2002 Ford explorer od flashing gas explosion


I have a 2002 Ford Explorer XLT. About 6 months ago I came to forums like this hoping to find out what that flashing OD light meant. If you don’t know it is the car’s computer telling you that the transmission has been put into "limp" mode. This mode gives the transmission limited functionality so that the car can be driven over a short distance (say a few miles) to a safe stopping point. It will go away temporarily if you stop the car and restart the engine, but may reappear if the computer still senses a problem (more on that later). For those of you who do not have a Ford Explorer, but do have a modern computer controlled transmission the OD flash or limp mode is a typical technique for dealing with transmission problems.

Because of my research I now understand that the 5R55 transmission (and its variations) is installed in the Explorer, the Ranger and in at least one Mazda SUV so this information may have some broad applications. So with that out of the way here is my story.

I do consider myself to have some mechanical skills at car repair. I have on more than one occasion fixed engines with blown head gaskets. However, until this last weekend I have never even looked inside an automatic transmission. So for me it was necessary to have a primer on automatic transmissions. I encourage everyone to take advantage of any web site that has educational pages on automatic transmissions.

For my story you need only focus on two aspects of an auto trans. The first is the so-called gears. These are often planetary gears that are always meshed (they are never decoupled) and are responsible for the various “gear” ratios you hear about. The second is that the actual shifting in an auto trans is through hydraulically operated clutches. This is what you hear/feel as the “gears” change; the various clutches engaging and disengaging. This arrangement, though complicated, makes for a fairly reliable method of transmitting power from the engine to the wheels. So what goes wrong?

Well, unless the fluid in the trans is failing the gears should always have proper lubrication and should not break (though they will wear over time). If they do break it is likely due to abuse (too much power for instance). That just leaves the clutches. If anything is going to go wrong with an auto trans it is likely with the clutches. The clutches will go bad either due to lots of use (high mileage for instance) or poor shifting. In either case if a clutch isn’t squeezed tight enough by the hydraulics it will slip and cause LOTS of heat. That heat has only one place to go, which is right into the fluid. If the fluid becomes too hot it will break down and no longer perform its intended functions, which includes lubricating moving parts. If the fluid is bad your headed for a rebuild because all those moving parts are now grinding instead of sliding.

So back to the story. . . With my transmission I did not know what had caused the computer to put the trans into limp mode when it first started happening (I read somewhere that there are over 100 different conditions to cause limp mode). The check engine light did not appear telling me there was an error code. By the way, go to HarborFreight.com and get a code reader. They can be had for around $40 and are invaluable when something like this comes up. Later as the OD would appear the code reader would show no codes. Great! Now what? Eventually the trans did start showing some real symptoms. Mainly not shifting into 2nd and overdrive.

It turns out that the trans was trying to shift into all of the gears. This was easily proven by carefully jacking the rear off the ground and onto jack stands and “driving” the car. I could hear and feel all five shifts, though some were not as firm as others. I thought maybe the fluid was bad and had a dealer change the fluid and filter. $200 later and nothing changed. Time for more research.

I check with a bunch of forums like this one and learned that 2nd and overdrive are handled by the same clutch. Ah, a single point of connection between the two gears. I also found out that Ford made the mistake of assembling a bunch of 5R55 transmissions with the wrong fluid. Over time this wrong fluid was to cause a shift solenoid to break. Oh, Oh. Time for another lecture!

On classic auto trans like the GM T-350 and T-400 all the hydraulic shifting logic is due to purely mechanical means. Things like governors, engine vacuum and a linkage to the throttle are all part of the mechanical logic to shift these classic transmissions. Ironically the so called computer trans is far simpler than a classic trans (ironic because hardly anybody actually fixes trans these days. They just have them rebuilt). The 5R55 has several hydraulic rams called servos that engage the various clutches. The servos are controlled by a valve-body (think relays if you understand electrical). The valve-body is controlled by a solenoid pack, which is directly controlled by the computer. As for sensors there are 4. A temperature sender for the fluid and 3 speed sensors. That’s it! Remember the part where I said the computer could sense problems. With only 4 sensors it can only reliably detect slippage and high heat. All other diagnostics are inferences (read that as educated ASSumptions).

So if the solenoids quit working the transmission can’t shift, so did mine break? Well, no. After about 35,000 miles I had the original fluid changed, which meant I then had the right fluid. Still, it sounded plausible that the pack was bad. Only one thing to do next!

This PDF is a wealth of information. It’s not a repair manual, but it’s very helpful. After I got the pan off and the solenoid pack off I mapped out the electrical connections to each of the 8 solenoids so I could test them. The 5 smaller solenoids are normally closed (they open with the application of electricity) and would pass compressed air when powered. The 3 larger solenoids are normally open and would close when powered. So the bad part was that I hadn’t found the problem and the good part was that I hadn’t needlessly bought a solenoid pack cause mine was good. So I kept reading through the PDF file. Then I saw the instructions for proper tensioning of the clutch bands. The 5R55 has 2 of them, one for intermediate and one for overdrive. Ah ha!

The best part about adjusting the clutch bands is that it is done by some bolts on the side trans case! So I put the trans back together and put the fluid back in (no easy task). Next I tightened the clutch bands as per the PDF instructions and tried driving the car. It actually shifted into 2nd gear, but began to slip with increased throttle. Ok! I’m on to something. I then carefully tightened the band a bit further than the instructions prescribed and drove the car again. Wow! What a difference. Not only would it shift, I could now force the trans into 2nd gear and floor it without slippage. And the overdrive? It is once again able to lug the engine when climbing a hill.

So the whole time my problem was a slipping overdrive clutch band. Remember what I said about the most common problems with an automatic transmission? Slipping clutches! So why was mine slipping? Easy. The trans had over 140,000 on it 6 months ago.

If you have a high mileage 5R55 and the shifting sounds bad, is slipping or skipping gears consider tightening the clutch bands. It’s easy and will only take a few tools (though the average do-it-yourselfer may not posses the necessary 5/16” square socket and a suitable torque wrench). If that seems to cure it then take it to a lube shop and have the trans fluid replaced as it’s likely ruined.