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Old 07-21-2023, 10:15 AM   #1
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Lost some accessory power - Thomas MVP EF

Does anyone have an accessory electrical panel like this? My windshield fans, coolant boost pump, and driver's heater fan stopped working recently. The fuse/relay panel shown on the right in the second picture is the only place they get power. I don't have any info on it so I don't know what fuse or relay does what. I know the panel is getting 12V ignition and battery power, and the strobe light and heated mirrors still turn on. The panel routinely gets soaked because the factory gasket on that panel door stinks, but reseating each fuse/relay didn't make a difference. Any help would be appreciated!
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Old 07-21-2023, 03:59 PM   #2
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I managed to get the tables showing the relay and fuse layouts from my lovely Thomas dealer here in Ohio. However all I can tell is that there's a problem with the whole module itself. SOME of the accessories that get ignition power work (like the strobe light), but most do not (like the fans, heater, etc.). Ignition power on the board is actually handled by a separate relay, not the big solenoid. That relay (number 5) was swapped out to no avail. I can see power on it, and again I know there's ignition power because the strobe works when turned on and ignition is on, and it shuts off when ignition is off. I'm guessing there's a printed circuit board inside this plastic module that's broken or so corroded it doesn't work.

(I'll post some links to the documents I received later and as requested.)
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Old 07-22-2023, 02:02 PM   #3
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I don't have a panel like that, so I can't help you there. But IMO some of those splices in the first picture you posted look questionable. Any chance they power the accessories you lost? If you haven't done so already I'd make sure they're good to go. I assume everything was working after those were made, correct? No recent changes/additions/modifications could have caused the problems you're having now? Were they literally working one day, then not working the next, with no work done in-between?

Second, just looking at it, it doesn't seem like there would be a circuit board behind that, though again I admit I really have no clue. Seems like just a DC distribution panel. If I were you, and other options had failed, I'd probably at least attempt to open it up (looks easy from the pics, maybe not in real life) and take a gander since you mention the potential for water infiltration. Just make sure to disconnect the battery negative first.

Also (after disconnecting the negative) it might not hurt to pull each connector and examine the terminals for signs of corrosion / damage.
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Old 07-22-2023, 02:42 PM   #4
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Originally Posted by TheHubbardBus View Post
I don't have a panel like that, so I can't help you there. But IMO some of those splices in the first picture you posted look questionable. Any chance they power the accessories you lost? If you haven't done so already I'd make sure they're good to go. I assume everything was working after those were made, correct? No recent changes/additions/modifications could have caused the problems you're having now? Were they literally working one day, then not working the next, with no work done in-between?

Second, just looking at it, it doesn't seem like there would be a circuit board behind that, though again I admit I really have no clue. Seems like just a DC distribution panel. If I were you, and other options had failed, I'd probably at least attempt to open it up (looks easy from the pics, maybe not in real life) and take a gander since you mention the potential for water infiltration. Just make sure to disconnect the battery negative first.

Also (after disconnecting the negative) it might not hurt to pull each connector and examine the terminals for signs of corrosion / damage.
I think I could open it if I unplugged everything from it and took it to a workbench. I may have to do that. Apparently these are very common points of failure, to the extent that I've received recommendations to just wire around it (as in create my own skoolie spaghetti of dangling add-a-fuses and relays).

I guessed it was a circuit board only because it seems to be a bit complex, but I could be wrong. It did occur to me why there's a separate "ignition" source on this panel: the main ignition solenoid stays on while you crank the engine (start position), but the items tied to this panel do not, so the "key" input to one of the relays for this panel must be tied to a position on the key switch that ONLY gets power when the key is "on".
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Old 07-22-2023, 02:44 PM   #5
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Another thought, has anyone found a way to mitigate water intrusion in these panels? This seems like a common issue. I think the water may actually be coming from the driver's toll window, that thing leaks like an absolute sieve but I don't yet know where the water gets in.
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Old 07-30-2023, 07:14 PM   #6
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Well, I called Mike and B's Bus sales earlier this week and got a replacement board from them for about $175 plus shipping. Slightly cheaper than eBay prices. I figured I would rather swap out the "bad" one for a "good" one, then analyze the bad one and possibly keep it as a spare.

The swap was easy, hardest part was removing the nylon lock nuts for the battery wiring, everything else was plug in connectors. I made sure they were all numbered, all but a few had numbers already on them so it was just a matching game to hook up the new board.

Looking at the old one, there are indeed printed circuit boards inside. The boards themselves are in good shape but I found a few spots where water and DC power created corrosion and potential shorts. I think I can just clean it off but I haven't tried yet. I am planning to find some sort of flexible plastic sheet I can put over these boards to try to shed water off of them since I'm not sure I'll ever fix the leaks in that compartment.
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Old 07-30-2023, 08:58 PM   #7
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Lost some accessory power - Thomas MVP EF

Sorry. Iím late to the game here. I have boards like that. Thanks for showing me what they look like inside.

Yeah my diagrams are stuck to the inside of the panel door. [emoji1743]


[emoji3522]Dave
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Old 07-31-2023, 07:42 AM   #8
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Originally Posted by Meathead View Post
Sorry. Iím late to the game here. I have boards like that. Thanks for showing me what they look like inside.

Yeah my diagrams are stuck to the inside of the panel door. [emoji1743]


[emoji3522]Dave
Thanks! The Thomas dealer was able to send me those diagrams too so now I know what is actually in each slot.
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