It's an epoxy resin filled with silica. It will come off with DMSO at three atmospheres or so. Sometimes a soak in DMSO for a couple weeks will make it peel off. This will also soften the PC board though.
--scott
I feel your pain. I have a 1980 MB 300SD. For the AC, MB used a part from Chrysler that they used on the Imperial. It's the size of a grapefruit and it controls the entier HVAC system. In that one widget you have a valve that's in the path of the engine coolant system that goes to the heater, about a dozen vacuum hoses that control the various flaps. You also have the electical outputs for the blower, AC clutch, etc. And all that is driven by a small electric motor that is inside the thing. The motor is part of a feedback servo system that moves in relation to the desired temp setting vs the actual. It goes from max cooling, to max heating. Oh, and a critical part of that sytem is the potentiometer inside that widget, the resistance value of which changes based on the current position of the motor that works the whole thing, moving it from max cooling to max heat.
So, you have hot engine coolant, vacuum, electrical, a potentiometer, all inside one widget. How smart is that? The typical failure mechanism is that the plastic housing cracks and it leaks coolant. They were up to $600 in the 90s for a new one. Some guy down in TN started a business rebuilding them. His uses an aluminum housing for the part where the valve and hot coolant are. I bought one 10 years ago for $500. It's been fine until recently. Now it's starting to have problems. Haven't had a chance to find out what's wrong.
Well, I only had lunchtime, and it got uglier & uglier as I went!
Actually, it's the same transistor, which broke in half while I was attempting to get the black rubber eraser stuff off of it to read the numbers.
It's really going to be HARD to read those numbers now...
Yes.
This quote below is verbatim from this location:
- the FSU works again
- there is no PWM , the gates signals are continuous voltage only , this is the reason why it heat so much its aluminium box... In fact there is no point on the board where square signals are present. Can somebody check its own FSU if it's the same ?
- the 2 bridges are in fact 2 resistors 10 milliohm used to balance the currents between the 2 MOSFET and balance the power also. The mesure of the DC voltage on those resistors can be used to evaluate the current of the blower and its worn state.
- I guess the principal duty of the computer on the other side is switching off the power transistors if the control voltage goes under 1V.
I put the FSU back in the car and it still work, I don't know if it will last long. because of the heat...
Olivier[/QUOTE]
I only found 1 transistor.
Ok, they are most likely transistors and the two straps are combining the emitters. Is the center junction of the two straps connected to the motor?
Does the red jumper connect to the 40 amp fuse? If both of those are a yes, then they are most likely NPN power transistors and the jumper ties the collectors together. Or power darlingtons. And it is just a linear supply.
In your other pictures, the spring clips just hold the transistors against the heat sink.
I think he is incorrectly calling the SMT IC a mosfet.
What went in the other three holes on the PC board? What was under the two spring clips on the heat sink?
You said you tested the FSU alone. If so, how can you say there is no PWM signal between the car and FSU?
the gates signals are continuous voltage only , this is the reason why it heat so much its aluminium box... In fact there is no point on the board where square signals are present. Can somebody check its own FSU if it's the same ?
urrents between the 2 MOSFET and balance the power also. The mesure of the DC voltage on those resistors can be used to evaluate the current of the bl ower and its worn state.
Say what? 10 milliohms is .01 ohms. How could that possibly balance the power to a motor in a 40 amp circuit?
ng off the power transistors if the control voltage goes under 1V.
Which makes no sense at all.
last long. because of the heat...
It seems they last at least a few years. Maybe it's like gas. You buy it and you go so far. I think you're in way over your head here;
The main points of his post, like the "two bridges that are .01 ohm resistors, that "balance" the motor don't concern you in the least?
Blind leading the blind
Sadly, I speak from experience, in both respects.
nate
I 'think' (but I'm not sure) that these are the resistors in series:
I'm still trying to figure out if there was only one or two:
If they are resistors, I've never seen any that look like that. Also, given that you want to thermally bond any components that generate major heat, why are they not heat sinked? With any power design I've seen, the key components, eg the transistors are directly bonded to the heat sink.
The main points of his post, like the "two bridges that are .01 ohm resistors, that "balance" the motor don't concern you in the least?
Blind leading the blind =============================================
It is common to place low value resistors in the emitters to balance the current in two paralleled devices.
0.01 ohms would be very realistic in a 40 amp system.As to blind, you exhibit that trait much better than anyone else involved with this thread. Asshole.
/You said you tested the FSU alone. If so, how can you /say there is no PWM signal between the car and FSU?
/ the gates signals are continuous voltage only , this is the reason /why it heat so much its aluminium box... In fact there is no point on /the board where square signals are present. Can somebody check its own /FSU if it's the same ?
/Say what? 10 milliohms is .01 ohms. How could that /possibly balance the power to a motor in a 40 amp circuit?
Not to the motor idiot. To the transistors.
/Which makes no sense at all.
Certainly not to you.
/It seems they last at least a few years. Maybe it's like /gas. You buy it and you go so far. I think you're in way /over your head here;
But not yours?
You can increase the sensitivity by winding multiple turns around the clamp on core.
Two turns makes it a 400 amp meter and so on.
I was never crazy about that style of heatsink.
You've never seen strips of nichrome in a space heater?
Two.
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