Disfunctional Cruise Control
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4th Gear
Joined: Dec 2008
Posts: 4
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From: South Texas
Disfunctional Cruise Control
I am a new owner of a 1980 928. The cruise does not function so I followed the troubleshooting in the manual and it came down to replacing the control module. Did that and the cruise does the same thing!!! So much for the troubleshooting guide. I can make it accelerate by actuating the level forward but it will not engage the cruise and hold speed. Based on this I assume the vacuum is OK and the servo is functional. I can not make it accelerate below about 40 mph so the pulse input for the speed sensor must be OK (I can see the input with a multitester). Has anyone had this issue? Any Ideas?
Thanks,
Bob
Thanks,
Bob
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Joined: Jul 2001
Posts: 3,291
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From: Midland, Texas
Hit the search feature -top of page- type cruise control ...
There are a number of hits, some are useless. But there are several on micro-switch ( throttle ?), connections, and lever base clearances, check it out.
Let us know results, bitte.
There are a number of hits, some are useless. But there are several on micro-switch ( throttle ?), connections, and lever base clearances, check it out.
Let us know results, bitte.
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Joined: Oct 2006
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From: Porterville, Ca.
Welcome Bob, on my '81 the computer had gone bad. It is located under the front consul and can be removed from the passenger side. After removing the side panel you will see a (approx.) 4"x6"x2" metal box held down by a single (8mm?) bolt. When removed you can open and try resoldering the connections that look bad. Personally I have never had luck doing this but others have. 928intl. has used for $75 or so. Actually I believe they may still have the half price sale going until the end of year, if so then only $37. Rog100 has this also I believe, but not positive and not sure of price. Hope this helps, and again welcome to the water!
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From: Northwest
Welcome Bob, on my '81 the computer had gone bad. It is located under the front consul and can be removed from the passenger side. After removing the side panel you will see a (approx.) 4"x6"x2" metal box held down by a single (8mm?) bolt. When removed you can open and try resoldering the connections that look bad. Personally I have never had luck doing this but others have. 928intl. has used for $75 or so. Actually I believe they may still have the half price sale going until the end of year, if so then only $37. Rog100 has this also I believe, but not positive and not sure of price. Hope this helps, and again welcome to the water!
H2
Team Owner
Joined: May 2004
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From: Philly PA
Hi Bob Welcome Happy holidays.
If you can please add more info about your car, auto, stick, model, usa or euro, try adding it to you profile it makes assisting you easier........ Or said another way guessing on your problem has a higher chance of success.
That siad you might want to check for micro switches on the clutch pedal if there is one, also check the hot post connections under the hood with the battery disconnected, and also the battery connections and the CE panel connections also pull all of the fuuses and check for corrosion, and look for water leak tell tales. If there are water leakes then the blower box seal should be renewed. Get a fuse chart here, www.928gt.com also pull all of the relays and check the pins for corrosion.
Any relay with corroded pins should be opened and inspected.
Also suspect is the CC switch
If you can please add more info about your car, auto, stick, model, usa or euro, try adding it to you profile it makes assisting you easier........ Or said another way guessing on your problem has a higher chance of success.
That siad you might want to check for micro switches on the clutch pedal if there is one, also check the hot post connections under the hood with the battery disconnected, and also the battery connections and the CE panel connections also pull all of the fuuses and check for corrosion, and look for water leak tell tales. If there are water leakes then the blower box seal should be renewed. Get a fuse chart here, www.928gt.com also pull all of the relays and check the pins for corrosion.
Any relay with corroded pins should be opened and inspected.
Also suspect is the CC switch
Thread Starter
4th Gear
Joined: Dec 2008
Posts: 4
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From: South Texas
Thanks for the input. By the way, mine is an automatic (originally from California) so the microswitch is bypassed - I don't know where that shunt is but I believe that I found the unused connectors tied back near the servo. I checked the resistance and found 0 ohms so I assume that the by pass is in place. I have already replaced the control unit (computer) and still get the same behavior. Soon after acquiring the car I pulled all fuses and cleaned all contacts on the fuse holders and the fuse contacts. I also removed the fuse panel and cleaned the ground contacts on seven brown wires connected to two terminals behind the panel. I've pulled the relays as well and cleaned what I could - overall no eveidence of water intrusion just oxidation that occurs over time. I think that the CC switch is probably Ok since I can actually accelerate the car using that switch.
