All of a sudden the cruise control will not come on. I see several posts where folks say a bad brake light, backup light, etc. will cause this. Is this true for a 1981 model which has not a single computer?

Anyone know?

The cruise controls is dependent on there being standard incandescent light bulbs in the brake light spots. Other spots don't matter.

The 81 300SD has a 14-pin cruise control system. I've done extensive work with them over the years. Did it all of a sudden stop working? How was performance before hand?

__________________
Cruise Control not working? Send me PM or email (jamesdean59@gmail.com). I might be able to help out.
Check here for compatibility, diagnostics, and availability!

(4/11/2020: Hi Everyone! I am still taking orders and replying to emails/PMs/etc, I appreciate your patience in these crazy times. Stay safe and healthy!)


82 300SD 145k
89 420SEL 210k
89 560SEL 118k
90 300SE 262k RIP 5/25/2010
90 560SEL 154k
91 300D 2.5 Turbo. 241k
93 190E 3.0 235k
93 300E 195k

JD - it had a mind of its own. I would set the speed and a minute or two later it would decide to work; worked fine the rest of the day after that and it always holds speed fine. As I remember once it started working that day, it would then turn back on again later that day as designed. Then over the last week it is just caput.

What the heck does incandescent lamps have to do with it; something about amps being pulled through the circuit? Over my head. Is it only the brake lights or how about a dud light in one of the other sockets?

It definitely sounds like your amplifier is was on its way out. These amplifiers suffer from cold solder joint and capacitor issues. I think that it finally gave up the ghost on you. I've seen quite a few unit behave like this.

The cruise control system expects to see a path to ground via the brake lights. If it does not see this path it will not engage. Sometimes folks put in LED lights to improve lighting however LED lights can be built in ways that eliminate the expected path thus disabling the cruise.

If you are interested in having the amplifier rebuilt do let me know. I'm happy to help out.

The cruise is only connected to the brake light sockets.

__________________
Cruise Control not working? Send me PM or email (jamesdean59@gmail.com). I might be able to help out.
Check here for compatibility, diagnostics, and availability!

(4/11/2020: Hi Everyone! I am still taking orders and replying to emails/PMs/etc, I appreciate your patience in these crazy times. Stay safe and healthy!)


82 300SD 145k
89 420SEL 210k
89 560SEL 118k
90 300SE 262k RIP 5/25/2010
90 560SEL 154k
91 300D 2.5 Turbo. 241k
93 190E 3.0 235k
93 300E 195k

Thanks; will keep that in mind. Good info.