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Cruise control won't control above 50 mph, now won't control at all

cruise control

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#1 madmanperez

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Posted 17 December 2018 - 03:08 PM

New problem, noticed it last time on I was on the highway. Around town, where speeds are in the 45 - 50 mph cruise control works (though a little sluggish to kick in). When I got on the highway (speeds upwards to 70 - 75 mph) when I tried to engage the cruise it didn't, or lets say it did but not I ended up slowing down to 50 mph. Luckily there weren't to many cars on the road. Just a couple who thought I was an old fart in the right lane. (they got the old part right)

Anyway, I realize the issue may be in the cruise control module in the engine bay. So basically any who could tell me if I need to replace or can it be rebuilt.

Thanks for the guys (or any girls that might be here) you all have never let me down.


update: total failure, no cruise control at all.

Edited by madmanperez, 18 May 2019 - 10:32 PM.






#2 CaliConquestAlex

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Posted 17 December 2018 - 05:06 PM

Check the vacuum line on the cruise actuator in the engine bay. Also check to ensure the cruise cable is properly connected to the gas pedal. Also, the cruise control requires the vacuum canister to work properly under boost.
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#3 ucw458

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Posted 17 December 2018 - 06:06 PM

View PostCaliConquestAlex, on 17 December 2018 - 05:06 PM, said:

Also, the cruise control requires the vacuum canister to work properly under boost.

The vacuum canister is for the HVAC controls only.  It doesn't store enough vacuum to operate cruise control.




3 things to check

The cruise control cable moves freely and doesn't bind in any way.

The filter on the actuator under the metal plate needs to be replaced if it's the stock one.  I used a cotton ball to replace mine.

Make sure the vacuum pump works and is connected.  There's a vacuum pump next to the radiator overflow bottle that is only for the cruise control actuator.  The relay for that pump is on the fender next to the wiper motor.  Vacuum line should go from pump to actuator then to a white check valve and finally to the intake.
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#4 madmanperez

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Posted 18 December 2018 - 01:49 PM

Thanks for the info UCW458. I'll be working on replacing a busted/snapped speedometer cable today (12/18) I'll check under the hood for per your directions, though I'll have to check on the placement of some items. I've been under the hood plenty of times but vac pump, to actuator doesn't ring a bell. I know what the cruise control actuator is and I'll crack it open to check/replace filter. I'll check back and edit this post with my findings

#5 ucw458

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Posted 18 December 2018 - 03:42 PM

Vacuum pump on the middle right, between the coolant resivour and the headlight.

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#6 madmanperez

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Posted 25 December 2018 - 01:29 PM

Thanks for the ucw548. i did find the vac. pump it does run to the cruise control unit, but, the unit is tee'd into that vav. line I'll have to check again to see where the rest of that line goes to.
How would I check if the vac. pump is working?

Also, dam that's one clean engine bay. (NICE).

#7 ucw458

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Posted 25 December 2018 - 10:09 PM

You can test the pump by putting 12v to it and holding your finger on the line.  Top line is in bottom is out.  Hose goes from pump to T for actuator.  Then to a check valve near the ABS then to the engine.  Between the check valve and engine the Vacuum canister and HVAC controls T in but they are not part of the cruise system.
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#8 Turbo Cary

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Posted 25 December 2018 - 10:32 PM

Anyone ever though of adapting a GM vacuum pump? I had a factory Conquest pump go bad causing me to chase down a intermittent vacuum leak. When I smoke tested it the diaphragm had failed. A GM pump is cheap, easy to find, and would only require a little modification to the vacuum line.

You'd have to clip the quick disconnect with the pump if acquiring from a junk yard, then get some cheap vacuum adapters to shrink the tubing down. Splice in the new connector for the pump and you're good to go.

Edited by Turbo Cary, 25 December 2018 - 10:33 PM.


#9 ucw458

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Posted 25 December 2018 - 11:32 PM

As long as it's a 12v pump with similar volume capability it will work.  But I'm sure people here have spare pumps to sell.  I have a spare for mine.  It's currently being used as a brake bleeder.


