Overdrive Shift Clunk

[Anticitizen1]

Hi all, been battling an issue with a new to me 2000 Mercury Mountaineer (161,x.. Miles, 5.0 AWD) where there is a distinct clunk when accelerating and making the transmission downshift out of overdrive. To be very specific, it does not appear in any other gear change, and only when tipping in and downshifting - flooring the truck yields a smooth acceleration with no clunk. Also, the clunk issue is NOT PRESENT when the truck is cooled off to ambient temperature, and steadily begins the OD downshift clunk once the truck has enough time to warm up while driving. My truck was factory equipped with 235/75r15 tires and I did move to 255/70/16s, while the speedometer is off approximately 5 MPH this was not an issue in two other 5.0 Explorers in the fleet. To date, I have serviced the following components:

• Synthetic ATF Fluid Exchange and Motorcraft Filter. Fluid did not appear dark or smell burnt, and there was no debris in the pan. The factory dipstick plug was also in the pan, indicating this was the first time the pan had been off since manufacture.
• Transmission Mount
• Motor Mounts Inspected, No Issues Found
• Tried 2 separate front and rear driveshafts with no change
• Drove without the front driveshaft, no change
• Installed new Universal Joints in the rear driveshaft with no change
• Checked the transfer case for chain slop/play by rotating the driveshafts in neutral and looking for excess slop/skipping. Seemed tight
• Replaced transfer case fluid
• Rebuilt the traction lock rear differential with Ford Racing clutch packs, put in 75w140 fluid with friction modifier. There was NO friction material left in the clutches when disassembled, but not the issue.
• Verified rear shocks have appropriate compression/rebound.
• Not necessarily related, but have also replaced fuel filter, pump, plugs, wires, cleaned MAF and Throttle Body, new engine air filter, leaf spring shackles, sway bar bushings front/rear, LED headlights and much more.

Overall the transmission shifts well in all other gears, but in my mind the next portion of the diagnosis will at least involve pulling the valve body and inspecting for damage and/or replacing the EPR solenoid, accumulators/springs or possibly seeking out a new transmission. This truck is in very good shape with minimal rust, body damage and the paint and interior are in very good shape for the age. This shift issue has been incredibly frustrating to diagnose, and I’m hoping someone here may have some advice or a thought that could help me out. I did search both the EE Forums and Google far and wide, and have been working with ChatGPT to help narrow down my fault tree. Thanks for everyone’s time!

 

[410Fortune]

When you cancel overdrive with the button do you still get a clunk?

This is a 4 speed automatic with a torque converter lock up … so it feels like overdrive but it is actually the converter locking up to make a 1:1 connection. From input to output

The converter locks up electronically, meaning the tcc solenoid inside the trans is responsible for the function it opens and allows fluid to pressurize the feed to the converter

If you have an issue with the tcc solenoid it could cause an issue

To solenoid can be ohm checked from outside the transmission

Using the od off button will disable the tcc solenoid. So if it no longer clunks with the od off… this would point to an issue with the tcc solenoid or converter

I did not see you mention the rear driveshaft slip yoke. The slip yoke should be cleaned and lubed. The slip yoke can cause a
Clunk like you describe. Especially when shifting from forward to reverse

If you determine it is in the trans you can try a few things. First inspect the converter. There is a converter access plate can be unbolted. You can then see the flexplate and converter nuts/ studs make sure the converter nuts are still tight and the flexplate has no cracks around the hub

Next you can install a bottle of lube guards dr tranny shudder fix.. this is a very good additive for your 4r70w trans fluid. This stuff actually conditions the converter by making the fluid more slippery: it works. I have solved converter clunk with this stuff in the past. If it changes your clunk then it’s likely an issue with the converter itself and you likely need a new one. Waaaaaay cheaper and easier then a whole new trans

Is the trans fluid bright red? Or does it smell burnt?

Going inside the valve body the tcc solenoid is suspect, not the epc
However you can replace all 4 solenoids while in there and the filter

Solenoids do wear out yours are now 25+ years old. A ohm test of the tcc is a good idea
Also running a scanner as you drive you should be able
To Monitor the pcm and see when it commands the tcc (converter lock up) and then see how the command coincides with the audible clunk

Often we will put a truck like this on the rack and actually run it up to speed.. this way we can pinpoint the area of the clunk noise precisely… yes it can be done safely… done at your own risk of course

160k is about half life of a 2000 4r70w
Any issues with a solenoid usually result in a trouble code and flashing od light
Have you checked for manufacture specific codes?

