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Old 10-22-2021, 06:02 PM
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Default Rear defroster blowing fuse

Hey guys

I have a 69 Grand Prix with a rear defroster and every time I turn it on it blows the 25 amp fuse . Anyone have any ideas why ?

Thanks

  #2  
Old 10-22-2021, 06:09 PM
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Electric grid or blower fan?

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Old 10-22-2021, 06:17 PM
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It’s the grid on the rear window

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Old 10-22-2021, 06:20 PM
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Here is some info .
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Old 10-23-2021, 01:13 AM
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Does it blow the fuse in Low or High or both?

Break the circuit up into sections. With the switch in the off position, disconnect the connector behind the rear seat. With a meter on the ohms scale check the resistance of both wires in each plug to ground. If you read 0 ohms on one plug or the other keep going to the next component, back towards the dash or towards the window, disconnect and recheck.

If the circuit reads good check the programmer leg and do the same.

Or if you have a handful of fuses keep testing at each break in the line that you make. If you don't blow the fuse when disconnecting the rear seat connector then you know the problem lies with either the relay or a possible short in the wiring leading up to the window.
Or if you do blow the fuse then disconnect the harness under the dash and recheck.
By opening up the circuit at different points you can isolate the faulty component

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Last edited by OG68; 10-23-2021 at 01:21 AM.
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Old 10-23-2021, 10:25 AM
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Try another relay. Quick way to narrow down its not that.

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Old 10-23-2021, 10:32 AM
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Quote:
Originally Posted by burd View Post
Try another relay. Quick way to narrow down its not that.
I’m looking for one but hard to find

Relay GM #9798875

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Old 10-23-2021, 11:25 AM
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Is it the same in a TA?
Rule #1, always buy 1 extra of everything lol
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Old 10-23-2021, 11:28 AM
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I don’t think so
I will take a picture in a couple hours for you

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Old 10-23-2021, 11:51 AM
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Lookup the new part #

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Old 10-23-2021, 11:55 AM
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Quote:
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Lookup the new part #
I don’t know how to do that

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Old 10-23-2021, 02:06 PM
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Quote:
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I don’t know how to do that
Stop by your local GM dealer, that number you have may have changed several times.
Have him run the numbers on the locator. You never know.
I recently found an EGR valve for a 78 ta recently by having my buddy run it on the locator.

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Old 10-23-2021, 02:22 PM
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I always like to troubleshoot before buying parts..but that's just me. OG68 is giving you good advice on using a meter.

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Old 10-23-2021, 03:41 PM
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You need spare parts. Lol

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Old 10-23-2021, 05:01 PM
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Quote:
Originally Posted by burd View Post
You need spare parts. Lol
Can never have enough of those!
Trying to find them in your stash is the hard part

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Old 10-23-2021, 05:19 PM
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You'll find that you need fewer of them if you troubleshoot before swapping them in! Especially in regard to electrical dead-shorts. That's a good way to let the smoke out of them.

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Old 10-25-2021, 01:44 PM
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I would trace the wires going to the trunk area. One or both are shortening out on metal. Are they going under the sill plates or do the wires travel down the transmission tunnel? It looks like wires come in at the center of rear seat, I would start in trunk and trace wires from there.

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Old 10-25-2021, 10:22 PM
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Hey guys I’ve narrowed it down to a bad relay . I have one coming so I’ll keep you posted . The bad relay is chattering when I switch to low and then pops the fuse

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Old 10-26-2021, 09:11 AM
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Awesome. Figured it was was relay. Those are like $35 if you can find one on the shelf.
Trying a new relay is still cheaper than taking it in to have the elec checked.
I’d look for a spare switch. BTW, I have decals for all the switches.

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