navy02ws6

Just blew my third rear defrost switch, can't seem to figure this out. I've heard in other threads that cheap rear hatch supports (like the ones I just installed) don't have the plastic bushing on the end that attaches to the hatch itself, which causes rear defrost switches to blow as soon as they're hooked up.

Looking at my old lift supports, which my original defrost switch worked fine with, they do have plastic bushings at the end, which are welded on. On my new lift supports, the bushing is metal. I tried wedging a piece of rubber between the electrical leads and the metal bushing, but I'm still blowing switches. I have a multimeter, but I have no idea how to use it to measure resistance, and I'm pretty sure the fuse in it for measuring resistance is blown as well.

See photo below of the fix I tried, which didn't work.

If no one knows how to fix this, does anyone know where I can get lift supports that don't blow defrost switches and don't cost an arm and a leg? Thanks!

G.Penrod

Maybe it's angle, but does your window have a rear defrost?

navy02ws6

I know, the glass is blurry in the photo because it's a close up, but I definitely have factory rear defrost, and it worked great until I replaced the lift supports.

paulo57509

My car doesn't have a rear window defroster, but I do have a couple of replacement lift supports sitting on the shelf. They're Sachs brand lifts with a plastic bushing/lock on the hatch end only, defroster wire tabs on both ends.

I checked the tabs on both ends and they do not have continuity between them and the lift support attachment points. They do have continuity between them (and the piston rod). Looking at the defroster schematic, the continuity of the lifts match what's on the schematic.

First thing I would do is get a DVM and confirm that the tabs on your lifts match the schematic and are not shorted to their ends.

Since it's the switch and not the fuses or breaker that's failing, I'd suspect you have high resistance someplace in the defroster circuit. If you had a dead short to ground the breaker or fuse would open.

Do you have both lifts connected to their wires?

navy02ws6

All 4 wires are connected on the lift supports.

This is my multimeter:


What setting do I use, where do I stick the probes, and what sort of reading should I look for?

someone972

iTrader: (2)

I'd recommend watching a short tutorial video like this one to familiarize yourself with the capabilities of your multimeter, they're very useful tools!

For this particular test, you want to have the black lead plugged into the center "COM" port, and the red lead plugged into the right-side, red bordered plug labeled with the "horseshoe". The "horseshoe" is the symbol for ohms, which is the measure of resistance. For the dial setting, I would probably put it on 200k. This sets it to a high resistance scale. You could also use the "-▶|-" looking one which should beep if there is a short, but I tend to like using the resistance (200k) because I can see if there is a partial short to ground which a lot of multimeters won't pick up on the other setting.

Once you have the multimeter set up, pull both wires off of the tabs on your defrost arms. Touch one probe to one of the tabs, and the other to any bare metal surface in contact with the mounting point. Sometimes you have to scratch at the surface a bit to get good contact. The meter ideally should show "OL" standing for overload meaning the resistance is too high to display and you don't have a short. If it shows a reading, and it's a low number or zero, then you have a problem. Do the same thing for the other tab and the other mounting point. Then, for good measure, check between the two tabs. You should see 0 on the meter.

Note that if you set it to 2000k, and have your fingers touching the metal on both probes, that you will see a high reading. The skin conducts a small amount of electricity, usually the resistance of the skin is in the megaohms range (2000k is 2 megaohms).

navy02ws6

Awesome, thanks for the how-to, that's exactly what I was looking for! Will give it a go this weekend.

navy02ws6

Okay, used the 200K mode, touched one probe to one of the tabs, and the other to the mounting bolt, and it showed 0.00. Put one of the old lift supports back in, did the same test, and it showed just "1" all the way to the left of the display, which is the same thing that it shows when it's not connected to anything. Tried moving my homemade rubber bushing to several different places like people said they did in the other threads to fix it, no change. Both the old and new lift supports look identical in terms of the materials that they're constructed from. Really the only difference I can see between them is that that both ends on the new one can be unthreaded to adjust where the tabs are, whereas the old ones were welded on both ends, leaving no adjustment.

Both the old and the new lift support show 0.00 when connecting a probe to each tab at the same time.

Now what?

paulo57509

Any luck with this?

navy02ws6

Don't really know what to do at this point other than replace the lift supports. I've tried everything I can think of to modify them.

knightfire

From the factory, the rear defrost switch in the dash does not control the heating grid directly.

The switch only controls a timer / relay assembly with very little current. It is this timer assembly that sends powers the grid with big AMPS.
If there were problems with your struts shorting out, this is the item that would be burning out, but more likely blowing fuses or circuit breakers.

If your having trouble with blowing switches, then your problem is with the timer or the wiring leading to it.

However, there is one possibility that makes sense in this situation... If someone added rear defrost to your car and did not include this timer / relay or any relay in the circuit, then it makes sense that drawing all those amps thru the dash switch would burn it up rather quickly.

paulo57509

I just noticed; are you having issues with the '91 Firebird or the '91 Camaro?

navy02ws6

Having problems with the Camaro. My T/A does not have rear defrost, which I always thought was weird.

knightfire -- Camaros (at least 1991-92) don't have a separate relay for rear defrost like Firebirds do. The relay is built into the defrost switch. Learned this the hard way when I tried to swap out the relay with a Firebird relay behind the dash of my Camaro, and spent 30 min upside down with my legs sticking out the door looking for a relay that's not there. Once again, everything worked fine before I replaced the lift supports, and the car immediately burned up an additional two good replacement switches from Hawk's the second I plugged them into the harness.

paulo57509

Obviously, there must be something different with the new supports. Based on your tests with your DVM, it seems like there's a short to ground between the tabs on the support and the attaching hardware to the body/hatch which there shouldn't be.

The weird thing is I would think if you have a short to ground, the PWR ACC circuit breaker would open (power locks and power seat would no longer work) before the defroster switch would fry.

navy02ws6

Yeah, you'd think the fuse would blow first, but apparently I'm not the only one this happened to: https://www.thirdgen.org/forums/elec...tch-keeps.html

I just can't figure out what they did to insulate the supports.

paulo57509

Look at the photo attached to your first post. Compare it with this:



The photo above is how my car left the assembly line; the supports are original (yes, they're dead).

Compare the plastic "bushing" in your photo with the one above. Notice that the bushing in your photo looks broken/damaged and the fastener has crushed the broken pieces. When new, the end and bushing should look like this:



I'm going to guess that the defroster circuit is shorted through the fastener because your bushing is broken/damaged.

NINÅ

patricks85z28

My defrost switch blew out today. Whats the fix?

navy02ws6

Did you just replace your hatch shocks? If so, fix would be proper hatch shocks, and a new (used) rear defrost switch.

As far as hatch shocks, I just installed a pair of Delco 510-509s, and they work fine with rear defrost. Only other brand I've tried is SanLong, and those definitely blow out rear defrost switches.