I have a 99 F150 that I have had for 20 years. It has 162K miles. Recently, when I am above 55 mph and let completely off the gas, it will coast normally for 2-5 seconds then there will be a clunk or shudder and there will be additional drag / engine braking slowing the truck a bit more than a normal coast. It almost feels like it is down shifting out of overdrive but it is not. The RPM's don't jump at all. In fact, if it coasts long enough before the clunk, like 5 seconds or so, the RPM's actually drop a bit after the clunk. It will do the same thing if overdrive is turned off. Once it "clunks" after coasting, as soon as I give it a very small amount of throttle, it clunks again and its back to coasting normally. It will only clunk if the throttle is completely off. Even the smallest amount of throttle will prevent the clunk. I am wondering if it is the torque converter lockout engaging after coasting a bit for some reason. It is not the slip yoke problem. I have removed the driveshaft and greased the slip yoke. Engine light has not come on. I took it to AAMCO for a diagnostic, and of course, all they said was the transmission needs a full rebuild. They really didn't do a much of a diagnostic. I am wondering if I can get away with a new torque converter or possibly the TCC lockout solenoid. Is there a simple way to unplug the TCC lockout to see if that stops the problem from happening? As an odd side note, this seemed to start sometime after changing the brake light switch. My cruise control stopped working at that time as well. Not sure if that is related or not.

 

Does it act the same with OD off or in second gear?

 

When I turn overdrive off, it will do the same thing in 3rd gear as long as the speed is high enough. Let off the gas, coast a bit, and clunk (TCC lockup?). It doesn't do it in second gear when letting off the gas, although I haven't tried holding it in second gear manually and trying it. Only in drive going through the gears normally at lower speeds. I will try shifting it to 2nd and holding it there, and see what it does. Obviously I won't be able to hit 55 mph. But 2nd gear will eliminate the possibility TCC lockout, right?

 

You can easily hit 55 mph in manual 2nd, that's only about 3500 rpm with stock gears.

 

Ok, I tested it by holding it in second gear. It does not "clunk" in second gear when coasting from 60 mph. I rechecked it in 3rd gear with overdrive off. It actually doesn't "clunk" in 3rd gear either, which I thought from before that it was doing. In 3rd gear when coasting, I do feel a slight shift or drag after coasting for a few seconds (TCC lockup?), but no "clunk" like I feel when in overdrive. Does the TCC normally lockup after coasting for a few seconds? Since it only "clunks" when in OD, does that mean something is wrong with the OD gear, snap ring, solenoid, etc...? It shifts normally all the way through the gears into OD when accelerating, only when coasting in OD at 55 and above will it "clunk". What say you? Thanks.

 

Turns out, it wasn't the transmission or torque converter at all! It was the brake light switch. I mentioned this suspicion in my original post. My brake lights went out earlier this year on a hunting trip. I found a Napa parts store in Council, ID, and he gave me a replacement (wrong one) which got my brake lights working again. As soon as I put the new one in, my cruise control stopped working, my fog lights stopped working, and I believe that is probably when the transmission started shuddering in OD. I put in the old switch, and my cruise control started worked again, the transmission stopped "clunking" in OD, and my fog lights worked again. So I picked up and installed another brake light switch, and it solve my transmission problem. Turns out the computer takes a reading from the brake light switch and must have been sending a faulty signal, causing TCC lockup at the wrong time. If I would have listened to AAMCO, I would have put up to $3100 into rebuilding the tranmission (although I probably would have DIY). The AAMCO guys don't do a serious diagnosis, this should have been easy to find. Their "diagnostics" is just a marketing tool to get you in the door. He basically told me since the truck had 162K, it was time for a rebuild anyway. Totally false. These things can easily go 200K without a rebuild as my Expedition has. I am glad it was an easy fix, but a little pissed at these repair shops that give you "free" or $90 diagnosis, but won't give accurate diagnostic information for those of us who like to DIY. I am willing to pay for the diagnosis, but expect accurate information as to what is wrong.

 

For your extra info, the lifting of throttle or braking at speed in OD at light throttle does signal the transmission to unlock.
It's been that way in Ford 'electric controlled" Transmissions since mid 80s.
It's by design intent to un-hook hard drive train connection to reduce stopping distance in panic braking situations.
The PCM has a timer to control the unlock time and does a relock up after a few seconds time if nothing more happens.
The PCM has a lead from the brake circuit to sense this action besides speed sensing.
Trucks with a Tach will show the action if the driver is a where of it.
You would see the the RPM rise about 300 rpm as throttle was lifted then reapplied. Rpm rise then drops back as the convertor unlocked and locked up again under brake signal and PCM timer action.
I was going to reply before you got it fixed but not got to it until now.
Thought you would like to know and for others to reads because it's a little known function even by the shops.
They would not see this as a pass or fail diagnostic unless they know and do a test drive.
Good luck.

 

Glad you got it fixed and cheaply too.
I don't know about Aamco now, but back in the late 80's they were ripping people off so bad that the attorney generals of 12 states got together and went after them. Their typical ploy was to call the owner and say they had to partially disassemble the trans to diagnose it and it would cost $100 plus to diagnose. Then they would call and say the bad news is that you need a rebuild but the good news is we have a nationwide warranty.
They pulled this on a coworker and before he authorized the diagnosis on his 74 Camaro, he talked to me and I told him it sounded to me that the vacuum modulator needed replacing and it only cost about $10 and you can unscrew it and replace it yourself. So, he declined Aamco's offer and when he went to pick it up, their mechanic told him he could fix it on the side for $35. So, he let the mechanic replace the vacuum modulator for $35 that night at a gas station.
I was camping at the beach with my kids and talked to a camper there who said he used to rebuild transmissions for Aamco and he had to quit because his conscience wouldn't allow him to keep rebuilding transmissions that didn't need it ripping people off.