Blew fuses on both sides of relay. Any ideas? - Hot Rod Forum : Hotrodders Bulletin Board
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Old 09-06-2010, 12:06 PM
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Blew fuses on both sides of relay. Any ideas?

Yesterday the cooling fan on my sedan delivery quit running while on my way to a car show. I swapped out the relay and the switch while at the show but that didn't solve the problem so I drove it home with no fan (cool here in WI, so never went over 190).

Today I discovered that I had blown the fuse to the main power to the fan as well as the fuse to switch side of the relay which controls that main power.

In further snooping around, I discovered the main line to the fan may have blown because of a wire which had worked itself loose from its tab on the fan harness. That wire is VERY close to the ground tab on the harness and it is altogether possible that the two touched and it went to dead ground, thus blowing that fuse.

My question is, could that also somehow be the cause of the fuse blowing on the switch circuit. These two circuits only meet at the fan relay and I just can't figure how juice from the main circuit contact could have had any effect on the switch circuit contact. Anybody ever had a similar situation or know if a relay can do that?

Could well be that the switch circuit blew for some totally unrelated reason. But I haven't been able to track down any evidence of a short or a possible short in the switch wiring. After replacing the fuses, securing the fan harness wiring, and testing both circuits for continuity, I hooked things back up and the fan is now working fine.

I haven't yet popped the top on the relay that was installed when the fuse blew to see if there is any evidence of damage inside. Perhaps after lunch I'll do that.

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Old 09-06-2010, 12:25 PM
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Just checked the insides of the relay, an no evidence of anything shorting. Things were clean as a whistle and operating normally.
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Old 09-06-2010, 03:36 PM
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If the short was after the relay and the relay was energized it would blow both fuses as it would really be 1 piece of wire at that point.

I am guessing the fuse closer to the fan is a lower value than the one nearer to the fuse block
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Old 09-06-2010, 05:00 PM
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The temperature control side of the relay (the coil) is a seperate circuit from the power side (the contacts).
The temp. control side is likely powered only when the ignition is on. And the power side is powered hot at all times thus reducing the current thru ignition switch.
You say the relay looks good internally so the only other thing is a wiring error. Easy now.
With the fan running check the current draw in the temp. control side of relay. If the measured current is only the coil load (.15 amps) it is wired correctly. The fuse issue must be a coincidence.
Replace the relay just for hell of it.

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Old 09-06-2010, 08:08 PM
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Quote:
Originally Posted by T-bucket23
...I am guessing the fuse closer to the fan is a lower value than the one nearer to the fuse block
Just to be clear, when I say I blew fuses on both sides of the relay, I mean I blew the fuse on the "switch/control" side of the relay and I blew the fuse on the "power" side of the relay. My OP may have not have been clear enough since I think your answer assumes I had two fuses on the high power wire, one before the relay and one after the relay.
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Old 09-06-2010, 08:17 PM
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Quote:
Originally Posted by vicrod
...You say the relay looks good internally so the only other thing is a wiring error.
That pretty well confirms my suspicion. I can test the switch/control side of the relay tomorrow but I believe it will show the proper current draw. What I now think happened is that the loose wire I found shorted the power side of the relay and blew that fuse. Then, when I opened up my console and started fiddling with the wires to troubleshoot things, I might well have shorted to switch/control side of the relay as well, thus blowing that fuse. I thought I had the battery switch turned fully off at that point, but possibly not.
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Old 09-10-2010, 08:03 AM
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So I know you were driving the car yesterday. Did you get home okay ? How's the fan now ?


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Old 09-10-2010, 09:17 AM
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Quote:
Originally Posted by 454 Rattler
...So I know you were driving the car yesterday. Did you get home okay ?
If I hadn't made it home last night, it would have been YOUR cell phone lighting up to come pick me up off the roadside.

Yes, the fan seems to be working fine now. Hopefully it was just that loose connection at the harness that created the problem and will now be permanently fixed.
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Old 09-10-2010, 10:36 AM
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Quote:
Originally Posted by cboy
That pretty well confirms my suspicion. I can test the switch/control side of the relay tomorrow but I believe it will show the proper current draw. What I now think happened is that the loose wire I found shorted the power side of the relay and blew that fuse. Then, when I opened up my console and started fiddling with the wires to troubleshoot things, I might well have shorted to switch/control side of the relay as well, thus blowing that fuse. I thought I had the battery switch turned fully off at that point, but possibly not.
Yup, that makes a lot of sense, you should consider the insulated connectors, they are great for stuff like relays that have the terminals close together
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