94 Explorer 4x4 oscillating RPM

[caddman6425]

Hey all,

My explorer just started having the RPM oscillate when put into drive, not park or neutral. The oscillation continue while driving and with Cruse Control activated. Nay ideas would be greatly appreciated.

 

[Rhett]

Sounds like a vacuum leak to me. I would check all of the vac lines and their connectors for cracks and looseness. Also inspect your air intake tube for holes and frayed ends.

I would do that first, before even considering IAC or TPS.

 

[Rhett]

Here's a vac chart for the 94 Explorer, hope this helps:

 

[caddman6425]

Thanks guys, it may be just that, since I had vac lines off trying to get cruse to work. I'll check and let you all know.

 

[caddman6425]

Well here is what I think. The next day when I started the Explorer and put it into drive, it didn't do it. I let it warm up to operating temp while I checked the vac lines. I found no problems with the line, BTW.

Today I took it out on the highway and got it really up to temperature and still no oscillation. Then I turn on the Cruse Control, and bingo, the oscillation started. Even after pushing the stop on the cruse, it still did it until I got home and pulled the cruse fuse.

Speed Control or Servo???

 

[caddman6425]

Correction, it doesn't stop when fuse is pulled. BTW, does this forum use infolinks???

 

[Rhett]

Well here is what I think. The next day when I started the Explorer and put it into drive, it didn't do it. I let it warm up to operating temp while I checked the vac lines. I found no problems with the line, BTW.

Today I took it out on the highway and got it really up to temperature and still no oscillation. Then I turn on the Cruse Control, and bingo, the oscillation started. Even after pushing the stop on the cruse, it still did it until I got home and pulled the cruse fuse.

Speed Control or Servo???

I was just looking at my 94 Ford wiring diagram. I am not that familiar with the cruise system, but I can (attempt) to tell you what that system seems to use as inputs.

From the diagrams, it appears that it is using the VSS (vehicle speed sensor), the cruise shutoff switch (I assume the one on the stalk), the brake/clutch shutoff switch, and the TPS (throttle position sensor). Finally, there is a servo motor, with an actuator connected to the throttle linkage. There is also a modulating valve which uses the vacuum reference, said vacuum being applied to the servo motor. That may be a good place to look. I assume the modulating valve is located in the cruise assembly, by the motor, but I can't tell from these diagrams with 100% certainty.

Hope that helps a little

 

[caddman6425]

I can confirm that the oscillation doesn't occur until the Cruse Control is engaged. Then to make matters worse, the oscillation continues after the Cruse is shut off and won't quit until the vehicle is shut off overnight.

I checked everything else, and all are within limits. The system didn't work after I built the vehicle from scratch out of a salvage yard. I checked the system and when I checked the cables at the Servo, I notice that one of the potentiometers had rust all over it, but the resistance checked OK. So after all the test on the system came back ok, I was left with activating the Servo remotely and at first it didn't work, but I figured that it was due too the fact that I didn't have three hands so I activated it at the servo and it engaged. After that, I had Cruse Control.


Never did think of the TPS. Though, I don't see it on my wiring diagram in my AllData. Could you, or would you show me a copy of it?

 

[natenkiki2004]

TPS is the little black rectangle mounted to the driver's side of the throttle body (hence throttle position sensor). It's a 3-wire sensor, it should be easy to find.

 

[caddman6425]

I know where it is, I just don't see it on my AllData wiring diagram.

 

[natenkiki2004]

I know where it is, I just don't see it on my AllData wiring diagram.

My bad. Check BBB Industries, they have nice wiring diagrams.

 

[92exp4x4]

Disconnect and plug the cruise servo vacuum hose. See if the problem goes away. If so then its in the cruise.

 

[smegun]

I can confirm that the oscillation doesn't occur until the Cruse Control is engaged. Then to make matters worse, the oscillation continues after the Cruse is shut off and won't quit until the vehicle is shut off overnight.

I checked everything else, and all are within limits. The system didn't work after I built the vehicle from scratch out of a salvage yard. I checked the system and when I checked the cables at the Servo, I notice that one of the potentiometers had rust all over it, but the resistance checked OK. So after all the test on the system came back ok, I was left with activating the Servo remotely and at first it didn't work, but I figured that it was due too the fact that I didn't have three hands so I activated it at the servo and it engaged. After that, I had Cruse Control.


Never did think of the TPS. Though, I don't see it on my wiring diagram in my AllData. Could you, or would you show me a copy of it?

he ha 280 sel mercedes oscillates at high altitude is a thing that happens for some injected ...could it be ?