• Welcome Visitor! Please take a few seconds and Register for our forum. Even if you don't want to post, you can still 'Like' and react to posts.

Speed Control Servo


85_Ranger4x4

Wallows in rivers

TRS Forum Moderator
TRS Event Staff
💻 TRS Socials
Article Contributor
TRS Event Participant
TRS 20th Anniversary
VAGABOND
OTOTM Winner
V8 Engine Swap
TRS Banner 2010-2011
Joined
Aug 7, 2007
Messages
35,321
Points
2,001
City
Frankfort
State - Country
IA- USA
Vehicle Year
1985
Engine
5.0
Transmission
Manual
For the most part mine works great, sometimes it surges though. On our trip to Ohio it seemed impact related, hit a big bump and surgy surgy until I pulled the vacuum line off the servo. One time I hit a big bump and it got to surging and then I hit another railroad crossing and it came out of it. It only did it thru Illinois on the way out (they have a crappy highway), across Iowa, Indiana and Ohio and then the whole way home it didn't miss a beat. It did it again after it sat all winter... until I pulled the vacuum line off the servo and it has worked perfect for two days now.

Vacuum reservoir and lines were new last fall, I have tried two different amplifiers (one for a 2.8 BII and one for a 5.0 F-150, it hisses when I pull the vacuum line off (engine not running) so the checkvalve and dump valve are holding... so I did some digging and found this:


Kinda makes sense, the valves are gummed up and stick if it gets bumped the wrong way. Since it works most of the time I wondered how hard it is to clean and lube the valves in what I have... I have a kind of crusty looking spare so I did an exploratory on it... and I don't see much and the two motor looking things don't want to come out:



How the system works:



Electronically the servo tests fine as per the EVTM, Ford shop manual and a neat thing I found here. The thread about fixing your servo looks like it might be in some other country (Australia maybe?) so they might have something different than what I have. I could keep grabbing them out of the JY hoping for something better or snag a NOS that might be a few years newer than what I have or get a $100 rebuilt one from who knows where... nothing really appealing and there is no actual valve test in any of the Ford literature I have found.

Has anybody else delved down this rathole?

@19Walt93
 
Last edited:
When it happens, the surging, if you touch the brakes, so turn off cruise, and then set it again does the surging stop?

Have you tried setting cruise on slower road with more bumps, like 35mph, to see if it happens

If you normally use cruise at 65mph it could be a wear issue, the valve or position "sensor"(50,000 ohm variable resistor)

Do you have a cable driven speedometer?
If so could be the VSS is the issue, which wouldn't be seen on cable driven speedo

Could also be engine vacuum, surge may be in response to intake manifold vacuum changes, i.e. PCV Valve
Cruise is just try to keep speed steady with changing vacuum level at its valve
 
I haven't run into that but it does sound like one of the valves may be sticking. If you can take it apart and clean them without destroying anything it would be worth a try.
 
I'd be replacing the VSS on the trans before digging into all that. They are more often the culprit, and when mine started to go the surge was intermittent.
 
When it happens, the surging, if you touch the brakes, so turn off cruise, and then set it again does the surging stop?

I can stop the truck, shut the truck off, restart the truck the next day and it will still do it... until I unhook the vacuum to the servo with vacuum in the system. Same for turning the cruise off and back on, no dice. It was doing it Sunday and Monday, I unhooked it Monday after I got home from work and it has been perfect since.

Have you tried setting cruise on slower road with more bumps, like 35mph, to see if it happens

If you normally use cruise at 65mph it could be a wear issue, the valve or position "sensor"(50,000 ohm variable resistor)

When it does it it will do it at any speed, it was doing it at 40mph Sunday.

The only time I have been able to get an impact to set it off was in Illinois. Around here sitting for extended periods seems to do it.

Do you have a cable driven speedometer?
If so could be the VSS is the issue, which wouldn't be seen on cable driven speedo

Yes, cable and VSS was new last year. VSS is a Motorcraft.
 
Last edited:
Well pressing down on the brake pedal should be the same as pulling vacuum line to intake
The vacuum dump valve(in cab) releases vacuum so vacuum valve has no "power" to hold open throttle
So should "start from scratch" as far as vacuum is concerned, unless valve is stuck, like you are thinking

The valves are just 12volt solenoids so you should be able to work them in and out with 9volt battery or jumpers from vehicle battery

"New" VSS, OMG!!!
Is that when the problem started, lol

I tell you, "new" does NOT MEAN what it used to
I am sure its fine but...............if there was no surging before it was installed...................
 
