surging cruise control 1991

[rabo]

My cruise has been surging ever since I got the 1991 explorer (now 5 months).
If I go uphill everything is stable so it is not a lack of vacuum power.
Also , the surging decrease considerable if the car is ran more than 30 minutes.
Anybody any suggestions?
Thanks in advance, Martin

 

[B94Sport]

Do you mean that the truck will drop RPMs and slow down and then speed up again and continue that process? My '94 does the same thing when I first set the cruise, but it goes away after a period of time like yours. Also, I have found that what speed you set it at makes a difference. In mine, if I try to set it around 65, it will surge. If I set it at 75, it will stay perfectly steady. I believe it may have something to do with the transmission and which gears it uses between 60-70 mph... Like it keeps switching between two gears and can't decide which to use.

 

[rabo]

No

I THINK MINE HAS TO DO WITH ENGINE LOAD; IF YOU DRIEV UPHILL OR FAST IT IS LESS CRITICAL.
MAYBE I SHOULD LOOK AT THE SENSOR.
Oops, sorry for capitals,
Martin

 

[Longs]

My '93 has been doing the same thing, as you describe. I have been checking vacuum lines, etc. without finding any leaks. I also pulled the vacuum modulator line from the transmission and didn't see any obvious signs or leakage or failure. The behavior is erratic - sometimes it acts up, sometimes not.

I don't think it's related to the transmission - I notice that mine is stabbing the gas pedal.

Someone posted that their acceleration control module was diagnosed as causing a problem with the cruise, but I'm not finding much information on this.

 

[Jason94sport]

Could be a faulty cruise control cable. It's the second wire that goes into the trottle body. I had a reverse problem. My gas pedal was sticking with the cruise off, when I put it on, it was fine. I replaced the cruise cable, & it went away. I also replaced the throttle cable while I was at it.

 

[Longs]

I thought about the cable, too. Visually, it seems OK, and the surging just has a different feel than I would expect if the cable were sticking.

I'm still thinking speed amplifier module, just trying to figure out the least expensive way to test my assumption...

 

[MattHarrell]

I've sometimes got the same problem but it seems to have gone away for the last three months. In mine the rpm's don't vary too much but I can physically feel the truck lurching forwards and backwards. I also don't feel it going uphills but mostly on semi-flat surfaces

I have done some work on the throttle body recently so maybe mine was just the cable.

 

[Longs]

I checked the cruise cable and lubed it, made no difference.

I have ordered a speed amplifier module from a salvage yard in Michigan, $35.00 shipped to my door, with a 90 day warranty. I'm hoping that fixes the problem.

 

[rabo]

Please let me know if new unit works,
Thanks in advance, Martin

 

[Longs]

The salvage yard sent the wrong parts - a cruise servo mechanism that didn't even have vacuum fittings, and some little module that I couldn't even find behind the glove box on mine. I'm trying to get them to send the right stuff now.

The module is actually just above the glove box opening. Mine if a light greenish color, with two connectors going into it. It actually mounts to the bracket that holds the glove box latch.

I'm hoping that if I give that description to the salvage yard, they'll get the right part.

 

[Longs]

Apparently not the Speed Amplifier Module

I received the part from the salvage yard, changed it, and it didn't fix the problem. Seems like maybe it's a little bit better, but there's still a noticeable surging effect.

I've now ordered the Servo from NAPA for $76.00 - see if that fixes it. If not, I'll just make my best attempt to return the Servo.

And here's a great link on the cruise control system...


http://www.dalidesign.com/hbook/ccmain.html

Longs

 

[phiple28]

Hello all,

I used to have a '91 XLT with the same problem. Whenever I tried to turn off my cruise control after using it for an extended period of time, the engine kept wanting to accelerate. This made braking very tough and dangerous. The problem immediately went away whenever I turned off the ignition and then started it again. Someone once told me it was a bad throttle position sensor. I replaced the part and NEVER had that problem again. True, I only had this problem happen a half-dozen times, so I'm not sure if it was a coincidence, or what... but it was a very cheap part and it's worth a try. I don't know how to check if they're bad... if anyone does, listen to them... but it took 5 minutes to change and like I said, the problem never came back. I hope this helps some of your problems.

 

[rabo]

Thanks

You are going to safe me a lot of time. Was looking into replacing the same parts.
For now, the overdrive of the tranny is pretty much going out; I can get a rebuild for 1200$ overhere (including labour). I still dont know if the car is worth it. The engine seems very sound, not using oil, the body is OK, who knows the car has another 100k left in it?
BTW , the car go,s fine without overdrive.
Martin

 

[Longs]

It would be interesting if it were the TPS, since nobody has identified this as a problem.

The link I provided shows a wiring diagram, and doesn't seem to show the throttle position sensor as interacting with the cruise control circuit. The module appears to decide whether to accelerate or let off the gas by sensing the input from the speed sensor. It then uses the solenoids to either add or release vacuum at the servo.

I'll post back when I know more.

 

[CADDYBILL]

I figured I’d add my two cents. Sounds like you guy’s may be describing two separate issues. The surging problem while the cruise control is set is probably the overdrive clutches slipping. Try and drive in at the same rpm and speed range and see if you have the same problem. If so your tranny is on borrowed time.

 

[Longs]

Caddybill - I thought exactly the same thing. I get absolutely nothing similar to the effect I'm experiencing when using the cruise.

When I drive without the cruise, the tranny seems fine, never has any feelings of slipping. That's whether I'm in OD or not, whether I drive to try to get the transmission to roam between OD and third, NOTHING.

I've disconnected the battery to reset the computer, checked for vacuum leaks, inspected the vacuum modulator for any signs of leakage, changed the speed sensor, replaced the cruise module with one from a salvage yard, no effect.

If you check that link in my earlier post, the author indicates that the module and the servo are both culprits in the surge. Based on the overall information on his page, he seems to know what he's talking about.

My new servo is on order - I'll post results when I have them.

 

[rabo]

TPS

Replaced the TPS without any positive results however.
Still surging cruise (especially when cold).
Lubed the cable as far as I could and left the cover off the cable assembly , again without results.
Anybody know where the speedsensor is located and if this part is expensive?
Strange enough my odometer has been off by about 9% from when I got the car. It works fine but is just 9% too low in its readings. Can this be part of the problem maybe?
Thanks for all input guys.

 

[Longs]

Rabo - as you know by now, I'm having the same problems with cruise. I tried changing the speed sensor first thing - it goes into the transmission, speedometer cable is driven by it. It's only about $20 - $25 so if you want to shotgun parts, it's not a bad place to start. Of course, it didn't fix my problem.

 

[rabo]

Please keep me posted on your vacuum servo fix.
Are the release valves located in the servo unit?
I cant understand why a warm engine is so much better.
Thanks in advance.

 

[Longs]

I've now ordered a new servo from Ford through a buddy that gets a discount (cheaper than Ford Parts Network). If anyone orders a replacement servo, be sure to get the original part number from the servo (you can see it without removing the servo, it's right by the throttle cable outlet). I had ordered one through NAPA that turned out to be the wrong one even though their website led me to believe they carried the right one.

My buddy has a code scanner, but we didn't find any codes. I thought maybe I'd find a throttle position sensor problem, which might have had to do with the problem being less noticeable or gone at certain speeds or RPMs or throttle positions.

I attached a vacuum source to the manifold side of the servo, as described in my other post, and it wouldn't hold vacuum or actuate the throttle cable. I'll have the replacement servo next week, and at that point I'll have changed just about everything I can think of. I'll keep you posted.