OD shifting oddly

[Paul Amestoy]

What controls the OD on my 1998k2500 6.5TD.. it wants to shift out of OD when I let off on the accel...then when I give it some fuel it shifts into OD ....it should go the other way around... is it the computer, or vac... :(

[JohnC]

What you describe may be normal. Many folks confuse overdrive with torque converter lockup. when you back off the throttle the torque converter unlocks. When you get back on it first the engine comes up to speed driving through the torque converter, then the converter locks, resulting in a perceived shift to a higher gear.

All this is controlled by the PCM.

[Paul Amestoy]

Thanks for that update... i will check, but it seems to be about a 300 rpm shift... it may be that the OD is not actually kicking in....

[JohnC]

Easy to tell. Accelerate very lightly from a stop and count the "shifts" In "OD" you should feel 1-2, 2-3, 3-4 then somewhere between 45 and 55 mph, TCC lockup. Try it again in "3" and you'll get 1-2, 2-3 then TCC lockup at the same mph as above.

Lastly, when you're in "OD" press lightly on the brake while holding the accelerator setting. The TCC should unlock and RPM's go up ~200 RPM. (Touching the brake will not cause a shift, so if anything happens it's the TCC.)

[moondoggie]

Good Day!

The OD ratio in your 4L80-E is 0.75. If your truck was at 2000 rpm in 3rd gear & it shifted to OD, you should see closer to 500 rpm difference, although the TC (torque converter) slippage would mask some of that.

Another procedure to check your TC lockup, or (in your case) determine if TC lockup is what

[Paul Amestoy]

Great posts...It is the TL and it increases RPM's about 200.... I tried the test in OD and when I depress the breaks ever so lightly...it does not dis-engage the TL.... The breaks are applied because I can see the lights..... Is this a potential problem.

[JohnC]

"TL"?

What is it that is increasing RPM's by 200?

TCC lockup will not occur unless you're over the minimum speed, which is 45-55 MPH depending on axle ratio and tire size.

A positive check for OD is to pull the shift lever to 3 at highway speed on a level road. If the trans shifts down it was in OD.

There are 3 sets of contacts in the brake switch assy. Just because the brake lights come on does not guaranty the TCC lockup switch has been tripped.

When the TCC unlocks it usually results in about 200 RPM increase. If it is locked, applying more throttle will result in road and engine speeds increasing together. If it is unlocked, engine speed increases first then road speed.

If the TCC or OD are not performing as expected, the PCM will store fault codes. Check them and let us know.

[Paul Amestoy]

john,
good question and it brings up yet another potential issue... when in a steady flat road in OD i find that my throttle is not available until I get to 2000rpms... when throttling back on the pedal when it hits 1900 the engine goes to idle..So it is either on with some level of throttle, or completely off when I am lower than 2000rmps... when i am going up a slight incline with a bit more throttle I do not have this problem... It also happens when in cruise, so I never thought it was a switch problem on the pedal or break...perhaps I should review the wiring down in that area.... I have found many ground related problems with these trucks...do you know where the ground post is located in the drivers side area? thanks for your help...here in so.Cal... I am stuck without any Diesel guidance...especially the dealers...