I would suspect the rear axle and driveline before I suspect the trans, and I am around these trucks often
The od off button would be a good test for this

Also check your exhaust mounts the one at the trans mount is very important
The 98-01 5.0 truck loves to spit out exhaust manifold studs.. especially the front ones on the pass side… why? Because the exhaust system is heavy and when the mound wear out it puts strain on these fasteners
Exhaust system can make aome
Clunks
So can rear e brakes and the rear shocks

Did you re do the rear e brake shoes and hardware when the rear axle was done? New axle seals? The rear axle is not leaking any fluid past the axle seals is it? I ask because we have seen a hard to find clunk noise caused by those damn e brakes a time or two .. also gear pine on your rear brakes can make some crappy noises

Make sure spare tire is tight to the frame and not moving back and forth on a loose cable

The 3rd shock and the vibration damper weight in the rear end can be checked for looseness as can the bushings in your traction bars

Food for thought, let us know what you find

Plenty of videos on ohm test solenoids for the 4r70w
The wires can be tested at the pcm or at the white Bulkhead connector on the pass side of the trans behind the heat guard. Easier at the pcm, but totally do able from underneath…

Dorman trans pans are cheap and good quality, they come with a built in drain plug. Makes fluid changes and pan drops so much easier…

 

[Anticitizen1]

When you cancel overdrive with the button do you still get a clunk?

This is a 4 speed automatic with a torque converter lock up … so it feels like overdrive but it is actually the converter locking up to make a 1:1 connection. From input to output

The converter locks up electronically, meaning the tcc solenoid inside the trans is responsible for the function it opens and allows fluid to pressurize the feed to the converter

If you have an issue with the tcc solenoid it could cause an issue

To solenoid can be ohm checked from outside the transmission

Using the od off button will disable the tcc solenoid. So if it no longer clunks with the od off… this would point to an issue with the tcc solenoid or converter

I did not see you mention the rear driveshaft slip yoke. The slip yoke should be cleaned and lubed. The slip yoke can cause a
Clunk like you describe. Especially when shifting from forward to reverse

If you determine it is in the trans you can try a few things. First inspect the converter. There is a converter access plate can be unbolted. You can then see the flexplate and converter nuts/ studs make sure the converter nuts are still tight and the flexplate has no cracks around the hub

Next you can install a bottle of lube guards dr tranny shudder fix.. this is a very good additive for your 4r70w trans fluid

Is the trans fluid bright red? Or does it smell burnt?

Going inside the valve body the tcc solenoid is suspect, not the epc
However you can replace all 4 solenoids while in there and the filter

Solenoids do wear out yours are now 25+ years old. A ohm test of the tcc is a good idea
Also running a scanner as you drive you should be able
To Monitor the pcm and see when it commands the tcc (converter lock up) and then see how the command coincides with the audible clunk

Often we will put a truck like this on the rack and actually run it up to speed.. this way we can pinpoint the area of the clunk noise precisely… yes it can be done safely… done at your own risk of course

160k is about half life of a 2000 4r70w
Any issues with a solenoid usually result in a trouble code and flashing od light
Have you checked for manufacture specific codes?

I would suspect the rear axle and driveline before I suspect the trans, and I am around these trucks often
The od off button would be a good test for this

Also check your exhaust mounts the one at the trans mount is very important
The 98-01 5.0 truck loves to spit out exhaust manifold studs.. especially the front ones on the pass side… why? Because the exhaust system is heavy and when the mound wear out it puts strain on these fasteners
Exhaust system can make aome
Clunks
So can rear e brakes and the rear shocks
Make sure spare tire is tight to the frame and not moving back and forth on a loose cable

The 3rd shock and the vibration damper weight in the rear end can be checked for looseness as can the bushings in your traction bars

Food for thought, let us know what you find

Plenty of videos on ohm test solenoids for the 4r70w
The wires can be tested at the pcm or at the white Bulkhead connector on the pass side of the trans behind the heat guard. Easier at the pcm, but totally do able from underneath…

Dorman trans pans are cheap and good quality, they come with a built in drain plug. Makes fluid changes and pan drops so much easier…

Hey 410! The clunk does not appear to clunk when OD is off, but definitely a good call. I did not in fact service the slip yoke, but I did have two different shafts in the truck with no change (and there was no clunk in the truck that the other shaft came out of. I may still disassemble and lubricate the yoke though.

I’ll have to look into checking out the TCC Solenoid and look through the TC inspection port, good call. I’ve also got no fault codes/DTC’s for absolutely anything in the truck which is part of my confusion here.