Danke
Bob
Danke
Bob
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From: Northwest
H2
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From: Park Ridge, IL (near Chicago)
Welcome.
I replaced my cc brain twice. The first was a dud. 928 Int'l sent me a replacement and it was fine. I seem to remember the WSM troubleshooting had about 20 steps to check function of the stalk lever, the servo and even speedo input. If these indeed all checked out (as they did for me) I would suspect a bad replacement brain and ask for another.
Good luck,
I replaced my cc brain twice. The first was a dud. 928 Int'l sent me a replacement and it was fine. I seem to remember the WSM troubleshooting had about 20 steps to check function of the stalk lever, the servo and even speedo input. If these indeed all checked out (as they did for me) I would suspect a bad replacement brain and ask for another.
Good luck,
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Thread Starter
4th Gear
Joined: Dec 2008
Posts: 4
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From: South Texas
Dave,
Thanks for relaying your experience. Since I did go through all of the troubleshooting steps as you did and replaced the control unit I have been searching for some other explanation but it seems that a faulty unit may be the case.
H2. I pulled apart my old control unit yesterday but couldn't find any obvious flaws at the solder joints - how difficult would these be to identify? I did scrape away some of the clear coat and checked the continuity between the solder joints and the pins and found all to be OK.
Thanks again,
Bob
Thanks for relaying your experience. Since I did go through all of the troubleshooting steps as you did and replaced the control unit I have been searching for some other explanation but it seems that a faulty unit may be the case.
H2. I pulled apart my old control unit yesterday but couldn't find any obvious flaws at the solder joints - how difficult would these be to identify? I did scrape away some of the clear coat and checked the continuity between the solder joints and the pins and found all to be OK.
Thanks again,
Bob
Racer
Joined: Nov 2006
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What's a good way to remove the coating on the circuit board if somebody wants to reflow all the solder joints? Will wiping it with alcohol or some other solvent remove it without damaging anything else?
Electron Wrangler
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From: Phoenix AZ
I'd test the function of all the switch pins on the stalk switch. If it accellerates but then does not hold a cruise set point - that is the same behaviour as if the cancel switch was active when the lever is at rest - I'd test for that specifically. Otherwise it must be a bad brain again which seems odd.
Alan
Alan
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4th Gear
Joined: Dec 2008
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From: South Texas
Thanks Alan,
I did check that cancel switch function as part of the troubleshooting listed in the manual and it tested OK. Meanwhile I have resoldered some joints on my old controller and will reinstall and test that one. While I've got it exposed I'll test the contacts again just in case. It did seem curious to me that both my original and the reconditioned replacement brain did exactly the same thing while all of the inputs tested OK and the servo seems to function!!!! I guess this is why we buy cars like this - such mysteries are better than watching CSI.
Bob
I did check that cancel switch function as part of the troubleshooting listed in the manual and it tested OK. Meanwhile I have resoldered some joints on my old controller and will reinstall and test that one. While I've got it exposed I'll test the contacts again just in case. It did seem curious to me that both my original and the reconditioned replacement brain did exactly the same thing while all of the inputs tested OK and the servo seems to function!!!! I guess this is why we buy cars like this - such mysteries are better than watching CSI.
Bob
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From: Boulder Creek, CA
You can reflow the solder joints, but you will need to add just a touch of solder to each -- I recommend 0.031" dia. Rosin Core -- the reason for this is the flux in the new solder will help the joint flow properly. IIRC I used Berryman's B-12 carb cleaner to clean the coating off the back side of the PCB. Just re-solder all of the joints, being careful to not apply too much heat, and don't bridge any connections.