Cruise Control is "Surging"

[materiamage]

Hey guys, let me start off with telling you my problem:

I have had my 91 525i for awhile now and, have been fixing a number of small problems as I locate them ( ie LKM, Thrust arms, heater control valve, and so on).
I have recently found a new "problem" or issue rather when using my cruise control.
I commute to and from work on the freeway daily as well as make a 115 mile trip nearly every weekend. It's during these drives that I will occasionaly use my cruise control on the car.
For the first few months everything was fine and worked great but in the last month or two I have noticed that my cruise control likes to "surge".
It doesnt happen everytime, and sometimes it may do it just once and stop, or it may do it repeatedly back to back for a matter of moment and then stop and run fine.
Please before you tell me to search I have spent quite alot of time searching the forums, and everything I have found Including this thread (http://www.bimmer.info/forum/showthread.php?t=5587&highlight=cruise+control+surge) have not been much of a help.

After browsing around on the NHTSA defect and recalls section (http://www-odi.nhtsa.dot.gov/cars/problems/recalls/results.cfm) I have found the following defect/recall (http://199.79.180.162/prepos/files/Artemis/Public/Recalls/1997/V/RC-97V131-NN.PDF).
While it seems unlikely that this is the culprit I am nearly at my wits end on tracking it down, and since I have been the victim of a VERY high speed cruise control failure related accident (http://www.team-integra.net/forum/display_topic_threads.asp?ForumID=16&TopicID=128895) in the past, I am becoming VERY nervous to drive my car anymore.

Please try to point me into the direction on what to do to correct this problem.

And I want to pre-emptivley say Thank you, for any help. It's appreciated.

~ RB

[SRR2]

Mine did that too. Not often, but enough to be annoying. The usual manifestation was a momentary push on the throttle that was strong enough to make anyone in the car say "what was that", yet the speed change was very small because the spike was so brief. I once caught out of the corner of my eye the speedo take a dip down at the same time the spike occured, so it appeared that the transient was due to a momentary dropout of the speedometer circuit. I was going to go after the speed pickup in the diff, but I ended up trading the car.

IMO, you can rule out the actuator, cables, switches and control unit. IIRC, the speed signal came from the diff, through the cluster, and then to the ECU and CC unit. I did refresh the caps in that cluster and it seemed (but it might have been wishful thinking) that the spiking happened much less often.

If I had to pick a culprit, it would still be the pickup in the diff. If you haven't replaced the caps in the cluster, you should do that immediately and rule out that potential problem. Yours are definitely gone by now.

[BigKriss]

my car has the same problem, I don't know how to fix it.

[SRR2]

Huh. Maybe this is a bona-fide case of "they all do that".

[Beemr750]

Mine did the hick-ups occasionally,replaced the speed sensor and that was the end of it.I bet if you see the speedo needle flickering at the same time, you found the logical culprit.Only less than $30 to fix it yourself.

[SRR2]

My thought precisely. When you think about what a stupid design that speed sensor is (magnets and reed switch) it's impressive that it lasts as long as some of them do. Reed switches DO wear out! The failure mode is usually a failure to make contact, which is consistent with the symptoms of the cruise glitch.

I think materia... should install a new pickup and report back. It's an *easy* fix on most of these diffs. Easy to get to and no flood of lube to deal with. Only trick is getting the safety wire right.

[materiamage]

Alright then, I will drop my the stealership tomorrow and pickup the switch and throw her in there. After I drive for a couple weeks I will get back with a reply and let you guys know if this is the problem or not. If that doesnt fix it then I will try the caps in the cluster, I just won't do them boht at the same time because then i have no way of knowing which one was the real fix.

Thanks alot fo all of the responses, please let me know if anyone else has any other thoughts on this matter.

~RB

[mikell]

If you haven't replaced the caps in the cluster, you should do that immediately and rule out that potential problem. Yours are definitely gone by now.

How would the caps in the cluster affect this? If the speedo is working okay, wouldn't it have to be the actuator or the cruise control module?

[SRR2]

Noise on the signal. When those caps go bad, which they all do, things get very ratty inside the cluster, and that would include the speed signal to the ECU and CC modules.