The fluid looked good when replaced, no burn smell or overly dark/debris found. Fresh ATF was added afterwards along with a new Motorcraft filter. Before replacing the fluid I did run Seafoam Trans Tune in the original fluid for about 150-200 miles.

Just went through the entire rear end, definitely not the differential or parking brake assemblies.

I did verify that the traction bars and bushings were both tight and had no excessive play, they were both okay. I have not tested the 3rd shock, but it is definitely OEM and probably in need of replacement. I’m not familiar with the vibration damper weight, where is it located?

I really appreciate the advice, you gave me a few other points to check I didn’t originally consider. Thanks!!

 

[410Fortune]

The awd t case

Drain it how does the fluid look? How quickly does it go dark? Meaning if you put in new atf how long before it’s brown?

I run Mercon iii dextron /
Regular atf in the awd t case

Not Mercon v

Your clunk could very well be in the awd case… I know you said no change with no front d shaft but…. These awd t cases are a wet clutch.. they don’t last as long as the rest of the truck

I hate them in fact hahaha
The awd sucks fuel and power and eats front tires and wheel bearings
It’s a 70/30 split and when the power isn’t being used it’s wasted energy in the form f heat…. In the 90s gas was cheap so were tires hahahaha

25 years, 160k miles… that t case would be scrap metal if it was my 00 mounty… 4406 manual shift t case for th e win

 

[Anticitizen1]

The awd t case

Drain it how does the fluid look? How quickly does it go dark? Meaning if you put in new atf how long before it’s brown?

I run Mercon iii dextron /
Regular atf in the awd t case

Not Mercon v

Your clunk could very well be in the awd case… I know you said no change with no front d shaft but…. These awd t cases are a wet clutch.. they don’t last as long as the rest of the truck

I hate them in fact hahaha
The awd sucks fuel and power and eats front tires and wheel bearings
It’s a 70/30 split and when the power isn’t being used it’s wasted energy in the form f heat…. In the 90s gas was cheap so were tires hahahaha

25 years, 160k miles… that t case would be scrap metal if it was my 00 mounty… 4406 manual shift t case for th e win

I did exchange the fluid in the AWD transfer case, and it was definitely dark and did smell a bit burned. The chain did not appear to have any slop in it when I checked the shaft play between the front and rear, but that’s not necessarily a guarantee all is well. I replaced it with Dex/Merc fluid and had no change to the clunk. I had a BW4406 swapped into my 98, great cases but got tired of the front shaft vibrations haha. This new truck will stay stock with the AWD system since it doesn’t see any hard wheeling.

 

[410Fortune]

The weight is bolted to the rear axle can’t miss it
You can live without the 3rd shock
In fact I take off all the weights, the traction bars, the 3rd shock off my trucks hahaha all that crap is a band aid. If your tires wheels are balanced the drivelines are in good shape and balanced then I’ve found no need for those things. The traction bars are
Limiting your travel… I live in the dirt so I don’t need street ability I need soft suspension travel

 

[410Fortune]

Should not have any front shaft vibrations if 4406 done well! I have a thread for upgraded trans mount…. (In my
Sig)
I run a regular double u joint f 150 front d shaft unless w me have a drop bracket lift then I switch to a double cardan type

Drain the t case again… I bet it’s dark and burnt again

 

[Anticitizen1]

The weight is bolted to the rear axle can’t miss it
You can live without the 3rd shock
In fact I take off all the weights, the traction bars, the 3rd shock off my trucks hahaha all that crap is a band aid. If your tires wheels are balanced the drivelines are in good shape and balanced then I’ve found no need for those things. The traction bars are
Limiting your travel… I live in the dirt so I don’t need street ability I need soft suspension travel

Interesting, had 4 of these trucks over the years and always seen the 3rd shock, NEVER seen a vibration dampener. Either they were long since removed or these particular model years never had them in the first place…I’ll have to check my factory service manual set for some clues. I could see these components definitely inhibiting an offroad build, but this truck is going to be mostly on-road so I’ll leave them on haha.

 

[410Fortune]

It could be me and my memory haha I work on a lot of ranger based trucks from 83-2011
It maybe was a gen 1 thing and the gen ii got the 3rd shock

You can still find a damper on some t cases and also some front diffs as well as rear… I like to shed weight and simplify my personal rigs. No real issues at highway speeds, even with modified rigs and big tires… so I figure it was for some vehicles that left the assembly line? I know ford had their reasons…

 

[C420sailor]

If you ever think you have an internal trans issue, driving around with a laptop with Forscan running will help with diagnosis. You a see exactly what solenoids are active, which gear is commanded, what the pressures and RPMs are of various components. Really aids in narrowing down what can be going on