by Sam Dillinger (comments: 0)

2003 Mercury Mountaineer Intermittent Shift Concern

2003 Mercury Mountaineer 4.6l 5R55W

2003 Mercury Mountaineer

For this article we’re going to take a look at a submission I got from a customer about a check engine light and intermittent shifting issue on a 2003 Mercury Mountaineer. The transmission in question on this unit is a 5R55W. This is the designation given to this specific transmission type. Each different transmission for all manufacturers has a designation similar to this. It’s important to be sure to know the correct designation for your transmission because it will be essential for everything from the correct diagnostic information to ordering the correct parts. NOTE: each digit of the designation and can be decoded. We’ll examine that information in a later article. So, let’s take a look under the hood!

The customer contacted me and described a check engine light and occasionally the loss of shifting. First we’re going to take a brief look at what exactly a loss of shifting means. All newer model vehicles have an electronically controlled, and shifted, transmission. This is for automatics only of course, as a manual is still shifted well… manually. By you. When the PCM, Powertrain Control Module (some manufacturers use the engine controller for engine and transmission functions. This is one of them, so it doesn’t have a stand-alone TCM), detects a failure, it will enter a “safe” or “limp” mode. Some manufacturers will call this “default” mode. It all means the same thing- the transmission now has a single forward gear and reverse. That’s it. This is intended to limit the operation of the unit to try and avoid a catastrophic failure. I’ve said in other articles, and will say here, catastrophic transmission failure = expensive repairs! This safe mode will also occur if the electronics of the transmission fail. We’ll look at the electronic possibilities for this in a different article. For today, the Pcode the customer found when scanning the TCM was P0760. This was the only Pcode present, and was responsible for the safe mode being activated.

P0760 has a few different names, but the generic definition is “Shift Solenoid C Malfunction”. This means that the C solenoid has given some type of failure. This can be in the form of a short or open circuit. Too high of resistance in the solenoid will cause it as well (extremely high resistance will be interpreted as an open circuit, but in this case we’re talking about resistance that’s slightly higher than normal). So what to do? The place to start with any of these Pcodes is to take into consideration what type of vehicle we’re dealing with. On this particular unit, the C solenoid controls the overdrive gear pairing. There is a button on the shift lever that will turn the overdrive gear off (this is commonly used for towing to avoid excessive back and forth shifting that will overheat the transmission). For this vehicle, the overdrive off button can cause this Pcode, but it’s not the most common problem found. It does illustrate the importance of being aware of the different situations that can apply to specific vehicles though.

After I had a chance to exchange a few messages with the customer I was able to determine that the vehicle would shift normally when the transmission was cold (as I noted in an article on transmission temperature cold is relative to environmental factors. So we’ll define cold here as: the transmission has sat long enough to return to ambient temperature and the fluid is cooled down) and after driving for a time and heating up the vehicle, the problem would start to occur. The check engine light would illuminate and the safe mode would be immediately activated. This is a particularly noticeable engagement. When safe mode turns on, the transmission will shift into the single forward gear, in this unit it’s 4th gear, and it will shift HARD. It will definitely be enough to get your attention. This is noteworthy because in any electrical circuit heat will cause resistance to increase. So the resistance was borderline to the point that the transmission would operate normally when cold, and when heated up, it would raise the resistance of the circuit enough to cause a failure to be reported.

The absolute most common cause for this is a failed shift solenoid. There is a “solenoid pack” that houses all of the shift solenoids in a single place. Usually it’s mounted next to, on top of, or directly onto the valve body. In this transmission, it’s mounted next to the valve body, onto the separator plate. The transmission pan must be removed in order to access the actual solenoid pack itself. There is a connector that goes through the transmission case and will have the wiring harness connector attach externally. Always start by removing the connector and checking the side that holds all the wires for any signs of damage, pushed out pins, corrosion/water intrusion. If none of these issues is evident, the next step is to test the power circuit to be sure battery voltage is reaching the solenoid pack. Assuming the power is present, the solenoid wire in question (solenoid C circuit in our case) must be load tested to make sure the signal is being transferred from the PCM to the solenoid pack. After that, you can test the resistance of the specific solenoid in the male side of the connector. This will give you all the info needed to make the correct diagnosis.

I’m going to take a moment here to explain a few things that I’m skimming over or leaving out. First is the PCM. This is what I’ve always called the module that controls engine functions. Ford service information calls it an ECM, Engine Control Module. They mean the same thing but the different name can be confusing if you're not aware of it. Next, anyone familiar with auto repair will see that I’ve skipped right past defining terms like resistance, short, or open circuits. That is being done on purpose to keep the article relatively simple. The explanations for these terms gets very technically detailed and will expand this article from an explanation of a specific problem into a masters class on automotive electrical diagnosis. The same applies to load testing a circuit. This is a really handy electrical diagnostic tool that any who attempt these repairs should learn how to do. Not to worry, that is in a different article, and there is a video for how to load test a circuit properly (it’s not nearly as hard as it sounds). So the omissions in this article are for the sake of readability and simplicity.

After the tests were performed, it was found that the C solenoid had higher resistance than was allowed. The solenoids can be replaced individually but this is a difficult repair. It’s more expensive to replace the whole solenoid pack, but for my DIY crowd, it’s what I recommend. Even for some of my more advanced readers, I don’t recommend trying to rebuild the solenoid pack. You have to have the right solenoid as well as the technical knowledge on how to replace it without causing any further damage. Far better to replace the solenoid pack. TIP: when replacing the solenoid pack, be sure to replace the transmission filter as well. It needs to be removed to replace the solenoid pack on this unit, and when you replace the transmission pan gasket, then fill with new fluid when you're done, you've now also done a standard transmission service at the same time.

After the customer replaced the solenoid pack, there were no further issues and they were able to resume normal driving with their vehicle. As always for my members, if you have any questions, need some diagnostic help, some more advanced tips on electrical systems, or just want to say hi, come see me at

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Meet Sam Dillinger

My name is Sam Dillinger. I've been a professional, dealership technician for 18 and a half years. My first introduction to mechanical repair was when my own vehicle broke down in the fall of 1995. I was 18 and couldn't afford to pay to have it fixed. So I borrowed tools and asked a ton of questions and, eventually, was able to replace the clutch on my truck by myself. During the course of that project, I found that I really enjoyed having a wrench in my hand and figuring out the puzzle of disassembly and reassembly of a motor vehicle.

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