Off topic but a good size glass jar like a pickle jar and a vacuum pump it makes a great brake bleeder.  Glue 2 pieces of brake line or similar size tubing to the cap of the jar.  One tube connects to the vac pump.  The other to a brake bleed screw.  The tube that gets the bleed hose also gets a small hose for inside the jar running from the cap to the bottom of the jar.  That way you can see the bubbles coming out of the brake lines.  I've used this to bleed brakes, suck up engine oil, drain a power steering system.  I've even used it to remove gear oil from a diff that didn't have a drain plug.  It's a handy little tool you can make practically for free.
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#10 Turbo Cary

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Posted 26 December 2018 - 01:01 PM

View Postucw458, on 25 December 2018 - 11:32 PM, said:

As long as it's a 12v pump with similar volume capability it will work.  But I'm sure people here have spare pumps to sell.  I have a spare for mine.  It's currently being used as a brake bleeder.

My main suggestion is due to the factory pump being 30+ year old plastic versus a newer aluminum bodied part readily found across the nation. Your idea for a cheap-o Mighty Vac is a good idea. MV's cost anywhere from 250-500 depending on capacity.

#11 vbrad511

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Posted 26 December 2018 - 10:08 PM

My '88 did this, until I changed all the vacuum lines.

#12 mikec

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Posted 26 December 2018 - 10:48 PM

The cruise control actuator gets "working" vacuum from two sources.  The first is from the engine intake manifold via the larger port below the "row of three" - it's the same port that feeds the dash vents and the vacuum reservoir canister.  Follow the vac hose from that port, around behind the engine, and you'll see it "T" a couple of times and you'll also run into two little white gizmos in the vac hoses.  Those are check valves (=one-way valves) that let air flow TO the intake manifold when the engine is running with vacuum... and they block boost pressure FROM the intake manifold.  Eventually one of those vac hoses reaches a small cylinder shaped thing attached to the firewall near the wiper motor; this is a vacuum sensing switch.  When vacuum is low AND the cruise control power switch (on the steering column) is ON the relay turns ON to send power to the vacuum pump at the front of the car.  Thus the vacuum pump only runs when engine vacuum is low or when the engine is on-boost - the vacuum pump is the second source of vacuum for the cruise control system.  Those check valves isolate the vacuum pump from the rest of the under-hood vacuum hoses - the vacuum pump can't affect the engine nor does it supply vacuum to the dash vent actuators.

Typical issues with the cruise control are:
1: obviously leaks in the vacuum hoses to/from the cruise actuator itself.

2: leaks at the vacuum pump.  I've seen the hose split/leak underneath the rubber cover piece of the pump.

3: that little vacuum sensing switch fails.  When this happens, the cruise will work fine when the engine is not loaded too heavily (i.e. when engine vacuum is high).

4: the relay controlled by the vacuum sensing switch fails.  So the switch can't trigger the relay to send electrical power to the vacuum pump.

5: air filters in the cruise actuator are clogged as ucw458 posted.

6: where the cable exists the cruise actuator you'll see a small plastic square-shaped thing... sometimes the cable gets frayed inside this two-piece plastic gizmo.  The cable passes through the firewall and makes a U-turn underneath the dash to grab the linkage above the throttle pedal.  Kinks and binding in the cable, or wires/air ducting under the dash that are hanging low can get in the way of this linkage.

7: Underneath the dash the end of the cruise control actuator cable is clipped to the dash metal structure... via a rectangular shaped piece of metal with a slot cut in it.  That clip fits over the metal part of the cable sheathing to anchor it to the dash/chassis structure.  If the clip is missing, or is in the wrong part of the sheathing, then the whole cable flops about when the cruise control actuator tries to pull on the throttle linkage.  With the engine OFF and the throttle pedal "relaxed" to the idle position, ideally there is very little slop between the cruise cable sheathing and the throttle pedal linkage.  The closer the clip is to the end of the sheathing the better - until you go one slot (you'll see slots in the end of the sheathing) too far and there is NO slack at all... you need some slack to make sure the cruise control cable doesn't keep the throttle pedal from relaxing to the idle position.

8: an incorrect replacement cruise control cable was installed - basically if the ends of the cable (the parts sticking out beyond the protective sheathing) are too long then most of the cruise control actuator's in/out travel range is wasted absorbing this slop.  This is not too likely; consider it a last resort once you've ruled out the other possibilities.

