Good news guys! The car is fixed…the XeModex guys diagnosis of TCM was “Full of cold solder on entire PCB”
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I’m thinking they meant ‘full of cold solder joints on entire PCB’
Good to hear. I was just back from basic training where I got my first exposure to electronics on teletypes. One on the problems we had to find before we could pass and go home was a cold solder joint.
So my dad had a new Plymouth and the radio would cut out. The dealer had it out for repair three times and never fixed it. Finally in desperation my dad let me take a look at it. Found a cold solder joint and worked fine after that. This kind of stuff straight from the factory.
Being a non certified technician I never rubbed it in.
thank you guys for all your suggestions and help! you guys are the best…THE BEST!
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I’m happy to hear your Mazda3 is back on the road & purring again. The TCM is presumably just another of your car’s small special purpose computers, constructed with integrated circuits & other small components soldered to a fiberglass printed circuit board. That type of soldering is usually done all at once using special purpose soldering equipment only pcb manufacture have, not hand soldered with a soldering iron. Cold solder joints are usually associated with hand-soldering. So it is sort of confusing how all of those joints could be cold soldered? Maybe their soldering equipment was on the blink at the time your pcb was constructed. But then we’d need an explanation how it worked from 2006 to 2014?
Aged pcb’s can develop faulty solder joints, but those are usually caused by the solder or metal traces cracking from thermal movement over time. The pcb material expands/contracts differently as a result of temperature changes compared to the solder, which eventually cracks the joint. In that situation, if all the joints remain accessible, the problem can often/sometimes be solved by just re-soldering all the joints. This is done with the pcb out of the car and on the electronics work-bench of course.
No
This is an extremely common problem for Mazda TCMs
I’m not clear what you mean. Are you saying that finding cold solder joints or all or most of the solder joints on a Mazda TCM printed circuit board is a common thing? If so, there must be some sort of theory for that, b/c it is defintiely not expected. Finding a slew of cracked solder joints on an older TCM, ok, I can believe that. Esp if the TCM is located in the engine compartment. There are big temperature variations inside an engine compartment.
It’s actually pretty amazing any pcbs at all can survive an engine compartment for years and years. Desktop computers have lots of solder joints which seldom fail, but they are not kept in engine compartments.
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The TCM was mounted on top of transmission some say that heat from the tranny causes it to eventually fail. I mounted the new unit on the firewall like some suggested.
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I personally don’t buy the theory that the heat from the transmission causes the TCM to fail . . . but I nevertheless relocated it, as suggested
I know of some designs that mount the TCM inside the transmission and they’re not failing at a high rate
I also know of some designs that mount the PCM directly on top of the engine and they’re also not failing at a high rate
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I agree, seems like if a transmission is getting hot enough to damage solder joints in the TCM mounted to it, that the transmission wouldn’t last very long anyway… Heat Kills transmissions…
But relocating it seems like a good idea anyway, but probably more for vibrations than heat…
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That’s definitely not true. In fact, when SMT assemblies experience cold solder joints (or voids), it is usually a majority of the joints that are bad. The reasons are many. Not the least of which is a process related issue- incorrect dwell times in either pre-heat or cool down, insufficient paste application, outdated paste, incorrect formulation of paste ratio flux to solder and on and on. These end up primarily being batch related failures. Where we see them most is when switching vendors and they do not have the profiles set correctly or they have very poor process controls in place. The result of material deflation efforts and cost downs for the supply chain folks.
The product in the OP is encapsulated. Go to the cheapest place to build them. They will only do final test after encapsulation. No longer able to do visual inspection at that point. Now mount the assembly on top of the transmission, subject to large thermal swings…I have a prediction for you 
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Not disputing your informative comment @TwinTurbo, but did you see post number 25 above? @db4690 seems very experienced w/this problem & says this is a very common problem & isn’t associated w/defective soldering.
OP, just out of curiosity, are you able to post a close-up photo here of some of these failed solder joints? .
I doubt that @db4690 would disagree with @TwinTurbo 's explanation.
They are a common failure item- it does not have to be for one single reason.
The way I read it, he does not believe it is heat related. I didn’t see anything disputing cold solder joints.
A properly designed and built TCM should be able to survive in its intended location. That does not preclude any failure mode or incident rate from design or process issues rearing its ugly head. Because one mfr does not have any failures with their unit mounted on an exhaust manifold (joking) does not mean another mfr unit is as similarly capable. At the same time, a historically solid unit can suddenly experience a rash of failures because of issues with the processes used to build them.
It’s not a matter of high heat exposure. IF there were cold solder joints in the repaired assembly, then thermal cycling can easily cause them to fail as the mechanical bond fails from repeated stress. Sometimes, this can take a long time to manifest itself but the weakness is built in and it’s usually just a matter of time and cycles…