Anyone with a car older than '95 should plan on opening up the cluster and replacing the caps. It's really that simple.

[SRR2]

You should plan on replacing the caps regardless. If they haven't already failed, they will soon.

[genphreak]

Alright then, I will drop my the stealership tomorrow and pickup the switch and throw her in there. After I drive for a couple weeks I will get back with a reply and let you guys know if this is the problem or not. If that doesnt fix it then I will try the caps in the cluster, I just won't do them boht at the same time because then i have no way of knowing which one was the real fix.

Thanks alot fo all of the responses, please let me know if anyone else has any other thoughts on this matter.

~RBThanks for doing it like this- its a great way to help everyone. I am sure it is a standard issue- mine has done it (in the 4 years I've had the car) probably 3 times, and every now and then I get the lightest of slight surges when on Cruise.

It is probably about to get worse, but again, this could be wishful thinking influenced by a touch of chaotic evil... Looking forward to hearing how it goes. :) Nick

[fujioko]

My 92 surges often when the cruse is on. The girlfriend punches me and tells me to “stop doing that”… “yeah, but honey it was the car.. not me”.

Anyway, I do see the speedo dip when the surge happens. I went under the car a few days ago and de-oxed the connector at the diff. Didn’t fix it, I suppose its time to replace the pickup.

[materiamage]

BTW, I don't suppose anybody here readily has pictures of the pick up and switch so I know EXACTLY what I will be into. I'm not at all worried, I am very well experienced with automotive work, and have torn down and rebuilt a few engines. I have just never messed with a speed sensor of any sort, besides pictures are always good.
Thanks again
~RB

[genphreak]

My 92 surges often when the cruse is on. The girlfriend punches me and tells me to “stop doing that”… “yeah, but honey it was the car.. not me”.

Anyway, I do see the speedo dip when the surge happens. I went under the car a few days ago and de-oxed the connector at the diff. Didn’t fix it, I suppose its time to replace the pickup.Maybe the car is tryig to spice up your relationship a bit! Yea, it'll be nice to find the real fix for this one me thinks...

[materiamage]

is this what that lil guy looks like?

http://img.eautopartscatalog.com/live/P401937882OES.JPG

Interesting I have not ever messed with a speed sensor.
Oh well, guess we will know in a week or so if it is the problem or not.
~RB

[SRR2]

Maybe. There are a few different flavors of it. You can see a couple different ones here at BavAuto: http://www.bavauto.com/se1.asp?dept_id=104#top

I dunno exactly where it's located on an E34. On my E30 it's readily available at the top rear of the diff. Changing it looks like 10-15 min tops. Of course what usually happens just when you think something ought to be easy, you find that it's located behind the diff's mounting bracket and you have to drop the drive shaft, half-shafts, and the differential to get to it.

[stx133]

i have had this problem since purchasing the car, i have stripped and cleaned. rebuilt the cruise motor to some benifit, my unit was so bad it would often not engage. there is no effect on the speedo needle while the surging is happening. when my unit is cold it generally works much better than when it is hot, i can watch the temp and guess accurately whether it will work or not.
am going to get a replacement module soon and try that..

[fujioko]

I went to the dealer and ordered the speedo pickup and a gasket for the oil leak on the engine. I decided to stop driving the 525 until I get the oil leak resolved.

Anyway I paid $47.00 for the pickup. I should have the Bimmer rollin’
again by the weekend. I hate driving my Saturn!

[tasman]

You should plan on replacing the caps regardless. If they haven't already failed, they will soon.

Maybe this is less than an intelligent question but what are caps? I am new to BMWs and am not sure what caps are; capacitors??

Thanks
TasMan

[materiamage]

Just a really quick update on the surging cruise control dillemma, It took the dealer almost a week to get in the correct E34 speed sensor, but after that I installed it the next day. Installation was simple and pretty easy, but best of all, I am very happy to tell all of you that this was in fact, the fix for the "surging" problems i did describe.

Anyways its been a long work night and its now time form bed, I will give another update when and if there is any status change with the sensor.

Thanks for all of those that helped me out and provided info for me, it is appreciated.

~RB