Oddball--but is it normal??

[Tedybear]

One to chew over on this Christmas Morning.

My 89' BII has always had a 'burp' when using cruse control. Sometimes worse then others.

Here's the details. Now my understanding is that when the cruse is disconnected via the brake pedal? The computer flags the transmission to disengage the TCC and the OD, kicking it back into 3rd gear for a moment. I suspect to aid if the driver wanted to use some engine braking.

Now in some cases? I tap the brake...engine RPM kicks up about 50 or so...the system does not see me using the brake pedal (sees my foot now on the gas), and the OD and TCC kick back 'on' and everything is normal. Takes about a second or two and it's back to normal.

Now if I hit the "Off" button itself? The gas pedal releases and I'm back in charge...no engine surge or engine braking.

Yesterday on the trip back home from work? (keep in mind....my other post about dodging a transmission bullet).. I tapped the brake pedal---and the engine went from 2500 RPM and kicked up to about 3200 RPM for several seconds before self-correcting. Now I did recheck the fluid in the transmission just to be safe. Level still good, pink...slight smell, but in decent shape. I'm fairly sure the transmission is working normally (or as normal as an A4LD can manage LOL)

I'm just concerned about the surging. Kicking up slightly I expect when I tap the brake and it tries to start engine braking by downshifting. This acted more like the engine RPM climbed up higher--like I mashed my foot on the gas and it went into 3rd gear to pass someone. Didn't feel like it was slipping--more like something spiked the gas hard for a moment.

Any of this making sense? LOL. It's been driving normal otherwise and the transmission has not changed any of it's quirky-ness or shown any (at least not yet) injury due to the slow leak that finally reared it's head last weekend)

Anywho...I'll be posting a different thread regarding doing the manual swap as a long term project. Something I can start picking up the parts for and plan out for my 'tax return' project I normally do.

Thanks in advance!!

S-

[xr75er]

I’d check for a vacuum leak at the pedal release. The cruse is maintained by engine vacuum that goes thru the fire wall to the brake pedal. Each time you step on or tap the brake the vacuum is disengaged and the cruse is released.That would make the engine rev.

[AllanD]

the cruise control is disengaged via the brake pedal by BOTH the brake light switch AND a "Vacuum dump valve" that is actuated by the pedal.

having redundant means to kick it out is desireable...

AD

[Tedybear]

the cruise control is disengaged via the brake pedal by BOTH the brake light switch AND a "Vacuum dump valve" that is actuated by the pedal.

having redundant means to kick it out is desireable...

AD


So if there is a (for lack of a better word) failed part--hitting the brake to cut off the cruse control could in fact not just dump the vacuum, but also create a huge vacuum leak at the same time? This would make sense, as the truck acted like it got a 2nd wind when it jumped up in RPM.

I've not used the cruse sense this happened. I'll settle for the brick known as my size 13 foot LOL. At least until it gets warm enough to dig into this issue a bit more.

S-

[Teddyzee]

The cruise control is completely separate from the engine ECU.

The cruise control box is connected manually to the throttle linkage with a separate throttle cable. It gets it's speed signal form the VSS (vehicle speed sensor) attached to the trans. Your'89 likely have a cable coming out of this as well for the dash speedometer. ('93-later have no cable, just the VSS...)

Most likely, your VSS signal is cut (by brakes, clutch, off switch, etc.) and the Cruise box does not react quite as quick, causing it to pull the throttle slightly.

If it's a problem, you may want to check the electrical connections, and check to see if the linkage is binding, not sure if lube is recommended.
On later trucks, the box is attached to the top of the passenger side fender. Not sure where on your beast.