intermitent cruise control issue.. HeLP - BMW Forum

Beeroh · 08-20-2009, 07:24 PM

1995 740iL..

cruise control works sometimes.. when it doesn't work I can still turn on the c/c button and the indicator light turns on but when I try to set the speed it doesn't work and then the c/c light turns off not being able to turn back on until the car is turned off, then itll turn back on...

What could it be?

darkhorse737 · 08-20-2009, 07:32 PM

last tune-up was?

mileage is...?

recent repairs done to car were...?

Beeroh · 08-20-2009, 09:18 PM

Last tune was... dunno..

mileage is 184k miles...

recent repairs were..
water pump replacement at 15k miles..
timing chain tensioners, belts, 10k miles..
just got the computer laser alignment and wheels balanced..

darkhorse737 · 08-20-2009, 09:58 PM

i suggest you do a tune-up with emphasis on checking for intake leaks. intake leaks... it causes the CC to perform erratically. From your description(and i assume it is an accurate description), the electronic component looks to me like it is in good order.

TIP: a good indication of an intake leak is when u pull out the spark plugs and the electrodes are too white; where they should be brown to grey. ;-)

a good shop will pull up stored codes and performance data stored in your DME as a means of determining what might be wrong, and u can't go wrong with that.

:-)

Beeroh · 08-21-2009, 05:03 PM

I assume that a tune up would be just like every other vehicle.. plugs, filters, cap and rotor, maybe wires.. I also noticed this morning that when my vehicle dropped to a certain temp.. below the first mark, 10oclock line, the cc dropped and wouldn't turn back on.. It might be a temperature issue, the cc drops after a certain temp? hmmmm...

darkhorse737 · 08-22-2009, 01:58 AM

Quote:

Originally Posted by Beeroh

I assume that a tune up would be just like every other vehicle.. plugs, filters, cap and rotor, maybe wires.. I also noticed this morning that when my vehicle dropped to a certain temp.. below the first mark, 10oclock line, the cc dropped and wouldn't turn back on.. It might be a temperature issue, the cc drops after a certain temp? hmmmm...

hardly likely a temperature issue(althougth your observation is good, i must say). In this case, what u might have is a small leak that develops when the engine is warm/hot. OR, the CC brain itself has gone bad.

It will help if you can discern whether ther is a difference in idle quality when a engine is cold as against when engine is hot.

I mentioned in my earlier post... "with emphasis on checking for intake leaks". Cruise control systems rely on vacuum measurement, along with other engine inputs to adjust throttle accordingly.

Beeroh · 08-23-2009, 08:38 AM

ok, on the way home this morning the temperature guage stayed at 12 oclock.. the cc stayed on the longest that I have seen it, around 15 minutes.. I did notice that the temp guage would drop a little then spike back up to 12 oclock.. and I saw the cc slow down simultaneaously as the mpg guage on the dash flikkered for a split second.. And I notice that the a/c heating unit turns on automatically even when the fans are turned all the way off.. then it shuts off.. the settings on the unit are not on Auto..

Ok, I realize i have electronic issues.. if only I had a wiring schematic..

What do you think..? I could be the cc brain like you said, and the temp gauge is faulty.. along with the a/c heating unit..

marti5 · 08-24-2009, 02:35 AM

It does sound like the cruise control unit decided that something was not right, and the brake switch is No 1 contender. The brake switch consists of two pairs of contacts, if they do not change state simultaneously then that is logged as an error. The cruise control system now cannot be sure if you have pressed the brake pedal, which pair of contacts indicates the correct state of the pedal? It doesn't know for sure so the cruise control system is switched off including the green light on the dash.

To be sure, diagnostics performed by BMW will find the fault is it is logged (I think)......

Beeroh · 08-24-2009, 11:19 AM

Thank you for your reply..

It could be the brake system, but shouldn't that only turn it off with the ability to be turned back on..?

My system can not be turned back on, only after the car is turned off and what ever trips the cc is reset..

dont these cars have sensors for everything, if the car has a brake issue shouldn't it throw a code?

darkhorse737 · 08-24-2009, 01:18 PM

This might help:

Good Luck

marti5 · 08-24-2009, 01:51 PM

Quote:

Originally Posted by Beeroh

Thank you for your reply..

It could be the brake system, but shouldn't that only turn it off with the ability to be turned back on..?

My system can not be turned back on, only after the car is turned off and what ever trips the cc is reset..

dont these cars have sensors for everything, if the car has a brake issue shouldn't it throw a code?

Not the brake system just the switch attached to the top of the brake pedal.

No it wont turn back on as the ECU has detected a fault.

It may not throw up a code for a switch as a switch does not have a sensor, same as it will not throw a code if one of your electric window switches dies.