Cruise control misbehaving

[gogmorgo]

On my ‘91 4.0. 
About a week ago on a long drive I noticed it wasn’t holding the set speed, sometimes it gradually lost speed, but if I barely poked the throttle it would accelerate back up to speed on its own. It did it sporadically over the course of a 600+km drive, stopping and shutting the truck off a few times through the day. Doesn’t seem to have anything to do with hills or anything.
After sitting overnight it was fine for a week and over 1000km of driving. Today it did it again a couple times early on but then was fine for 400km or so, but then it started gaining speed. Sometimes gently, just picking up 5-10 km/h then and slowing back down to set speed, sometimes wide open throttle right up to ~145km/h when it shuts off the cruise.
 

Anyone have any insight? Things to check? I thought the cable might have been sticking but that shouldn’t pull the throttle open. I don’t think it’s a sensitivity adjustment either because it works fine most of the time, including through the mountains. Doesn’t seem like the buttons are sticking either. 

I’m not sure what I like less, having it sneakily scrub off speed so it takes longer to cross Saskatchewan, or having it randomly put the hammer down with no warning. 

[fiatslug87]

Vacuum? Just guessing.

[gogmorgo]

Wouldn’t surprise me if there was a vacuum leak, but it doesn’t have other vacuum leak symptoms like vents going to defrost. I also don’t really see a vacuum leak pulling the throttle wide open. 
After some reading online I’m leaning towards the servo unit. I’ll have to find some diagnostic procedures first though. Not many available online. 

[pizzaman09]

Might be worth making a little adjustment to the dashpots on the yellow cruise control module.  Take note of your original settings then make some adjustments, there is a centering adjustment that to me sounds like you need to turn up slightly.

 

This is of course only going to help if it isn't a vacuum leak.

[gogmorgo]

5 hours ago, pizzaman09 said:

Might be worth making a little adjustment to the dashpots on the yellow cruise control module.  Take note of your original settings then make some adjustments, there is a centering adjustment that to me sounds like you need to turn up slightly.

 

This is of course only going to help if it isn't a vacuum leak.

I don’t think this is the problem. 99% of the time it works just fine and holds speed without an issue, so I’d rather not mess with the adjustment. And considering it just rolled over 540,000km this morning it doesn’t make sense to me that it would suddenly need adjusting after all that mileage. I don’t see a centre adjustment problem making it randomly accelerate wildly beyond where it’s set at either.

In case the metric is confusing, I’ve had it set around 70mph and when I let it run up on its own it was pretty near full throttle up to 90mph where it disengaged the cruise on its own. If memory serves that’s around the fastest it would let me set the cruise so that at least seems consistent. 

There was nothing obviously wrong under the hood this morning, and it’s done almost two hours of highway driving over rolling terrain without the speed wavering. I pulled onto the highway behind a Lincoln Corsair and it stayed locked on it for 175km, over 100miles. I’ve got another 450km this afternoon before I get home, and can give it a more thorough look. 

[pizzaman09]

On 2/25/2024 at 2:37 PM, gogmorgo said:

I don’t think this is the problem. 99% of the time it works just fine and holds speed without an issue, so I’d rather not mess with the adjustment. And considering it just rolled over 540,000km this morning it doesn’t make sense to me that it would suddenly need adjusting after all that mileage. I don’t see a centre adjustment problem making it randomly accelerate wildly beyond where it’s set at either.

In case the metric is confusing, I’ve had it set around 70mph and when I let it run up on its own it was pretty near full throttle up to 90mph where it disengaged the cruise on its own. If memory serves that’s around the fastest it would let me set the cruise so that at least seems consistent. 

There was nothing obviously wrong under the hood this morning, and it’s done almost two hours of highway driving over rolling terrain without the speed wavering. I pulled onto the highway behind a Lincoln Corsair and it stayed locked on it for 175km, over 100miles. I’ve got another 450km this afternoon before I get home, and can give it a more thorough look. 

Though I do agree with you in concept, electronics can drift over time as the capacitors dry out and such.  A little adjustment could be helpful.  I found that it was pretty easy to do, my brother drove down the highway in cruise and I adjusted the dashpots just a bit at a time until it ran perfectly.

[schardein]

This is a 91, correct?

There is no yellow cruise control module- not used in the HO years.  No adjustments at all that I know of?

 

The FSM has a trouble shooting chart.  The only problem description that is close is "Hunting Surge Or Speed Variations At Low Speeds" the chart says 1. Lock up torque converter roughness 2. Amplification of engine surge 3. Defective servo 4. Defective cable 5. Defective engine controller

 

I'm not sure that will be much of a help.

 

There is more to the chart, most of it leans towards vacuum leaks, defective vacuum check valve, defective vacuum reservoir, defective cable, defective switches, defective servo.

 

The FSM also suggest checking for codes.  That's a simple check and you might get lucky and get a lead. (the Key On-Off three times procedure)

 

I had to replace the servo on my 91 XJ when I got it, to get the cruise to work.  That was around 2010.  Since then I've had to replace it again.  Both replacements came from the junkyard.  Based off this experience, my first thought is a defective servo.

 

There are also some electrical checks on the servo listed in the FSM.

 

If you would like to me to post screenshots (or message you) of the FSM pages, just let me know.

 

Other than that, I suppose some frayed or damaged wires from the control switch on the stalk could cause such an issue.  Maybe they are pinched somewhere, or a mouse chewed them, or there is wear in the wires at the pivot points near the turn signal stalk, or at the pivot point of the tilt column, if equipped.  But it seems to me that would more likely result in no operation, rather than the symptoms you have.

[gogmorgo]

Yeah I was wondering about adjustability. I was sure it’s just an ECU function on a ‘91+
Haven’t had a chance to do much digging yet after getting home. Its setting up to be a busy week, and neither my brain or body are at 100% right now.

But about 440 of the 450km I had left were done with no cruise. Brutal driving across Saskatchewan without it. 
It progressed from the ocasional random acceleration to being pretty well all it does. Within 5-10 seconds of setting the speed now it just pulls the throttle wide open. Sometimes it lets go and coasts back down to the set speed but it usually starts pulling again before it gets there.

 

It cuts out if I touch the brake pedal and then won’t resume on its own which makes me think it’s not an issue with the resume/accel switch being stuck or damaged wires in the column. Although I was in there this fall changing the wiper switch so I did wonder about that. And just, general age and usage of components.

No codes beyond 12 and 55 any time I checked, although I haven’t checked since it’s started only accelerating. I was wondering more for throttle position sensor possibilities and that wasn’t it but I see now there’s also a cruise circuit code on the list. I’ll have to check in the morning. 
 

From the limited info I found online I’m also leaning heavily towards the servo.  Test procedures for it would be helpful, yes. 
 

I noticed a couple NOS servos on eBay so I’ll probably swing for one of those if I need to replace it. We had a decent junk yard here but they quit letting us plebes pull our own parts a few years back and they have no interest in pulling small stuff for anyone. Frustrating.

 

 

 

 

[pizzaman09]

I had missed that this was a 91.

[schardein]

From the 91 FSM.

EDIT: these were much more readable before I uploaded them.  If you care to msg me your email, I could try that.

[gogmorgo]

11 hours ago, schardein said:

If you care to msg me your email, I could try that.

 

No this is good enough for me. Thanks!