I think you can test the vacuum pump and the vacuum sensing switch by unplugging the hose at the base of the throttle body.  Cap the port but not the hose.  This should force the cruise control system into using the vacuum pump only.  So go for a drive and try using the cruise control... if it works at all then the vacuum switch and pump are at least trying to work... the bug is elsewhere.  (hoses, jammed cable, etc.)  You'll notice during this test drive that your dash vent controls may not work either because we've disconnected their vacuum source.  (they may work for a little while based on the vacuum trapped in the reservoir/canister unit)

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#13 Indiana

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Posted 29 December 2018 - 10:11 PM

Yes dead filters.
Check valves are rubber flappers.
Break down the actuator to get to the check valves and clean the goo (that used to be a filter but now is not) off.

#14 madmanperez

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Posted 10 January 2019 - 05:34 PM

I want to thank everyone for their input. Believe it or not, it took all this time to take care of the speedometer cable problem. RockAuto has always had the parts I needed but this time, though the part picture was right, the part was not. Not once but twice. My local Autozone only had a "universal" cable, but it didn't have the right type of end to fit into the transmission. In the end leave it to Amazon to have the right part. Same image that was found on RockAuto, same part number, but the RIGHT part.

Mid you all I do is strip out the cable core and fish it through the existing sheath. Works well when you got the right part to start with. So, make note that RockAuto does not carry the right part for our speedometers. Regardless of the image and part number ATP Y-861. Amazon has the same part number but actually has the right part.

I've pretty much lost daylight as of this writing, but I have made note of all of the info and will endeavor to start figuring out the source of the fault. I'll check back hopefully with good news, if not with more questions.

Edited by madmanperez, 10 January 2019 - 05:36 PM.


#15 madmanperez

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Posted 21 January 2019 - 05:00 PM

ucw458: checked the filters under the plate of the unit and they were pretty much goo. replaced with cotton balls as you mentioned. didn't make a difference, my next step is vacuum lines and vacuum pump. last couple of days have been a little to cold to work on the car. will carry on with better temps in the next few days.

#16 ucw458

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Posted 22 January 2019 - 11:36 AM

If you try everything else and it still doesn't work then check for vacuum leaks inside the actuator.  Mine had an issue where the diaphragm inside the actuator wasn't sealing to the housing.  So I bent up the metal tabs to disassemble the actuator.  A tiny bit of rtv on the sealing surfaces stopped the leak.  The actuator works by using a controlled leak regulated by the valves to overcome the spring and pull the cable.  Any leaks in the system will cause it to not work.

You can test the actuator for leaks by using a 9v battery to close the valves and putting a vacuum gauge inline with the actuator.  Put 12v to the onboard vacuum pump then shut it off and see if the system holds vacuum.
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#17 ucw458

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Posted 22 January 2019 - 11:40 AM

View Postmadmanperez, on 21 January 2019 - 05:00 PM, said:

last couple of days have been a little to cold to work on the car. will carry on with better temps in the next few days.

LOL, too funny.  You are in central Florida complaining about cold.  I'm up in the mountains around 8,000 ft and it's snowing.  Wanna trade weather?
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#18 madmanperez

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Posted 03 February 2019 - 01:36 PM

ucw458: That's a "NO" on the weather trade.

Sorry to ask, but could you provide a little more detail for the set of instructions. This is just a little bit out of my comfort zone. (read not much of a mechanic. more computer geek. Starion 3yrs +-, PCs 22yrs+-)

greatly appreciated

#19 ucw458

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Posted 03 February 2019 - 09:49 PM

The actuator works using the valves as a controlled vacuum leak.  In their off state they provide a path from inside the actuator to open air.  If you power them then the actuator is sealed except for the vacuum line running to it.  Try powering the valves then sucking on the vacuum line running to it.  If you can get air from that line continuously then the actuator is leaking.  If you have a vacuum test tool like a handheld pump or a gauge then you can test it that way.

The reason for using a 9v battery is because it's current limited to what the battery can provide.  If you hook the car battery to it for more than a few seconds there is the potential for destroying the valves.  All they are is an electromagnet and a plunger.  A car battery may not hurt the valves but better safe than sorry.
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#20 madmanperez

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Posted 18 May 2019 - 10:29 PM

Since the cruise problem has gotten worse I'm gonna start with replacing the vacuum hose that runs from the pump to the control module and see what happens. I'm starting with that since its a 31yr old hose and its the cheapest part to replace.

Edited by madmanperez, 18 May 2019 - 10:35 PM.






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