Last edited:
Well pressing down on the brake pedal should be the same as pulling vacuum line to intake
The vacuum dump valve(in cab) releases vacuum so vacuum valve has no "power" to hold open throttle
So should "start from scratch" as far as vacuum is concerned, unless valve is stuck, like you are thinking

The valves are just 12volt solenoids so you should be able to work them in and out with 9volt battery or jumpers from vehicle battery

"New" VSS, OMG!!!
Is that when the problem started, lol

I tell you, "new" does NOT MEAN what it used to
I am sure its fine but...............if there was no surging before it was installed...................

Well, that was back before I knew I had to have a vacuum reservoir so all it did was surge.

Truck started out without cruise from the factory, I added it.

And then when I swapped transmissions/transfer cases I left the VSS in the t-case to plug the hole. Coming out of the truck the trans/t-case did a barrel roll and crushed the VSS.

New VSS did the same thing until I figured out I had to have the reservoir and then it has worked for the most part.

The vacuum dump is on a different hose coming out of the servo. I think it is downstream from the valving in the servo.

 
Yes, you do need a vacuum reservoir, just the small ones that sit on top of the heater box on the 1994 and earlier are fine to use, with a check valve

Not sure if that diagram is correct about the vacuum dump valve, but maybe I am reading it wrong
"Vacuum dump" means valve is closed when foot is off the brake pedal, pressing down the pedal "dumps" the vacuum pressure which releases cruise controls ability to hold throttle cable so basically an emergency back up to electrical deactivation of cruise control

Found this description of the older Ford cruise setups: http://www.thorssell.net/hbook/ccexpl.html
 


I played with it tonight. I started it and let it run for a bit and shut it off. I pulled the vacuum dump hose off the servo (far left side) and nothing happened. I pulled the main vacuum line off the right side and it hissed. I think the vacuum dump dumps the servo itself (I am picturing some sort of bellows in there) while not draining the vacuum to the valves on the other side.

And yeah, I added the "correct" coffee can reservoir on the PS fender late last summer. It wouldn't work at all without it, constant surging.

 
And the reserve can has a check valve between can and intake(source)
 
And the reserve can has a check valve between can and intake(source)

Yup, you can see it in the first picture in my last post. It is the white disk in the main feed hose. If not for that there wouldn't be anything to hiss when I pull the main feed off the servo.
 
OK, thats not where a check valve goes

It needs to be between the can and intake, not between cruise and can

Intake(vacuum source)----------check valve-----------can-----------cruise

The can then has vacuum reserve for the cruise to use
Right now you just have the length of that hose as the reserve
 
Last edited:
All this nonsense just to try and replace this?

Foot-Xray-1024x683.jpg
 
OK, thats not where a check valve goes

It needs to be between the can and intake, not between cruise and can

Intake(vacuum source)----------check valve-----------can-----------cruise

The can then has vacuum reserve for the cruise to use
Right now you just have the length of that hose as the reserve

Its been 4 years but I am pretty sure the donor had it where I have it.

The hoses to the vacuum can are 3/16, the ones to the servo are 5/16, I don't remember them adapting the hose bigger and then back smaller again.

I can't find a straight up vacuum diagram for the cruise system though.

All this nonsense just to try and replace this?

View attachment 40692

On 10+ hour road trips it is nice to have.

For the first 19 years I had the truck it didn't have it.
 
Because you generally set cruise once you are at speed the vacuum in the intake is stable, so you probably wouldn't notice a difference
But because you are having an issue I would put a check valve between can and intake, you can leave the white one there, and that may have been stock location as a way to make vacuum more stable as the Cabs Vents and EGR systems shared the reservoir

And when you put in the can its check valve was left in donor vehicle, but I would get one
 

Sponsored Ad


Sponsored Ad

Special Events

Events TRS Was At This Year

TRS Events

Member & Vendor Upgrades

Become a Supporting Member:

Or a Supporting Vendor:

Recently Featured

Want to see your truck here? Share your photos and details in the forum.

TRS Latest Video

TRS Merchandise

Follow TRS On Instagram

TRS Sponsors


Sponsored Ad


Sponsored Ad


Amazon Deals

Sponsored Ad

Back
Top