notaguru

'83 US spec, auto trans, 68k miles

When a speed is set, it holds for a moment and then begins falling off. I assume there's a 33 year old vacuum line somewhere, or a diaphragm, that's begun leaking. Where should I look and how can I test to identify the fault?

Thanks!

Randy V

Add that info to your sig line so you don't have to type it every time.

Have you tried a search on your cruise control problem?

Fogey1

These are most of the threads I'd collected as of march of this year. Your problem is common, Search is your overly chatty friend. There are multiple failure modes, the workshop manuals have a detailed test procedure.

https://rennlist.com/forums/928-foru...the-place.html

https://rennlist.com/forums/928-foru...cruise+control

http://members.rennlist.com/sharkski...ugeGremlin.htm

https://rennlist.com/forums/928-foru...ion-fixed.html

http://forums.pelicanparts.com/porsc...ml#post3789513

https://rennlist.com/forums/928-foru...cillation.html

https://rennlist.com/forums/928-foru...-a-change.html

https://rennlist.com/forums/928-foru...l-problem.html

http://members.rennlist.com/pirtle/svc_cc.html
Porsche 928 S4 Service - Cruise Control

***** http://jenniskens.livedsl.nl/Technic...0/MyTip022.htm

https://rennlist.com/forums/928-foru...e-control.html

https://rennlist.com/forums/928-foru...the-place.html

https://rennlist.com/forums/928-foru...cruise+control

https://rennlist.com/forums/928-foru...ke-lights.html

http://forums.pelicanparts.com/porsc...ght=tim+polzin
10/07 - 2/08 LEADS TO .........................

http://forums.pelicanparts.com/porsc...ml#post3789513
Understanding the VDO Cruise Control Amp SCHEMATICS & PIX
2/08 -1/14

https://rennlist.com/forums/928-foru...cillation.html

https://rennlist.com/forums/928-foru...-a-change.html

https://rennlist.com/forums/928-foru...l-problem.html

http://members.rennlist.com/pirtle/svc_cc.html
http://jenniskens.livedsl.nl/Technic...0/MyTip022.htm

http://www.nichols.nu/tip003.htm

https://rennlist.com/forums/928-foru...-actuator.html

https://rennlist.com/forums/928-foru...-the-oven.html

https://rennlist.com/forums/928-foru...trol-woes.html
https://rennlist.com/forums/10007721-post7.html

https://rennlist.com/forums/928-foru...-actuator.html

https://rennlist.com/forums/928-foru...trol-help.html

https://rennlist.com/forums/928-foru...t-working.html
Cruise Control - Not working - Oven Method
5/13 -

https://rennlist.com/forums/928-foru...l-rebuild.html
Cruise control rebuild
12/13 -

https://rennlist.com/forums/928-foru...ntrol-amp.html
Naïve idea: is it possible to re-fabricate VDO Cruise Control Amp?
3 pp now with schematics, pix, part #'s and so on.
1/14

http://www.silvercircuits.com/index.htm
http://www.americancircuits.com/aci/...6/Default.aspx

https://rennlist.com/forums/928-foru...ain-stuff.html
Interesting Cruise Control Brain Stuff

https://rennlist.com/forums/928-foru...l#post11853249
Mine had the same symptoms as the one on the left. Would accelerate but not hold. Sent it off to Beckman Technologies . For $160. it was repaired and hasn`t missed a beat since. They do rebuilding of MB parts and VDO parts.

https://rennlist.com/forums/928-foru...e-control.html
Cruise control 10-11-2014, 09:54 AM

prescott

replaced all my vacuum lines and got nothing. replaced my cruise brain and got nothing. replaced the brake pedal switch (PITA) and got nothing. replaced the actuator and BAM BABY! the thing works!

Hope this helps!
Brian

mark kibort

could it be the leaky bubble air container in the fender?

notaguru

Hey Fogey... that's an impressive Cruise Control Library. Thanks from an OLD Fogey.

Fogey1

You're welcome. You can thank me by taking LOTSO PIX of the stuff you do, in case I ever get properly motivated to get off my *** at the computer and do some real work.

I'm bettin' I'm older than you. Next year I have to start MMD's.

granprixweiss928

Mine does the same.

81 5spd 60K

MainePorsche

Sounds like a vacuum loss. Trace line from the 'Y' off the master cylinder, through the fender wall, to the appliances behind the driver front tire in the wheel well.

granprixweiss928

Ive done that, and it holds vacuum fine.

MainePorsche

Your Bowden cable in good shape ? Cable moves freely ?
I'm not versed in diagnostic algorithm for cruise units. Just some basic ABC's.
I once lost my cruise while cruising in the 70's. Heard/felt a pop - car immediately decelerated - ran fine though - I was panicked for a moment and expected to see Mr. [!] - got home 150 miles later - looked under the hood and at once I see a broken/split cruise Bowden cable. For it to break like that it must have already been kinked/damaged and obstructed. I remember using the cruise stalk to accelerate at the time of the event - it wasn't just setting the cable tension, it was actually drawing on it.

supercedar

If it is in the control board then re solder every spot on circuit board. Colin just did mine last week on the 86.5. He said if you have vacuum then it needs the re solder. Made mine work !