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Old July 30th, 2020, 03:46
tempest411 tempest411 is online now
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Fan Diode Melted Down On Me Tonight

I work the graveyard shift...At lunch I went out and started my '88 XJ and heard a ton of belt squeal, and the volt gauge was way down to about 9V. I opened the hood as it ran to try and see what was going on and saw something emitting an orange glow at the front DS corner of the engine bay, followed by smoke and the smell of burning insulation. I shut it down right away of course and after a little bit of searching the tubes of the interweb I identified the part that melted down was the 'radiator fan diode'. I found a schematic and am confident I can repair the damage with a couple off the shelf diodes and some wire, but what I don't understand is what could have caused this to happen in the first place. From what I can see, even if one of the diodes shorted through itself, or to an adjacent diode, the circuit might behave a bit weird, say by the AC activating any time the fan switch activated the radiator fan, or applying 12V from the AC clutch to the already-energized radiator fan switch circuit. Or, if either went open then the fan would not come on. Any thoughts on what would cause thing to melt down in spectacular fashion?


I was going to attach a few pictures, but for some reason there is no way to do that
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Old July 30th, 2020, 04:47
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cruiser54 cruiser54 is offline
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Re: Fan Diode Melted Down On Me Tonight

Bad grounds in the engine bay and/or wonky fan motor.
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Old July 30th, 2020, 04:59
tempest411 tempest411 is online now
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Re: Fan Diode Melted Down On Me Tonight

Quote:
Originally Posted by cruiser54 View Post
Bad grounds in the engine bay and/or wonky fan motor.

The ground is through the the primary coil of the radiator fan relay, which is otherwise intact. I hope to see more in the daylight when I get home...Crossing my fingers that some other weird damage wasn't done, and I DO make it home ok. I have the burnt wires separated and taped out of the way. I have the radiator fan relay jumpered. I kinda wonder if I inadvertently damaged some of the wiring from removing and reinstalling the air filter housing multiple times the last few weeks as I was working on it.
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Old July 30th, 2020, 14:37
lawsoncl lawsoncl is offline
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Re: Fan Diode Melted Down On Me Tonight

I'd look real close at the wiring from the diode to the fan relay. If there's a short to ground there it'll pull current through the diode from the ignition switch +12v.
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Old July 30th, 2020, 19:30
tempest411 tempest411 is online now
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Re: Fan Diode Melted Down On Me Tonight

I made it home just fine this morning with the patch job I did in the dark last night. I only had about 15 minutes to look at this today before heading to work in one of my wife's cars, but it looks like the main fault is on the lead coming from the AC relay, the leg that triggers the compressor's clutch. That leg is shorted to ground. Even though the AC was off last night when I started it, it must have somehow contacted the adjacent lead at the diode, which on my particular XJ is hot 100% of the time. About fifteen years ago when I switched from a closed to an open cooling system, I installed my new fan sensor at the thermostat housing on the ground leg of the radiator fan relay's primary coil, and jumpered the connector that previously went to the Renix style sensor in the radiator. So...I have some work to do this weekend. Fortunately the hot lead from the ignition switch doesn't seem to have suffered any damage outside of about four inches from the diode. My immediate suspicion is it's at the AC relay. Something seemed a bit wonky with that circuit when I was tracing everything while trying to solve a crank/no start problem I was having.
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Old July 31st, 2020, 09:38
tempest411 tempest411 is online now
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Re: Fan Diode Melted Down On Me Tonight

Just an update, if it will help anyone in the future that may find this. I have found enough information to explain to myself what happened. The fault started at the diode when the hot leg from the key switch shorted to the leg that comes from the AC relay. Initially I thought 'why is the lead from the AC relay grounded??' The answer to that is right in the schematic...which we aren't supposed to post here, nor do I see a way to even post a simple drawing...For what ever strange reason when the AC relay is DE-energized Jeep elected to take the center terminal of the relay, which the orange AC clutch wire/circuit goes to when it's the AC isn't on, and route it to ground. Stranger still, this is only on the L6 models. L4 equipped Jeeps do not have this 'feature'. I still don't know what the nature of the failure was on the diode...whether it was a break down of the diode, or if two terminals were somehow unplugged and touched together. I had the air filter housing next to where this diode is located in and out a number of times over the last few weeks, so it is possible I disturbed or damaged it without realizing it. It's small and just kinda hangs out loose on the harness. I'm not sure why the wire goes to ground when the AC is off..do you? When I repair this mess-the whole run from the AC relay to the radiator fan relay is trashed-I'll put a fuse on the lead coming from the key switch, which was the source of current. I also have some much more heavy duty diodes I can use. I'm also tempted to just use a four terminal relay in this position, but there may be some reason for the use of the five terminal relay and the ground path it provides. Maybe something to do with 'flyback' voltage from the compressor clutch?



Comments anyone?
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Old July 31st, 2020, 19:32
tempest411 tempest411 is online now
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Re: Fan Diode Melted Down On Me Tonight

My education continues...the new AC compressor did not have a diode on it. I didn't pay too much mind to it. I figured the manufacturer (it's a new Sanden part) knows what they're doing, so it's either not needed on the current design, or perhaps it does have one built into it somehow. I'm thinking it doesn't have one. Not all do. I suspect the flyback voltage overwhelmed the smaller diode on the radiator fan circuit, causing breakdown and then a short circuit...
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