I am suspecting something weird in the bulb failure circuit in my turbo diesel which has messed up my cruise control. Is there any way of bypassing that circuit? Is the cruise grounded that way? Can I then ground it elsewhere? Any ideas most welcome!
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George Holmer, Belgium, Europe, 1987 745 GLE Turbo Diesel Intercooler (D24TIC/M46) 200k; 1988 745 Turbo Intercooler (B230FT/M46) 290k; 1988 745 GL (B200E/M47) 190k

The Volvo and Hella engineers who designed this cruise control system didn't want it to work if it wasn't connected to the brake light circuit as an Off input. It wouldn't be safe to have a cruise control working that didn't shut down when you hit the brakes. So, they designed what is called a "supervised" circuit. The cruise control imparts a very small voltage to the brake light circuit. If no current flows through the circuit, the cruise control will not work. This is covered in the 700/900 FAQ, and I may be quoted there.

I have had a Lamp Supervision Relay failure, and the brake lights did not work. The cruise control didn't work either. Yes, you could bypass the Lamp Supervision Relay, but that is more work than just replacing it. The brake light input is 54S (Bl-R), and the outputs are 54L and 54R (both yellow). Those are terminals 9, 10, and 11 respectively.

My experience has been that when the bulb out sensor relay malfunctions all of the brake lights as well as the cruise control quit working as well. Check to see if you have brake lights. The bulb out sensor can somtimes be resoldered- examine all the solder joints and resolder any that look suspect.

Randy