Cruise control question
08-22-2004, 09:08 PM
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Cruise control question
My 1984 928S has cruise control. When I activate it it engages, the throttle moves forward etc. However it slowly loses the speed setting. When I use the stalk to accelerate, it works. My question is that it appears that it is functioning correctly from an electrical standpoint, but may be losing vacuum. Anyone else seen this and am I correct in this assumption?
Karl
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08-22-2004, 10:05 PM
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I'm also a part of that club. Who ever finds the culprit first. please post. I will be getting into it when i'm done with the timing belt/water pump/cam cover gaskets etc.
08-22-2004, 10:07 PM
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Might want to check for cold-solder joints in the control brain. I had the same problem: set the cruise, it would hold very briefly, and then gradually the speed would fall off. Looked for vacuum leaks for a while, couldn't find any, and gave up. Eventually took the brain out and resoldered any joints which looked even remotely bad, and some that looked fine. Cruise works great now. There's a check valve on the line to the vacuum resevoir which is fairly easy to check if you want to start there. The resevoir is huge, so I think you'd need a big vacuum leak to cause any problems.
Courtesy of Wally Plumley at 928 Specialists:
Courtesy of Wally Plumley at 928 Specialists:
If the cruise control doesn't work, the first thing to try is to find the rubber cross connection near the vacuum brake booster. One line runs from the check valve on the brake booster to a small black/blue check valve, and from there to the rubber cross. From the cross, one line runs to the firewall - this is the main vacuum feed for the HVAC system. Another line runs to the front fender - this is the line to the vacuum reservoir and the cruise control servo (tempostat). The fourth line is a vacuum test port, and should be plugged.
Disconnect the vacuum line betweent he black/blue check valve and the rubber cross. Crank the engine, and make sure that there is a strong, steady vacuum on the line.
Disconnect the line going to the firewall, and plug that leg of the cross. If the cruise control now works, you have leaking vacuum servos in the HVAC system under the dash.
If the cruise control doesn't work when hooked directly to the vacuum source, the problem is more likely to be electrical than vacuum.
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Wally Plumley
928 Specialists
Disconnect the vacuum line betweent he black/blue check valve and the rubber cross. Crank the engine, and make sure that there is a strong, steady vacuum on the line.
Disconnect the line going to the firewall, and plug that leg of the cross. If the cruise control now works, you have leaking vacuum servos in the HVAC system under the dash.
If the cruise control doesn't work when hooked directly to the vacuum source, the problem is more likely to be electrical than vacuum.
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Wally Plumley
928 Specialists
08-23-2004, 02:40 AM
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Dave A.-
I checked out your site last night, and I was wondering what you used to dissolve/clean the coating on the solder joints. I ended up scraping each joint with an X-acto knife, as I couldn't find anything that would dissolve the coating. If I ever have to repeat the process, I'd like to have an easier solution.
Ron- Thanks for the parts. Hopefully my shipment made it to your doorstep by now.
I checked out your site last night, and I was wondering what you used to dissolve/clean the coating on the solder joints. I ended up scraping each joint with an X-acto knife, as I couldn't find anything that would dissolve the coating. If I ever have to repeat the process, I'd like to have an easier solution.
Ron- Thanks for the parts. Hopefully my shipment made it to your doorstep by now.
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08-23-2004, 03:06 AM
#8
Instructor
Sounds like vacuum leakdown to me. Isolate parts of the vacuum circuit and do a vacuum check on them with the aid of a Mitivac or other vacuum pump tool. The hose where it meets the diaphram is notorious for hardening with age and developing a small crack, sometimes on the side where you wouldn't notice it.
08-23-2004, 04:24 AM
#9
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Originally Posted by Andrew Schauer
Dave A.-
I checked out your site last night, and I was wondering what you used to dissolve/clean the coating on the solder joints. I ended up scraping each joint with an X-acto knife, as I couldn't find anything that would dissolve the coating. If I ever have to repeat the process, I'd like to have an easier solution.
Ron- Thanks for the parts. Hopefully my shipment made it to your doorstep by now.
I checked out your site last night, and I was wondering what you used to dissolve/clean the coating on the solder joints. I ended up scraping each joint with an X-acto knife, as I couldn't find anything that would dissolve the coating. If I ever have to repeat the process, I'd like to have an easier solution.
Ron- Thanks for the parts. Hopefully my shipment made it to your doorstep by now.
