Transmission slipping in low gear only, high rev before truck moves

COUNTING09

MEMBER
1998 FORD F-150
4.6L
V8
4WD
AUTOMATIC
266,000 MILES

Transmission only slips when starting from a standstill like a stop sign. Revs to 2,500 RPMs or more before moving. Only a slight jerk, no banging. Runs through the rest of the gears fine. No leaks or bad smells. Fluid still has red tint but has been 100,000 miles since a change. Tried a bottle of Lucas stop slip to no avail. Is it possible it’s something simple like a plugged filter? I can change all that myself but don't want to if there’s no chance that will help. Guessing anything more involved will cost me more than the pickup is worth. Any thoughts on what’s wrong? Thanks for your time.

Thursday, April 14th, 2022 AT 9:51 AM

5 Replies

JACOBANDNICKOLAS

MECHANIC
110,176 POSTS

Hi,

If it is the original transmission, it may simply be the clutches are worn in the low/reverse clutch pack. Are there any issues when placing it in reverse? Also, if you place it in low and pull it out, does it slip?

Also, let me know if all other operations are normal and if you have scanned the computer for diagnostic trouble codes. If it is slipping, there should be a variation between the input and output speed sensor code.

Let me know.

Take care,

Joe

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Thursday, April 14th, 2022 AT 8:31 PM

COUNTING09

MEMBER
3 POSTS

Thanks for responding, sir.

No troubles in reverse, and yes, still acts up when I start with it in low/first.

No codes when read with my cen-tech brand reader.

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Thursday, April 14th, 2022 AT 9:42 PM

JACOBANDNICKOLAS

MECHANIC
110,176 POSTS

Hi,

Since we are not slipping in reverse, it sounds like the clutch pack is good. If the clutches were bad, it would likely slip in reverse. The planetary low gear clutch is a one-way clutch. Either it is failing or there is a lack of pressure for engagement.

I'm going to try explaining how this works and what leads me to my theory.

See Pic 1
The (A) reverse clutch couples the (B) input shaft to the (C) reverse sun gear. It is applied in reverse range only.

See Pic 2
The (A) planetary (low) one-way clutch is a roller clutch that holds the (B) planetary gearset in first gear, (OD), and D ranges. During automatic coasting downshifts into first gear (OD) and D ranges), it freewheels so that there is no engine braking.

The only other thing I can think of is if the reverse servo is leaking. That can happen due to wear, and they have been known to leak. However, the servo may not fix it if the band and drum are worn or damaged.

I hope that made sense. Here is what I would suggest. First, service the transmission and the filter. Additionally, flush the converter and the transmission oil cooler.

Let me know if this helps or if you have other questions.

Take care,

Joe

See pics below.

Images (Click to make bigger)

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Friday, April 15th, 2022 AT 6:37 PM

COUNTING09

MEMBER
3 POSTS

Thanks so much for taking the time to respond and explanation of how stuff works.

Changed pan fluid and filter - took bolt out of bottom of torque converter and let it drain until it quit.

Fluid was still very red - barely any sludge on the magnet.

Refilled and added a bottle of Lucas stop slip.

Still not working right.

That’s about all I’m able to tackle outside on my driveway, lol.

Any other thoughts?

Might be time to just give up on her - I know It’s done good to get that many miles.

Thanks again.

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Sunday, May 1st, 2022 AT 5:07 PM

JACOBANDNICKOLAS

MECHANIC
110,176 POSTS

Hi,

At this point, the transmission should be taken apart and inspected. If I had to guess, it is related to a servo issue.

Let me know if you change your mind or if I can help in any way.

Take care,

Joe

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Sunday, May 1st, 2022 AT 7:07 PM