Disassembling an automobile PCM - Page 1
Forum section: electronics / general

author: Skimask
author_rank: Super Contributor
timestamp: May 04, 2014, 05:19:19 am
content: So, I got an '09 Odyssey. A/C quit working last fall. Let it ride until this spring. Take it into the shop to get a new timing belt, ask the guys to diagnose the inop A/C while they're in the area.They came back with a bad PCM diagnosis. I'm thinking, WTH??? Almost unheard in a Honda. So, I take a closer look, do more indepth troubleshooting. Sure enough. The line that gives a ground to the A/C compressor relay coil isn't pulling low. Bad MOSFET according to few schematics I've seen for that PCM. I can watch the resistance between that pin and ground change from infinite resistance to something less than infinite but nowhere close to ground when commanded by the PCM to change states.I go to take the PCM apart and it's held together with a righteous amount of 'goop'. I got the case of the PCM split apart. Now I'm left with a PCB that's held in the other half of the case with the 3 connectors 'goop'ed to the rest of the aluminum. The PCB itself isn't 'goop'ed to the aluminum housing...just the plastic around the connectors. I can bend the PCB up just far enough to see all the components.Question...anybody know of any solvents and/or methods that'll help loosen up the 'goop' without damaging the PCB?

author: Rerouter
author_rank: Super Contributor
timestamp: May 04, 2014, 07:22:29 am
content: soak for a few minutes then brushing the very top layer off in metho is how i have had to attack a few types of goop in the past, note this is the slow way...Alternaivly as there is a measurable change you could make up an external module with a comparitor to drive your own mosfet and save possibly damaging something else,

author: Skimask
author_rank: Super Contributor
timestamp: May 04, 2014, 08:02:44 am
content: Quote from: Rerouter on May 04, 2014, 07:22:29 amI thought about that, but as I said, resistance readings go from infinite ohms or at least "OL" on the BK2709B with the A/C switch off, to tens of megs with the switch on. Volts shows ground with noise on it regardless of switch position. Maybe I should try injecting a low level signal in there thru a series resistor, kinda like a curve tracer, and see if anything changes.?.?.? If I can see a change on a 'scope or a meter, surely I can build something to detect such changes...If/when I get the PCB out, and if/when I figure out which component it is that failed, and if/when I can't figure out what to use as a replacement, I'll trace the base/gate drive pin back to the MCU or wherever it goes, see if there's anything usable off of that, and drive something external to the PCM itself...even if I gotta drill a hole thru the case! As long as I keep it isolated (optocoupler + MOSFET + etc), should be good and not wreck anything else.If I knew the input conditions for cycling the A/C compressor in this particular vehicle...if I knew if it was more than just the evaporator output temp or maybe ONLY the evaporator output temp or maybe a relatively large combination of things (evap output temp, high side psi switch closed, not too high, not too low, batt voltage good, throttle < WOT, and so on), I'd build my own A/C compressor relay driver/controller. But, alas, it seems to be information that Honda, or at least their authorized technicians, keep close to them. I can easily tap into the "A/C ON" LED for an 'arm' input and can easily get to everything else, but don't want to let the smoke out of anything either.Incidentally, a new PCB from Honda runs about ~$600. Used on ebay, anywhere from $50 to about $200 or so. And a guy still has to get the new one reflashed to match the vehicle and keys at around $100+ at the local Honda dealer.???metho??? methanol?

author: andtfoot
author_rank: Unknown
timestamp: May 04, 2014, 08:07:39 am
content: Quote from: Skimask on May 04, 2014, 08:02:44 amMethylated Spirits