'72 Rear Defroster Resurrection... Ugh...

[BumbleBus]

Hi. I'm basically re-wiring my entire bus as nothing really worked when I got it. Tonight I was working on the rear defroster, important in Montana where I live. I've got some issues.

I'm seeing 11.9-ish volts across the switch terminals, like when I hook up K to ground or 15 to ground (31) I get good voltage readings (watch out crossing 15 and 31.. zap.. fuse blown.. doh). The green bulb is active and lit, all good signs I assumed. I ran a fresh 14ga wire directly to the back defrost grid, but I'm not reading any voltage in the back no matter what I try. I've got the ground hooked up to the frame on the left side, both spade connectors are fresh, the grid itself looks good with all traces intact etc. The grid does not get warm at all to the touch despite that I let it "run" for 15+ minutes. What am I missing? How can I get a reading back there? + to spade - to frame?!? Thanks for any tips! I know the rear defroster is kinda silly and not really important, but it sure would be nice to have her running this winter and having an OG defroster in a '72? Worth the effort in my opinion!

[dtrumbo]

You may have already done this, but perhaps I missed it in your post. Put the red lead of your meter on the end of your newly-run wire that is connected to the right side of your defroster grid. Then put the black lead on a GOOD ground (the screw head of the hatch latch on the rear deck of the bus is good). With the switch on and the green light lit, what do you get then? If 12-ish volts, good. If not, the laws of physics have been suspended or your newly-run wire is bad or the ground you found for your meter isn't so good.

If you do have voltage on the right side of your grid, then the next thing is to check the quality of the ground on the left side of the grid. Switch your meter to 'ohms' (200 in the ohms section on your meter) and put the red lead on the connector attached to the left side of the grid and the black lead on your known-good ground. You should read close to zero ohms. If not, check the integrity of the ground wire that travels from the left side of the grid following the hatch hinge to where it's screwed to the bus.

If that's all good then you need to check the integrity of the defroster grid in the glass. Turn off the defroster switch and disconnect the newly-run wire from the right side grid connector. With your meter still on ohms, put the red lead on the now vacant spade connector on the right side of the grid. Put the black lead on the left side of the grid. What is the reading? Let us know what you find.

[BumbleBus]

Thanks for the reply! After fiddling around a bit I got voltage back to the glass and all grid traces are getting power, but it's uneven and saggy at best and by the middle of the glass it's pretty low... sometimes <5v... at least it'll defrost the outer right part of the glass. Haha! I've heard you can get trace repair kits so I may just try that since it more or less sorta kinda works now.

Okay, on to wiring up all the rear taillights etc. Ugh. At least the body of this beast is in awesome shape. I keep telling myself that's all that matters.

[Oregon72]

Thanks for the reply! After fiddling around a bit I got voltage back to the glass and all grid traces are getting power, but it's uneven and saggy at best and by the middle of the glass it's pretty low... sometimes <5v... at least it'll de-frost the outer right part of the glass. Haha!

Well done! that is 100% more than my rear defroster. I should try to figure out a fix for mine too.

..At least the body of this beast is in awesome shape. I keep telling myself that's all that matters.

It really is important. Not having a bunch of body work to do is a huge plus. It's great you are jumping right into this head first. Keep up the good work - can't wait to see your progress.

Where are you at with the engine for now??

[dtrumbo]

After fiddling around a bit I got voltage back to the glass and all grid traces are getting power, but it's uneven and saggy at best and by the middle of the glass it's pretty low... sometimes <5v...

That's GOOD! The grid wires act as a resistor to make heat (albeit undetectable) to clear the window. At the right side you have 12 volts, at the left side you have ground (zero volts) so in the middle you should have half the voltage which is about what you have. Yippee! You are the only person I know who has an intact rear defroster grid. Congratulations! Now see if you can repair/reuse the original wire coming from the switch to the rear window.

I've heard you can get trace repair kits so I may just try that since it more or less sorta kinda works now.

Don't even bother. I've tried the Permatex brand repair kit and it was useless. Yours works, so don't touch it! Enjoy seeing clearly out your back window this winter!

[Bookwus]

Hiya Bum,

You are the only person I know who has an intact rear defroster grid. Congratulations!....

No kidding! This must be one of the Holy Grails of the VW world.........the intact rear defroster grid. They often look OK on a cursory inspection but if you look very closely you'll see that the baked on metal composition has decomposed or flaked off in parts. The paint is still there but you now have just about no conductivity.

If, indeed, yours is good, treasure it.

[Ritter]

At least the body of this beast is in awesome shape.

Now keep it that way. Get a poptop seal on there pronto. Mine was missing for lord knows how many years and it will eventually wear away your roof paint in a cool little halo of surface rust....

Good work on the defrost!

[BumbleBus]

Get a poptop seal on there pronto.

Will do! Good eye! Thankfully I got a new seal already (came with the bus). I wanna clean the top first though, somehow, gotta research that. So far one of those Mr. Clean Magic Eraser sponge things seemed to work well, but I can't imagine doing the whole thing with those! Definitely doesn't need to be re-resined or anything though thank goodness.

[vdubyah73]

For those that have a worn out grid on the rear window, this might work. It seems the wire grid is more like a decal instead of the plastic mat they used to be. I haven't seen one up close and personal, just something i found on the interweb before I decided not to worry about what was behind me.

http://www.frostfighter.com/prt2600.htm

[LiveonJG]

For those that have a worn out grid on the rear window, this might work. It seems the wire grid is more like a decal instead of the plastic mat they used to be. I haven't seen one up close and personal, just something i found on the interweb before I decided not to worry about what was behind me.

http://www.frostfighter.com/prt2600.htm

Nice find Bill.

-John

[dtrumbo]

For those that have a worn out grid on the rear window, this might work. It seems the wire grid is more like a decal instead of the plastic mat they used to be. I haven't seen one up close and personal, just something i found on the interweb before I decided not to worry about what was behind me.

http://www.frostfighter.com/prt2600.htm

Nice find Bill.

-John

X2. It looks like their "stick kit" would be the ticket since we all have these very cool green illuminating knobs with the Hostess cup cake logo on it and the associated wiring that otherwise, doesn't do anything. They're very reasonably priced and appear to be a nice product. Hopefully one of us will be the guinea pig and provide a review.