B_Sharp

4Runner 2000 5vz Engine

PROBLEM
The Rear Window Defogger on my 4Runner never defrosted the window clearly. I never troubleshot the problem that has existed for several years because it was only a problem a few days/weeks per year. That is, only when very cold outside or even during snow. When cold, the rear (and right side) windows always stayed mostly foggy.

CONSTRUCTION
The defogger construction is a resistive coating of lines across the window that heat up thus melting the window moisture. What is not obvious though is the electrical schematic would look like one resistor in series (THICK line across BOTTOM edge of window) with several parallel resistors (several THIN lines across window MIDDLE). The ohmic ratio is roughly 40% / 60% on my window.


So roughly:
Resistor Series - WINDOW BOTTOM
40% heat. My guess is to melt snow that accumulates at bottom of window.

Resistors Parallel - WINDOW MIDDLE & TOP
60% heat. Melt moisture across remainder of window, bottom up.

MEASURED
Factory manual says 10 volts should be across window resitive lines. My window only has 7 volts (middle resistors). And all lines are connected. Actually, I measured 11 volts to the window but 3.7 volts across BOTTOM resistor and 6.5 volts across parallel middle resistors.

ADJUSTMENT
I would like to adjust the voltages to factory spec, eg 10V across parallel and remaining 1V or 2V (12V battery, 13.5V when charging) across bottom. There is no designed way to adjust the bottom resistor so it takes less heat, thereby more heat across window center. The exact current would have to be measured and a large wattage, low ohm resistor bonded across the bottom thick flat resistor element.

Another possible small gain could be running a parallel low AWG, high current capacity wire, from Defog fuse to the window. Even then, with 11 Volts already at the window (non charging), I didn't measure the battery voltage but at 12 Volts, then an extra wire is less than 10% benefit max (1V/12V).

A defogger adjustment would be a useful feature.

This is a rare nuisance for an otherwise well designed vehicle.

COATING
Does anyone have experience layering a thick resistive coating on top of the already bottom strip?? There are no gaps, I checked. I would have to wait for proper cold weather to layer a coating to tweak its effect. Whereas an adjuster switch would have been better since it could be tweaked when the problem occurs in bad weather.

eric-the-red

Never had a problem with mine, it's an 2001 4Runner. I've never checked the voltage but it defogs and defrosts very quickly.

And btw the right side window isn't a defroster, it's an antenna.

B_Sharp

Yeah, I searched this site over many years but I noticed that no one actually MEASURED the right side to be conclusive. An ohmic check indicated around a couple ohms which could also be a heating element or maybe measurement error on using flat tin foil. It is definately an antenna because of the etching pattern, a couple lines do not complete the round circuit. So a floating wire end means an antenna.

BUT one single etch does circle around so it is possible that a few inch wide strip is BOTH heating and antenna. Maybe tomorrow I'll just verify so not to assume. Darn i used to have an oscilloscope for checking high frequency RF.

Or the loop could be like those UHF circle antenna. RF (radio freq) tends to have strange configurations.

AC Check
It could be checked with a DMM test of AC first. The antenna etches 'might' show up as millivolt AC. Admittedly, a DMM is only designed for low frequency AC like 60 hz.

DC Check
The closed loop 'possible suspect' heating element etch must show up near 12v edge, 0v at other edge plus 5V middle. If that ain't there, it's all antenna.

I'm an Engineer by trade. I'm trained only to believe observation. Sorry
Or read it in a spec somewhere.

Then if the AC and DC tests were compared, that would be proof.

It's definately an antenna (etch configuration). But in the site search, no one quoted a source that it is NOT also part heater.

It's doubtful a heater because legally etches would have to cover the entire window. Still, I gotta believe observation or spec. Academic stubborness LOL!

eric-the-red

I kind of figured you were an engineer from your first post

That testing stuff is way too much work for me, my defroster works and the radio works, that's all that matters.

dz

that is far too much work for a mostly unnecessary window.

eric-the-red

True, not much point having a defroster on a window you never look out of

dz

actually, on what I'm driving around most of the time, I don't even have a rear window to defrost

B_Sharp

Side window is definately 100% antenna because no DC volts 0 thru 5 thru 12v. I gotta go outa town right now for my niece's high school graduation.

MOVING HEAT AROUND
But yes the rear window IS a big problem. It is one of those problems that if sent to a repair shop they would spend months and big bucks and still not fix it because it is not black and white like straightforward broken etch.

I had to take DMM measurements, even the amperage at the fuse. Pulled my back doing it, lol, cuz you gotta squat walk inside the 4runner with engine running and window down for defrost to work. Worth it though.

The solution is actually to coat over, on top of the 'hidden' bottom thick strip about only 10% to 25% at first, but no more than 50% metallic paint coating. That should send 10 to 30% more heat to the middle window and less at the bottom.

If I get motivated when I get back in a few days, I'll upload a clarifying picture for anyone with the same problem. That should make better sense.

LOL being an engineer is both a blessing and a curse.

B_Sharp

Rear Window ADJUSTMENT



Where:
V12 = Voltage rear window connector
Vm = voltage middle
V0 = near ground voltage

The voltage drops for the 4Runner rear window are:
Battery +12v => V12 => Vm => Vm' => V0' => V0 => Gnd

Voltage measurements are one end at (-) ground chassis and positive lead wrapped in small (1 inch square) tinfoil on top resistor etch. Engine running.

VOLTAGE DIVIDER
This is an equivalent circuit. About 12 parallel heating line etches in glass middle (left diagram) are equivalent to 1 resistor heater (right diagram).



POWER GRADIENT
This is the simplified circuit showing power distribution.
There is a bottom heat resistor and equiv. middle heat resistor.



Pt is total power to window in watts. Pb = power bottom. Pm = power middle.

It is important to measure the rear defogger current. Remove defogger fuse, put DMM inline, measure current. If any heat resistors are adjusted by layering more paint compound on top of etches, the current (I) should not change radically. Hence the Pt, overall total power remains the same.

The heat power to the window is a gradient function. The resistor etches should not focus radically more heat on a focused area so that in hot weather, inadvertant defog switch turned on, won't damage the window. Maybe 10% or 20% increment. Only enough heat should be applied so that a clear window is mostly uniform under adverse (cold) weather condition.

SPECIFICS
My window is roughly the heat of a 100 watt light bulb. 60 watts in middle and 40 watts at bottom. With a current about 7 amps. Vm about 7 volts but should be about 10 volts. This window defogs poorly though, don't go by these specific numbers. What is needed is good I and Vm values, engine running, from some toy owner in cold climate (canada? northern US?) with uniform clear window in cold snowy weather.

Joyota

Just one small recommendation (something I thought of while checking my wife's defroster). You can put a pan of hot water in the back of the runner to create your fogged up windows.