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Discussion Starter · #1 ·
I feel stupid for asking this, and I've tried getting a for sure answer, but here it is: Can I just put a junkyard TCM in and have it work, or do I NEED to have it programmed? Dollar for dollar if I need to have it programmed I might as well just drop the $250 on a pre-programmed one, but I've seen a few videos of people just swapping them and they don't really mention having to program it. It's a 2003 Town and Country if that matters
 

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Edit: Sorry, thought immediately this was about the main computer, or ECM. You said TCM, which is the Transmission Control Module. Yes, you CAN just swap that out and not need programming. The computer will "learn" your transmission values as you drive it. You can even swap in TCM's from different models or years, but the base programming may be different. Best to look at the part number on the TCM sticker and try to match that. It's not crucial to match it, but like numbers will likely have the same software values programmed in already.

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What matters is if you have the security system/chipped key. The computer and wiring is very similar between 2001-2003 vans and the later 3rd gen vans.

The security feature is programmed into the ECM (engine computer), BCM (body control module), and initiated with the SENTRY module under the steering column with antenna around the ignition to sense the chipped key. The ECM, Sentry module and chipped key all have to stay together as a unit when swapping into a different van. The odometer will change with the swap, so may not be legal to do so if inspections are required. This is why people have to go the preprogrammed route.

The BCM learns the security feature if it doesn't have it from the factory. Once learned, it cannot be erased. If your van does not have security and you add the ECM, Sentry module and key to it, the van will "learn" or turn on the security feature in the BCM. Now the van will always have to have the security system in it.

You can't just add ANY sentry module to the van, it has to be paired with the ECM and key. You can't swap parts out with the battery hooked up either, or you risk connecting the sentry module to the wrong computer. There have been people who have tried adding security to their van without having all the matching/paired up parts and locked themselves out of their van. The fix was towing it to a dealer and getting computers replaced and programmed costing hundreds of dollars.

The main thing is look for that sentry module under the lower steering column cover to first see if you have security.

Why do you want to swap the computer? Being it's a 2003, have you checked for the infamous melted injector wiring harness yet? There's a thread dedicated to it at the top of this section. If melted and shorting for too long, it can damage the computer which is when people start thinking replace the computer before checking that harness, and then they risk damaging the next one.
 

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it's all plug and play, the only difference is the shifting and clutch pack values which are learned over time

you can also just remove yours completely and the transmission will still function hydraulically on its own in limp mode up to 2nd gear
 

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Discussion Starter · #6 ·
Okay so I've tried 3 different TCMs and still have limp mode and P1684 code. Is there possibly some reset procedure that I'm missing, or is the next step to start probing wires and looking for a short?
 

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Always good to post the actual problem early on — for this code it’s unlikely that it’s the TCM.

This code means that the TCM has lost battery power. It sounds like it never has it if it’s in limp in mode. Missing power or ground to the module are likely causes. Do you have a wiring diagram?
 

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Discussion Starter · #8 ·
I don't have a diagram, no. What makes you say that it's unlikely that the TCM as failed?

EDIT: Let me expand upon this: The van did lose power so the code might be completely unrelated or the cause of the problem.
 

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I don't have a diagram, no. What makes you say that it's unlikely that the TCM as failed?

EDIT: Let me expand upon this: The van did lose power so the code might be completely unrelated or the cause of the problem.
You’ve replaced it twice and nothing changed. You wouldn’t think the odds are good that all three have the same problem, would you?

The TCM (like any electronic module) is always the last thing you replace after you’ve eliminated everything else. They don’t go bad that often, and you can’t really test them.

You should try to clear the battery disconnected code. If it doesn’t clear, chances are pretty good that it’s the real problem. Troubleshooting it requires checking powers and grounds. I’ll post the first part of the troubleshooting process. Sorry about the quality — it’s pictures of a monitor.
 

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P1684 doesn't really mean anything on these vans, it just means the battery was disconnected "recently", i still have it on mine after months and months with no issue

have a shop pull codes direct from the TCM, these are different than normal OBD codes that trigger the check engine light
 

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That’s not how it should behave. That code should go away or be able to be cleared. I understand that’s how yours is behaving, and that there don’t seem to be other issues.

Regardless of codes, I’d for sure check the power and ground to the TCM. If the module has no power the transmission will be in limp in mode.
 

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That’s not how it should behave. That code should go away or be able to be cleared. I understand that’s how yours is behaving, and that there don’t seem to be other issues.
it should be able to be cleared with the right scanner yes, but if i remember correctly it should be 60 "drive cycles" before the computer clears the code itself, it sticks around for a long time
 

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Discussion Starter · #15 ·
P1684 doesn't really mean anything on these vans, it just means the battery was disconnected "recently", i still have it on mine after months and months with no issue

have a shop pull codes direct from the TCM, these are different than normal OBD codes that trigger the check engine light
Using a Matco Maxlite scanner it was unable to communicate with the TCM which is what originally lead me to believe it was the TCM.

Couldn't I just probe the connector using a pinout such as this:
 

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you can probe for power or various sense voltages, but it wouldn't tell you much other than basic power, i think it's more likely that the scanner and TCM just don't get along or don't meet whatever standard is required for communication
 

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You could — just get the right diagram (that’s for a RWD). I don’t know how much of this kind of thing you do, so sorry if I tell you things you already know. You really need a wiring diagram to go along with this to know what you’re seeing. You also don’t want to stick something in the terminals — you run the risk of opening up the terminal and creating an intermittent connection that’s a beast to diagnose.
 

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First, is the van in a corrosive environment? I've seen TCM's badly corroded with the housing mounting bolt tabs breaking off, and the steel cover rusted through. I finally found a stainless steel cover on one at the junkyard and grabbed it. The corrosion could cause a loss of ground.
 
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Discussion Starter · #19 ·
you can probe for power or various sense voltages, but it wouldn't tell you much other than basic power, i think it's more likely that the scanner and TCM just don't get along or don't meet whatever standard is required for communication
Okay, but the van doesn't shift and the speedometer doesn't work so I'm lead to believe the TCM isn't working.

First, is the van in a corrosive environment? I've seen TCM's badly corroded with the housing mounting bolt tabs breaking off, and the steel cover rusted through. I finally found a stainless steel cover on one at the junkyard and grabbed it. The corrosion could cause a loss of ground.
Yes, I live in the rust belt and the original TCM has completely corroded over and the stud the bolts go into are crumbling. You're saying it doesn't ground out through the cord that goes through it?
 
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