Cruise control

[Rick T]

1978 Chrysler New Yorker 440. Hi my cruise control has a mind of its own! If I set it at any speed it will engage but then lose speed then smash on the gas hard to get to speed again. This will happen over & over and repeat. Any clue what’s causing this and what part will I need to fix this issue? Thank you.

 

[CBODY67]

You can download a Chrysler factory service manual from www.mymopar.com . There might not be a listing for your model year, but the latest one in their list will work.

The earlier cruise control transducers had two little Allen head set screws on the housing. One was for the "set" function, for when you punch the button on the end of the turn signal stalk to initiate things. The other one was for "cut-in" functions, as to how far the speed would drop before the cruise would seek to initiate actions to return to the set speed. As in drop more than 2mph. Later versions deleted the "set" adjustment, I believe, leaving only the "cut-in" adjustment screw. Other than some cosmetics, that is the only difference I know of between the earlier "'68 model year" and "later" versions.

Which ever adj screw is left, make then adjustments in small increments to see how functions might be affected. Progress from there. Worst case scenario is that a rebuilt transducer might be needed. Perhaps yours might need a new diaphram or "cleaning" to make it right again? Otherwise, the might be some internal electrical contacts that have become grungy?

What apparently seems to be happening is, for some reason, the unit perceives the brake pedal switch has been tripped, then the "Resume" function happens, which results in the fast acceleration to get back to the originally-set cruising speed as quickly as possible (within the allowable accel pedal linkage movement). So you might check the brake switch adjustment/switch to see if something might be kicking it off. Yet then what is triggering the "Resume" function?

Then you can inspect the wiring in the steering column, by unplugging the connector near the base of the steering column and checking for continuity of the switch's wiring. Might be a chafed wire in that harness?

To further inspect the harness, you'd need to remove the steering wheel and probably the lock plate to get access to the mounting screw of the turn signal handle to the turn signal switch. Then tie a piece of small wire around the harness connector, rotate the end of the connector, and then gently pull it to the top for inspection. Then reverse to pull it back down the column to plug it back into the body harness.

Just some thoughts,
CBODY67

 

[Rick T]

You can download a Chrysler factory service manual from www.mymopar.com . There might not be a listing for your model year, but the latest one in their list will work.

The earlier cruise control transducers had two little Allen head set screws on the housing. One was for the "set" function, for when you punch the button on the end of the turn signal stalk to initiate things. The other one was for "cut-in" functions, as to how far the speed would drop before the cruise would seek to initiate actions to return to the set speed. As in drop more than 2mph. Later versions deleted the "set" adjustment, I believe, leaving only the "cut-in" adjustment screw. Other than some cosmetics, that is the only difference I know of between the earlier "'68 model year" and "later" versions.

Which ever adj screw is left, make then adjustments in small increments to see how functions might be affected. Progress from there. Worst case scenario is that a rebuilt transducer might be needed. Perhaps yours might need a new diaphram or "cleaning" to make it right again? Otherwise, the might be some internal electrical contacts that have become grungy?

What apparently seems to be happening is, for some reason, the unit perceives the brake pedal switch has been tripped, then the "Resume" function happens, which results in the fast acceleration to get back to the originally-set cruising speed as quickly as possible (within the allowable accel pedal linkage movement). So you might check the brake switch adjustment/switch to see if something might be kicking it off. Yet then what is triggering the "Resume" function?

Then you can inspect the wiring in the steering column, by unplugging the connector near the base of the steering column and checking for continuity of the switch's wiring. Might be a chafed wire in that harness?

To further inspect the harness, you'd need to remove the steering wheel and probably the lock plate to get access to the mounting screw of the turn signal handle to the turn signal switch. Then tie a piece of small wire around the harness connector, rotate the end of the connector, and then gently pull it to the top for inspection. Then reverse to pull it back down the column to plug it back into the body harness.

Just some thoughts,
CBODY67

Thank you I do have all the service manuals. I’ll look at them.

 

[Welder guy]

It sounds like you don’t have enough vacuum to hold your servo’s rubber diaphragm . It’s quite possible you have a hole in the diaphragm. Maybe you have a vacuum leak in the vacuum rubber hose, check valve or grommet leading to your brake booster . But it’s most commonly the diaphragm in the servo so then the servo will need to be replaced. I’ve probably picked up a dozen of the servos at the auto wreckers for about 20 bucks apiece. That same servo was used from 1970 to about 1980 after 1980 about 89 a black plastic version was made . It looks the same and can be used but may have a slightly different mounting bracket. There are a few threads on this very subject if you do the search .

 

[Welder guy]

A guide to your Speed Control Servo This is a very informative thread

 

[Rick T]

It sounds like you don’t have enough vacuum to hold your servo’s rubber diaphragm . It’s quite possible you have a hole in the diaphragm. Maybe you have a vacuum leak in the vacuum rubber hose, check valve or grommet leading to your brake booster . But it’s most commonly the diaphragm in the servo so then the servo will need to be replaced. I’ve probably picked up a dozen of the servos at the auto wreckers for about 20 bucks apiece. That same servo was used from 1970 to about 1980 after 1980 about 89 a black plastic version was made . It looks the same and can be used but may have a slightly different mounting bracket. There are a few threads on this very subject if you do the search .

Thank you I didn’t think of that. My car has a big cam built engine so vacuum has been an issue. I also have little issue with the brakes, however, once it’s warmed up, the problem goes away with the brakes.

 

[Welder guy]

Brake light switch /w cruise control fix

Last couple of days I've been working on trying to get my cruise control working again and found that in fact the cruise part of my brake light switch had gone bad. When looking online th non cruise switches are readily available, but the cruise ones are hard to find. The one NOS I found was...

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