Wiring up rear window defroster, need tips

[351FUN]

My rear window broke recently, and I had it replaced with one with the defrost strips. This did not come on my truck factory, and I'm not having much luck finding information on wiring it up. Does it just take full power from the battery, or do I need to limit the current it gets? The glass guy said they can overheat if it gets too much power, but I'm not finding what it does need.

Truck is a 1992 Blazer Silverado.

 

[1998_K1500_Sub]

Here's the schematic for the 1998 trucks. Yours may be similar if not identical except for switchgear.

Looks like they're wired directly to B+ and GND, but with a time-delay device in series to turn them off after a period of time. See the first page, and then see the section titled "Circuit Operation" on pg. 8A-61-6

I suspect the delay device acts as a means to turn it off so that it doesn't stay on indefinitely, e.g., due to user forgetfulness. I doubt, but can't say with certainty, that the delay also prevents it from "overheating". Clearly, the window has a lot of surface area and I can't imagine the heater grid can put enough heat into such a large surface to make it "hot".

A sunny summer day in Phoenix would probably make tinted glass a lot hotter than any defroster grid.

 

[351FUN]

That's extremely useful, thank you. I may try to track down that relay and put it in circuit too, just for peace of mind.

 

[1998_K1500_Sub]

That's extremely useful, thank you. I may try to track down that relay and put it in circuit too, just for peace of mind.

Yes, a trip to the Treasure Yard would probably yield one for you at a low, low price.

 

[351FUN]

I need to get a few other things anyways, so it's time to take a pilgrimage soon.

 

[1998_K1500_Sub]

I need to get a few other things anyways, so it's time to take a pilgrimage soon.

It's always a treat. I usually go looking for one or two things and leave with the Suburban full of treasure.

 

[351FUN]

It's always a treat. I usually go looking for one or two things and leave with the Suburban full of treasure.

I'm hoping to find a w/t I can grab the whole front end off so I can upgrade to the 5x7 headlights cheaper than LMC or Ebay prices.

 

[DerekTheGreat]

I got tired of all the whistling my split window did and so I replaced it with a defrost window too. I wired mine up using factory wiring & a switch pilfered from a treasure yard. However, it doesn't work well at all. It's getting battery voltage, but I suspect there's too much resistance within the grid so not enough current actually flows through the thing to heat it up properly. Damn aftermarket trash. Least it's cool to look at- I certainly haven't seen many early regular cab short box GMT400's. Only one, and it was a long box.

 

[1998_K1500_Sub]

Damn aftermarket trash.

It was an aftermarket window w/ grid? Is that what you mean?

You measured across its terminals and it’s no less than 0.4V below system voltage?

 

[DerekTheGreat]

Yes, it's got a grid. Looks factory but isn't.

I measured right where power comes up to the grid, it was in the low 14's if I remember right with the engine running. This was over a year now though. I didn't know how else to test it.