up any readible faults however the issue may not be stored in the fault resigter (quite often not).

Its a starting point though.

I could read your + and - ibus codes to see weather theyre inverted for some reason ? maybe an issue with your steering wheen button electronics.

[R3CON 0]

Thanks guys.

Firstly I know the difference between the normal surge produced by engauging the CC system as normal (as I said 1 out of every 4 times or so it behaves as it should) there is definately an issue with the system when its faulting as whether I press the minus or plus it throws a wobbly (often accelerating further) the word surge may have not been descriptive enough basiclly when its faulting and I engauge the CC the accelerator goes to full open position for about 2 seconds (propelling the car atleast 20kms higher than when I engauge it). I have been in the position to just allow it to do its thing (going down motorway and engauging at about 80km) it doesnt settle down either. It is either full on the gas pedal or off all together there is no fine movement of the accelerator. The issue "feels" mechanical like the cable is sticking or incorrectly set up. The real baffling part of the whole episode is that sometimes it works perfectly noramlly....random

[M5V8 337]

does your e39 have fly by wire throttle control?

it would be more an electrical issue in that case, there is no physical throttle cable.

Scan may show up what the issue is. try checking the cruise button operations, make sure they are clean & have a even feel. A faulting or sticky +/- button would explain this behaviour.

Edited June 15, 2011 by M5V8

[R3CON 0]

does your e39 have fly by wire throttle control?

it would be more an electrical issue in that case, there is no physical throttle cable.

Scan may show up what the issue is. try checking the cruise button operations, make sure they are clean & have a even feel. A faulting or sticky +/- button would explain this behaviour.

No fly by wire. It a mechanical throttle and CC unit. Have thoroghly checked for a sticking button etc, its pretty weird.

Cheers for the help though

[hybrid 1040]

The button is attached to a digital encoder. It may be an issue with that. The only way to check that is to hotline the IBUS and check to see if the button is sending a repeditive signal to the DME.

Other things are the stepper motor attached to the throttle body.

Another thought is there is something going on with your speed sensor. A simple resistance test or diagnostic scan monitoring the live data will determine if its in range. Same goes with your TPS.

[R3CON 0]

Thanks guys. I have it booked in Monday so will get the low down then......hopefully. lollies