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1978 300d
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395 Posts
Discussion Starter #1
i've only gotten it to work 2 or 3 times since august. when i push and hold up (accel. set) the car accelerates until i let go. when i push up while driving and then pull towards myself (resume) it does nothing. from what i remember, i've gotten it to work using a push up/pull towards myself combo maneuver a few times, but that's it. can anyone tell me what i'm doing wrong? or if my cruise isn't working right? thanks!
 

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1981 240D
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185 Posts
It definitely sounds like your CC is not working correctly. Up is accel/set and down is coast/set. You should only have to lift up or push down and release in order to engage your CC. Resume is for resuming your last cruise speed after hitting the brake. It is difficult to say what is wrong with your cruise control. Is your 300D a manual or automatic transmission? Your problem could be as simple as a worn/dirty contact point in the control switch, or it could be as complicated as a vacuum problem, speedo problem, or CC control module circuitry problem. If you search this thread, there is a wiki on troubleshooting your CC system. Does your speedo 'bounce'? If so, under what circumstances?
 

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1978 300d
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395 Posts
Discussion Starter #4
It definitely sounds like your CC is not working correctly. Up is accel/set and down is coast/set. You should only have to lift up or push down and release in order to engage your CC. Resume is for resuming your last cruise speed after hitting the brake. It is difficult to say what is wrong with your cruise control. Is your 300D a manual or automatic transmission? Your problem could be as simple as a worn/dirty contact point in the control switch, or it could be as complicated as a vacuum problem, speedo problem, or CC control module circuitry problem. If you search this thread, there is a wiki on troubleshooting your CC system. Does your speedo 'bounce'? If so, under what circumstances?
my car is an automatic with 146,000 miles. the only time my speedo bounces at all is when i'm moving slow in traffic..10-20mph.
 

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when i push and hold up (accel. set) the car accelerates until i let go.
Sounds like the cruise control amplifier is bad. Mine did the same thing before I had it rebuilt. This is a common failure symptom. In normal operation, you should either lift up or push down momentarily to set, pull back for resume, push forward for off, push down and hold for decel/set, or lift up and hold for accel/set. They operate pretty much like any modern car, except you can't tap up or down to changed the speed by only one or two MPH like on new cars.

Check out the FAQ section on this site, General Development Laboratories (GDL):

General Development Laboratories - Table of Contents

This is where I had mine repaired. There is also a lot of information about the cruise controls systems, how they work and troubleshooting.

Some people have luck resoldering the circuit board. But if you do that and it doesn't work, places like GDL will not repair them.
 

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1978 300d
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395 Posts
Discussion Starter #6 (Edited)
Sounds like the cruise control amplifier is bad. Mine did the same thing before I had it rebuilt. This is a common failure symptom. In normal operation, you should either lift up or push down momentarily to set, pull back for resume, push forward for off, push down and hold for decel/set, or lift up and hold for accel/set. They operate pretty much like any modern car, except you can't tap up or down to changed the speed by only one or two MPH like on new cars.

Check out the FAQ section on this site, General Development Laboratories (GDL):

General Development Laboratories - Table of Contents

This is where I had mine repaired. There is also a lot of information about the cruise controls systems, how they work and troubleshooting.

Some people have luck resoldering the circuit board. But if you do that and it doesn't work, places like GDL will not repair them.
how much did it cost you for repair? my brother is a mechanic, so he might be able to help me out..

edit: never mind, found the cost on their site..not worth the money right now, still have to get a muffler and other fun stuff
 

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'79 300TD - '74 Triumph Spitfire, '86 Alfa GTV6 & Spider
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My car was doing the exact same thing. Re-soldering the Cruise Amp circuit board fixed it. Not hard, just tedious and you need steady hands and a SMALL pencil soldering iron, I have a 20w one from Radio Shack that I use for this sort of thing. Cost ~$10, I think.

-Kevin

'79 300TD
 

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1978 300d
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395 Posts
Discussion Starter #8
My car was doing the exact same thing. Re-soldering the Cruise Amp circuit board fixed it. Not hard, just tedious and you need steady hands and a SMALL pencil soldering iron, I have a 20w one from Radio Shack that I use for this sort of thing. Cost ~$10, I think.

-Kevin

'79 300TD
great! is there a tutorial somewhere i could use to aid my attempt at this project? i'll have to get another soldering iron, mine's ancient and the tip resembles that of a sharpie marker :D
 

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1981 240D
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185 Posts
how much did it cost you for repair? my brother is a mechanic, so he might be able to help me out..

edit: never mind, found the cost on their site..not worth the money right now, still have to get a muffler and other fun stuff
You don't need a mechanic so much as an electronics repairman. I am an electrical engineer and I have discussed this very topic with a fellow engineer at work who is a BMW fanatic. You might get temporary results by reflowing the joints, but you risk damaging any fragile electronic components or printed circuits (if there are any). Additionally, there may be capacitors/resistors that need to be replaced. The only way to truly fix your problem is to go through the schematics of the module and make sure each intersection of the circuit is electrically sound. KCM is right about them not repairing amps that you attempted to repair and a lot of places will not accept a botched repair job as a core. For most people, the CC system is very low priority. I would focus on higher priority things your car needs. Also, you need to address the speedo bounce before the CC will function properly anyway. Also, I would make sure my vacuum system is tip-top. Keep researching other CC threads. There is plenty of information here. I am not 100% sure about the function of an A/T actuator on your 300D, my only 123 is a manual trans.
 
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