Ok, this is a weird one...I have a 86' 300SDL with 141K on it. When I bought it, the guy told me the tranny was rebuilt a fairly short time ago. Never trust that someone has done all proper general maintenance on a car...so I changed the tranny filter and fluid at 129K. However, before I did this, I noticed the tranny had a funny little slip to it.

When cold, the tranny shifts fine...you have your typical erratic shifts here and there from the throttle but thats normal with these trannys ive been told. (erratic meaning it shifts at weird RPMS but still shifts)

When this tranny gets warm though...it slips from 2 to 3. Not a terrible slip...but makes the engine RPMS go up a little before shifting. Its definately best described as a slip but only when warm and only from 2 to 3. I checked the fluid today...clear clear red and no funny smell (the smell of burning from overheating in the fluid).

What do you think this could be...Lucas Stop Slip didnt seem to remedy the situation.

Brian Carlton...I know them gears in yer head are turning, I could be more detailed if need be!

Just to help narrow the problem, check all the vacuum lines. There is one that goes to a pod on the driver's side fender well. If leaking, or cracked, (or off) it will cause erratic shifting.

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87 300SDL - 215K Miles !!
99 F-350CC Dually PSD - 190K
86 300SDL - 189K
All on B-100

Now, it all depends on what "not a terrible slip" means.

I've noticed the 2-3 is definitely the softest shift and the rpms will typically rise very slightly when it occurs..........but, you'd really need to be looking for it.

If it rises such that you lift off the pedal because it's very obvious, then you've got too much vacuum.

I dont think its the pod but I wouldnt know how to check it...however the lines look ok on it. Where would I look under the car for leaking vacuum lines? Where else could be subject to leaks? It very well may be a vacuum leak knowing these cars.

Darn it Brian, you totally ROCK! Thats EXACTLY why it slips...it will get up to speed and I'll let up on the pedal slightly and WHAMMO, thats why...however I only let up on the pedal SLIGHTLY.

A slight slip is about a solid second between shifts. (second as in "time")

What would I do to reduce vacuum?

I beleive turning the adjuster on the vacuum modulator clockwise at the transmission (try 1/4 turn increments) will reduce vaccum.....

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91 350SD
14 F150 Eco
19 Fusion Hybrid
11 GT500

One second second is too much slip. I'm talking about 1/2 second or less as "normal".

Put a vacuum gauge on it and see what you have in the transmission line at idle.

I'll pull the old mity vac out of the cabinet soon...by the way...in lieu of paying the more than usual attention to my cars tranny...Ive noticed it downshifting when Im approaching a stop.

At first I thought it was bumps in the road or my brakes...but its my tranny downshifting...what would cause this?

Like I said...it doesnt CHUNK when downshifting nor does it lurch...it just feels like hitting bumps in the road. Whaddya think? Is it in association with the vacuum issue?