Rear defrost issues (1 Viewer)

[Seth_O]

The rear defrost on my '93 has never worked since I purchased the truck. I finally dug into it this week and found that I have +12v at the connectors, and good continuity across the lines for the first few inches on the glass. Moving from d-side to p-side I get ~10v about 1/3 of the way across the rear window, then its down to about 3v getting out to ~2/3 of the way across, and by the time I reach the negative lead (p-side) I have ~.01v registering on the MM. This is consistent across all the lines of my defrost circuit, with the defrost turned on, the -ve lead of the MM grounded and testing for 20v DC with the +ve lead.

Any ideas? Seems weird to me that its so consistent across all the lines unless that's by design, but then it would function, no? Appreciate any help, I have searched and read a handful of threads but nothing seemed to address this particular issue.

 

[toyotaspeed90]

Does your rear wiper work consistently?

I'm curious to see what people say.... in my 93, both my rear defrost and rear wiper have some odd functionality. One of them is, if it's acting up and defrost is on, when the key is taken out of the ignition, sometimes the defrost button will carry the ignition "on" load, not running the vehicle, until I turn the defrost button off or open any other door. My AE95 does something similar - which is when the rear hatch is opened (and if the blower motor switch is set to "on") it will essentially turn the ignition to "on", and you can hear the blower motor turn on. Again, opening another door with shut the circuit off.

I brought this up a while ago and someone mentioned looking into the grounds for the back half of the vehicle.... I haven't gotten around to it with mine, to be honest.

 

[Seth_O]

Wiper motor works fine, although I cant get my rear wiper fluid to spray. That may just be a clogged line/nozzle though.

 

[toyotaspeed90]

that would lead me to think that any common ground send to the upper hatch is working.... but.... have you sent a new/separate test ground to the ground side of the defrost to see if it's functioning then?

 

[Seth_O]

I tested the voltages across the glass both using the ground wire in the system, and using a body ground on the truck. Same results both times.

 

[ppc]

Are you testing with the hatch open or closed? Check both ways.

 

[Sarmajor]

The voltage from the heater element to negative (chassis/ground will decrease across the window as you get closer to the negative (chassis/ground end of the element. That is one of the fundamental laws of electricity. Check from the negative terminal of the heater element to negative/chassis ground on the chassis not the door and you should get real close to zero ohms and real close to zero volts. Looking at the wiring diagram for the rear demister it looks like the ground wire (white with black trace) may exit the rear door and ground somewhere else on the vehicle.


Sarmajor

 

[Seth_O]

Are you testing with the hatch open or closed? Check both ways.

testing with the top hatch closed, bottom open.

 

[Timpanogos Slim]

The voltage from the heater element to negative (chassis/ground will decrease across the window as you get closer to the negative (chassis/ground end of the element. That is one of the fundamental laws of electricity. Check from the negative terminal of the heater element to negative/chassis ground on the chassis not the door and you should get real close to zero ohms and real close to zero volts. Looking at the wiring diagram for the rear demister it looks like the ground wire (white with black trace) may exit the rear door and ground somewhere else on the vehicle.


Sarmajor

Yeah, this.

Seth, you just performed a classroom experiment on voltage dividers.

Disconnect the lead at either or both tabs and measure the total resistance across the defroster. I think you'll find that it's too high to produce much heat.

I have the same problem. my guess is that it used to have tint, and whoever removed the tint film wasn't very careful, and took a lot of the grid material with it

 

[Seth_O]

Yeah, this.

Seth, you just performed a classroom experiment on voltage dividers.

Disconnect the lead at either or both tabs and measure the total resistance across the defroster. I think you'll find that it's too high to produce much heat.

I have the same problem. my guess is that it used to have tint, and whoever removed the tint film wasn't very careful, and took a lot of the grid material with it

this makes sense to me, tks for spelling it out. when I disconnect and measure continuity from one side to the other I get practically zero.

I have a circuit pen from working on the arcades, can I just redraw the lines?

 

[Sarmajor]

Continuity of the heating element should be low otherwise it would be a crap heater. I will check mine tonight to give you a value to compare too.


Hedley Berge

 

[Seth_O]

Continuity of the heating element should be low otherwise it would be a crap heater. I will check mine tonight to give you a value to compare too.


Hedley Berge

Tks!

 

[Sarmajor]

Checked mine just now and this is the results:

V = 12 volts across the element.
I = 7.5 amps current flowing through element.
Ohms law means that the resistance is V/I.
Resistance is ~ 1.6 ohms.
Power of the element is ~ 90 watts.
If you have 12 volts at one end of the element and 0 volts at the other end and low resistance similar to the above figure then the element should work.
I don't have a Toyota Factory Manual to tell you where the various connectors are that service the rear window demister but my manual shows at least 4 different connectors in the circuit.
Looks like plugs BT1, BV1, BW1 & BW2 are involved and are also involved in the rear window wipers.
You can repair cut sections of the elements but do not replace long sections as the conductive paint will be a better conductor than the element and be a short circuit.


Sarmajor

 

[Seth_O]

Checked mine just now and this is the results:

V = 12 volts across the element.
I = 7.5 amps current flowing through element.
Ohms law means that the resistance is V/I.
Resistance is ~ 1.6 ohms.
Power of the element is ~ 90 watts.
If you have 12 volts at one end of the element and 0 volts at the other end and low resistance similar to the above figure then the element should work.
I don't have a Toyota Factory Manual to tell you where the various connectors are that service the rear window demister but my manual shows at least 4 different connectors in the circuit.
Looks like plugs BT1, BV1, BW1 & BW2 are involved and are also involved in the rear window wipers.
You can repair cut sections of the elements but do not replace long sections as the conductive paint will be a better conductor than the element and be a short circuit.


Sarmajor

Tks for taking those measurements. Am I understanding correctly that I should get 1.6 ohms resistance across the filaments with no power on? I know I'm getting the voltage reduction. Should I be measuring from one lead to another?

 

[Sarmajor]

You should measure about 1.6 ohms between the terminals at each side of the element.


Hedley Berge