Just an update. After a long trip, stopped working again towards the end. Checked the switch I put in, all good. Checked the +12v wire and the set switch still operating ok, all good. Checked the brake / clutch switches having an input into the cruise control, all good.... couldn't work it out.

I happened to notice while I was testing the wiring that I could hear a noise under the bonnet, similar to a solenoid moving. I popped the bonnet and had a look at the throttle cable and associated assembly coming from the cruise. I noticed that the throttle plate when I first moved it was a little 'sticky'. I moved it up and down a few times, then tested it, and it started working again! I lubbed it up a little to try and ensure it doesn't get stuck again.

I've had an incident before in a BMW where I drove through small flood water and the accelerator stopped working. Turned out that the throttle plate was stuck in the closed position. I inferenced that some humidity got into the inlet and closed it stuck. Same result there, lubed it up and all good.

 

A further update.

The cruise control seems to work now but on long journeys it will eventually stop after I try to engage it after it being disengaged (such as when pressing the brake).

When the car then sits for a while and you turn it on, I can hear the solenoid making a noise (like re-setting).

Any ideas how to fix it?

 

A further update....

so I opened the aftermarket cruise control unit box up, it's a Rostra brand one. Sort of figured out how it basically works. Tried lubricating it but then realised that the servo inside the cruise control module box is pretty powerful and a lack of lubrication wasn't the issue. From reading online, an aftermarket Rostra cruise control fitted on a motorbike and having similar problems, someone suggested that it might be interference, like a type of electrical interference, and they suggested to either put a capacitor between positive and negative going into the control unit module or use a car audio electrical noise filter. So I did that and it appears to be working again, but I'll have to see how long for!

 

Thanks for the updates. I have trouble getting the cruise control to engage. I press the set switch many many times and finally it will engage. But your updates are appreciated as I will look to seek out a solution.

 

I discovered on my car that I already had an aftermarket electric cruise control instead of the stock vacuum one, but I believe the set switch is a problem on these models, as in they wear out over time. I've bypassed mine altogether.

 

So it worked well for about an hour then the same problem. It's like the servo inside the box just turns to custard. Not sure what's causing it. Now when I turn the car on, I can hear the servo/solenoid moving but it still doesn't work. I'm going to open it back up and fiddle with the settings (via dip switches) on the circuit board. Some settings I don't understand which I can adjust on my cruise control:

Pulses Per Mile - 2000, 4000, 5000, 8000.
Control Switch - Open Circuit Control Switch, Closed Circuit Control Switch.

Also, the dip switch currently has it set to 5 or 6 cylinders instead of 3 or 4 cylinders. I left this setting as it was, as the cruise control worked when it was working, but I might change it to 3 or 4 and see if it solves the ongoing problem.

Anyone got any ideas on the correct settings for Pulses Per Mile and/or Control Switch as outlined above.

 

So after reading online and looking at Rostra manuals, one manual mentioned that the blue wire which connects to the tachometer is not used on a manual car, if you have a clutch switch installed on the clutch pedal. I wasn't the original installer of this aftermarket system on my car, but when previously testing, I worked out that there appeared to be a switch installed on the clutch pedal. The Rostra manual mentioned that if a clutch switch is installed, the blue wire should be grounded to prevent "trashy" signals being received. Testing the voltage of the blue wire with a micrometer displayed variable voltages going down (sort of between 6v going down to 3v). This would explain why the cruise control stopped sometimes but not others. I grounded the blue wire to a constant 12v grounding. Test drove it again and it's all working...for now. I'll have to see how it goes on my next long drive.

In the process of trying to fix this, I lubricated the shaft that carries the electromagnet with WD-40. It appears that this also lubricated the drive shaft out of the electric engine connecting to a rubber belt, which connects to the gear pulley which moves the electromagnet. This has caused the belt to slip! I degreased it but it still slipped. I shoved a piece of dishwashing sponge in their to try and guide it and stop it from slipping off the engine shaft. I moved the gear and belt backwards and forwards a number of times and it appeared to stay. A pretty dodgy fix to getting the belt to stay on I suppose but I couldn't think of any other way of keeping it on there.

 

Hello,

I am writing from Poland. I have a problem with CC in my A4B5 1997 2.8L ACK.
In that post you wrote, that the problem was speed sensor. I have a question, because it is not clear - did your speedometer work fine despite the sensors malfunction which caused the CC not work? Because mine speedo seems to work fine, no problems with speed, odometer etc so I am not sure if there is any point of replacing that sensor.

 

I performed those tests using paper clips (I jumped pins in order to start the vacuum pump etc). Everything worked fine.
Then I started to measure resistance according to internet instructions. Pin number 9 showed 11.20Volts, so I decicded to replace the speed sensor.
The voltage is good now but CC still does not work. I gave it a last try and I looked at circuit breakers one more time. I have discovered, that sth has blown fuse number 31 out. Also - it burns (almost) one route on the CCM PCB. I've checked on another CCM after replacing the fuse (I used 10A instead of 15A to be on the safe side), but it has ended the same way - blown fuse and one route almost destroyed (visible signs of heat/fire).
Now I have to figure out what is killing that fuse.
No idea for now.
"Fuse 31: 15A Reversing lights, cruise control, automatic transmission, diagnostic socket"
This is a manual transmission, reversing lights are not a problem, OBD socket also works fine...
I will try to track some wires behind the steering column,radio etc... Will see...
BTW - that route on PCB goes from pin number 12. 10mm from the connector. I wonder if it might suggests problem with the "Vac pump solenoid valve" (pin number 12 covers signal from there).