Cruise control troubleshooting

[Elwood P Dowd]

I've got a 87 Diplomat SE that the cruise is not working in. Speedometer works fine so I'm assuming the speed is getting through it and too my speedometer and there is a strong vacuum coming from the power brake booster. Anyone have any tips for troubleshooting these things. I have to admit with all I've done to vehicles I've never had to troubleshoot a cruise control system and I'm a bit stumped as to where to start here. I appears there are only 3 wires going to the unit under the hood. Is it possible to test the vacuum system that holds the throttle?

 

[Oldiron440]

Every one I've messed with it has been the controller but I haven't had much to do with them and its been years.

 

[Aspen500]

Is it the same style as the older ones were? I'm not familiar with what the '87 is like.
What I have seen in the past is, broken wires near the speed control switch, a faulty brake light switch (the portion for the cruise) and a broken actuator cable. There is a fuse (20 amp I believe).
Check the cable from the servo to the carb first, just to be sure. I'm not aware of any way to test the servo unit itself other than to eliminate all other possible causes. Yeah, not real helpful.
There is a diagnostic tree and test procedures in the service manual but the one I have is on CD-ROM so you can't copy and paste (that I'm aware of).
This is the style servo I'm referring to:

 

[Elwood P Dowd]

This is what mine looks like.

 

[Elwood P Dowd]

Which basically looks like what you have but mine has more plastic

 

[Kernel Sanders]

There is not much that can go wrong with the cruise circuit... If your speedo works then your cables are good

I've only ever seen these not work because of chewed up/disconnected wires (and there's really only a few wires) and a servo gone bad (which is what I've seen most often)

The big difference between the 70's and 80's servo's is only the material they are made of went from aluminum housings to plastic housings and they were a little bigger in the plastic form... they are interchangeable physically and electrically so just find a pick n pull one or two and give em a try.... after inspecting your wiring anyway.

If you need specific pictures of the wiring let me know. This is a full system that out of the car (I have another out of an 80's truck as well)

There is a cruise schematic out there as well but I cant find it at the moment

 

[Elwood P Dowd]

Kernel, looking at those pictures makes my chuckle. Could you imagine in this day and age a dealer installed cruise control! I can't imagine the look on the average technicians face when told they need to add wiring to the steering column.

 

[Kernel Sanders]

I know I wont touch a steering column on any car with an airbag... scares the crap out of me

 

[Aspen500]

I installed dealer installed cruise (and A/C) numerous times back in the '80's. Air bags are nothing to be afraid of, you just need to know what to do and, more importantly what NOT to do, like check the wiring with an ohmmeter except when and how specifically instructed in the service manual. The battery in the meter will ignite the bag or bags. (auto tech since 1984).

I recall having problems with the cruise (which was dealer installed ironically enough) on my '78 Cordoba and it was always the connections at the servo. The terminals would get loose and even though they were slid over the pins, there was a poor connection. Cleaning and squeezing the terminals together made it good for a year or so. Something else to check anyways.

 

[Elwood P Dowd]

I installed dealer installed cruise (and A/C) numerous times back in the '80's. Air bags are nothing to be afraid of, you just need to know what to do and, more importantly what NOT to do, like check the wiring with an ohmmeter except when and how specifically instructed in the service manual. The battery in the meter will ignite the bag or bags. (auto tech since 1984).

I recall having problems with the cruise (which was dealer installed ironically enough) on my '78 Cordoba and it was always the connections at the servo. The terminals would get loose and even though they were slid over the pins, there was a poor connection. Cleaning and squeezing the terminals together made it good for a year or so. Something else to check anyways.

That's funny that you should say that, I've been playing with those connections thinking about how they don't feel like they are making good contact. I'll probably check that tomorrow, tonight I ended up putting all new bushings in the windshield wipers. You need 4 in case anyone is wondering.
Also my dad had a 78 Cordoba that I wanted so bad when I turned 16, he didn't let that happen. I used to love listening to that 4bbl singing it's song, I wouldn't have been a bit surprised to see it suck the hood in. I've wanted a Cordoba for years but I've never been in the right place at the right time.

 

[Aspen500]

Not to hijack your thread but, I loved that Cordoba and even though it's been gone since 1996, I still miss it. What happened to it you ask? Wisconsin salt happened to it in all the wrong places, like the front spring hanger bracket which decided it wanted to be inside the car one day. Body was still pretty good and the interior looked darn near new yet. Sigh........... (horrible photo, sorry)

 

[Elwood P Dowd]

Not to hijack your thread but, I loved that Cordoba and even though it's been gone since 1996, I still miss it. What happened to it you ask? Wisconsin salt happened to it in all the wrong places, like the front spring hanger bracket which decided it wanted to be inside the car one day. Body was still pretty good and the interior looked darn near new yet. Sigh........... (horrible photo, sorry)
View attachment 37253

I'm sure dad's would have suffered the same date here in Ohio. I'm always on the lookout for one but even those are becoming cost prohibitive anymore

 

[BudW]

The Two cruise servos: the ‘70’s (mostly metal) and the ‘80’s (mostly plastic) will interchange - but the fender bracket is different between the two. Everything else will interchange except differences between the years with switches and stop light switches.

The plastic version has more “steps” to it, so when you push button to speed up/slow down, it will do so in smaller increments than the metal version servo does. Also, the vacuum pod part is larger, mainly for smaller engines (4-cylinders) and will make a speed change faster than the metal version will.

I installed cruise onto my ’67 Plymouth Satellite using the plastic version servo (4-cyl version). The car was automatic 340 4-bbl. Driving down an interstate someone pulled from on ramp right into my path. I quickly changed lanes – so no problem. No cars beside me and I was alert to the possible conflict.
What scared the bejeevers out of me was the cruise control put car into passing gear and opened the 4-bbl. I was not expecting that, as the car literally took off without me doing a thing. The lane change didn’t slow the car down that much (I thought).

The fender servo bracket is an FMJ-body only part. There is one version for all metal design (early) and plastic version (late).

I attached (below) the diagnostic flow chart from my ’77 FSM (Factory Service Manual) – which should be the same up to ’89. If any differences, it will be minor.

This bracket is a 4-bbl (only) part and not so easy to find. The 2-bbl bracket is “L” shaped.

BudW