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Re: Blew a fuse again
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Posted by Ari [Email] (#2847) [Profile/Gallery] (more from Ari) on Thu, 18 May 2006 06:01:18 Share Post by Email
In Reply to: Blew a fuse again, Ben, Wed, 17 May 2006 22:07:33
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From the schematics I can find-
Fuse 25 drives the A/C compressor clutch, coolant fan, and A/C fan. Let's say that one of those fans is pulling more current than it should. The sum of the two fans and the clutch pop the fuse. But any combination of two will be by definition less current, and that seems to be just under the limit to pop the fuse. My guess is that one of the fans is pulling more current than it should, which is the sign of a dying fan. Try spinning the blades of each fan - does one seem 'stickier' than another? That might be a sign. You could measure the current (make sure your meter can withstand a 30 amp surge - not a common meter!) and see if one fan is pulling a lot of current. What's a lot? My guess would be in excess of 20 amps.

The condition of the compressor won't affect the clutch current. That is based on the clutch current itself and nothing else.

However, how is your battery and alternator? what is the voltage at idle, when I assume you're doing these tests? If the voltage is low (say under 13 volts) at idle, the fans and clutch take more current to engage than at even a slightly higher (say 13.5) voltage. Not a big thing, but something.

As to the diode. It is a diode with the cathode connected to the switched 'hot' lead of the compressor clutch and the anode connected to ground. It is not in series with the clutch coil, but in parallel.

That diode is a surge supression diode. The AC clutch is a big coil of wire (an inductor). When engaged, a lot of current is flowing in it. When the AC relay opens to disengage that clutch, that current wants to keep flowing. The result is that the voltage on the clutch changes very fast and high - it can hit a few hundred volts. Interestingly, it goes negative. If there was no diode, this would cause some arcing on the relay contacts. The diode squelches this voltage surge.

Another way to think of it is that when the clutch is engaged, the coil is a big electromagnet with a big magnetic field. When you stop the current into it, the magnetic field collapses. When a magnetic field and a wire move in relation to each other, voltage is created (see Alternators). That is the surge voltage.

Diodes have two basic failure modes - short or open. Open is the most common. You wouldn't see any symptoms other than wear and tear on the AC relay contacts. If you have the right kind of meter, you could check the diode, but you'd have to take it out of the circuit, since it is in parallel with the AC coil. A shorted diode would be obvious - the AC clutch wouldn't engage and you'd blow a fuse. There is no such thing as a partially shorted diode, so I don't think that's the issue.

I doubt the diode is your problem. If you were to replace it, all you need is a simple diode from Radio Shack. I'd say any diode with at least 2 amp capacity and at least 600 PIV (peak inverse volts) would do just fine. Switching speed isn't critical. The diode sees nowhere the currents of the coil, in reality.

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