Electrical Rear Defrost - '87 - Volts, Amps: Yes. Deice: No.

Eulers · Jun 7, 2015

Fox Body Electrical Gurus, Hi!
I'd appreciate your suggestions with my hatch window defrost problem described below.
CAR: '87 hatchback

Problem: Turning on the rear defrost button does not even make the slightest difference in any fog or glazing of ice on the rear window.

Volts: 12+v measured at the two tabs that are mounted to the glass (The fine wire grid is fed from these tabs)
Volts: 12v measured on each of the fine wires in the grid in the main backlight (glass)

Amps: 55-77ma measured from each grid wire to ground

Grid Wires: I've visually inspected the fine grid wires and they do not seem to have any breaks along their length.

Tint: There was "temporary" tint on the glass. I label it temporary b/c it was NOT held on by any adhesive. It was held up perhaps by static electricity or something. I had bought the tint solely because it did not have adhesive. The tint was on for a few years. I easily peeled it off last autumn by just getting my fingernail under one corner. It came right off. The DEFOGGER was indeed effective on fog & ice glazing with the tint present. But last winter, after the tint was removed, there was no noticeable defogging or de-icing.

I had read through the following on a 94/95 but it didn't lead me to discover anything on mine.
rear defrost | Mustang Forums at StangNet

Any suggestions?
Thanks
Eulers

jrichker · Jun 7, 2015

You at least one thing wrong; current is AWAYS measured with the meter in series with the load. Try to do in like you did and you'll blow the fuse in the meter or damage it. Why? because you created a direct short between power and ground. With voltage the meter is a very high resistance, and uses microamps (1/100,0000} of an amp.
With current , the meter is a very low resistance and will use all the current the circuit will provide. The meter is scaled to measure small currents and then larger ones. Put the meter in parallel with the load and you will put all the current the circuit can deliver though the meter. If you are lucky, blows the fuse inside the meter. If you aren't, it damages the meter.

Step 1.) Check the fuse. Fuses will sometimes look good but not pass the rated current.
Diagram courtesy of Tmoss & Stang&2birds

Fuse box layout

Step 2.) One side of the circuit will have voltage when the circuit is energized and one side will be at ground when the wiring is working correctly. Your post said you got 12 volts on both wires. That indicates a bad or missing ground.
The following diagram is one I copied part of from a Ford wiring manual

Eulers · Jun 8, 2015

jrichker said:
You at least one thing wrong; current is AWAYS measured with the meter in series with the load. Try to do in like you did and you'll blow the fuse in the meter or damage it. Why? because you created a direct short between power and ground. With voltage the meter is a very high resistance, and uses microamps (1/100,0000} of an amp.
With current , the meter is a very low resistance and will use all the current the circuit will provide. The meter is scaled to measure small currents and then larger ones. Put the meter in parallel with the load and you will put all the current the circuit can deliver though the meter. If you are lucky, blows the fuse inside the meter. If you aren't, it damages the meter.

Step 1.) Check the fuse. Fuses will sometimes look good but not pass the rated current.
Diagram courtesy of Tmoss & Stang&2birds

Fuse box layout

Step 2.) One side of the circuit will have voltage when the circuit is energized and one side will be at ground when the wiring is working correctly. Your post said you got 12 volts on both wires. That indicates a bad or missing ground.
The following diagram is one I copied part of from a Ford wiring manual

See the following website for some help from Tmoss (diagram designer) & Stang&2Birds (website host) for help on 88-95 wiring Mustang FAQ - Wiring & Engine Info Everyone should bookmark this site.

94-95 Mustang wiring diagrams
Mustang FAQ - Wiring & Engine Info

Complete computer, actuator & sensor wiring diagram for 91-93 Mass Air Mustangs
http://www.veryuseful.com/mustang/tech/engine/images/91-93_5.0_EEC_Wiring_Diagram.gif

Complete computer, actuator & sensor wiring diagram for 88-91 Mass Air Mustangs
http://www.veryuseful.com/mustang/tech/engine/images/88-91_5.0_EEC_Wiring_Diagram.gif

Ignition switch wiring
http://www.veryuseful.com/mustang/tech/engine/images/IgnitionSwitchWiring.gif

Fuel, alternator, A/C and ignition wiring
http://www.veryuseful.com/mustang/tech/engine/images/fuel-alt-links-ign-ac.gif

O2 sensor wiring harness
http://www.veryuseful.com/mustang/tech/engine/images/mustangO2Harness.gif

Vacuum diagram 89-93 Mustangs
http://www.veryuseful.com/mustang/tech/engine/images/mustangFoxFordVacuumDiagram.jpg

HVAC vacuum diagram
http://www.veryuseful.com/mustang/tech/engine/images/Mustang_AC_heat_vacuum_controls.gif

TFI module differences & pin out
http://www.veryuseful.com/mustang/tech/engine/images/TFI_5.0_comparison.gif

Fuse box layout
http://www.veryuseful.com/mustang/tech/engine/images/MustangFuseBox.gif

87-92 power window wiring
http://www.veryuseful.com/mustang/tech/engine/images/mustang87-92 PowerWindowWiring.gif

93 power window wiring
http://www.veryuseful.com/mustang/tech/engine/images/mustang93PowerWindows.gif

T5 Cutaway showing T5 internal parts
http://www.veryuseful.com/mustang/tech/engine/images/5_Speed_Cutaway_Illustrated.jpg

Visual comparison of the Ford Fuel Injectors, picture by TMoss:
http://www.veryuseful.com/mustang/tech/engine/images/Ford_Injector_Guide.jpg

..........................................................................................

JRICHKER: THANK YOU!. Thank you. Thank you. Your instruction is excellent. Yes,I'm still learning electrical. I will remember to measure in series from now on instead of shorting to ground for current measurements. Superb explanation. I understand now.

I am on the road this week, so I will use your procedures once I return. I will post again this upcoming weekend with an update.

Thanks again!

7991LXnSHO · Jun 15, 2015

Tint is a good way to ruin a defrost grid. The tint folks know it and still tint over them. I hope you are lucky with the static cling one.

If you find a break in the lines on the window, it is fixable. I have sold and used a Loctite or 3m product that looks like a bottle of metallic fingernail polish. You clean the window however they say, use two pieces of masking tape to make a new line where the old one grid stripe is gone, then paint away. Dry, pull the tape and the lines all work.

Eulers · Aug 5, 2015

7991LXnSHO said:
Tint is a good way to ruin a defrost grid. The tint folks know it and still tint over them. I hope you are lucky with the static cling one.

If you find a break in the lines on the window, it is fixable. I have sold and used a Loctite or 3m product that looks like a bottle of metallic fingernail polish. You clean the window however they say, use two pieces of masking tape to make a new line where the old one grid stripe is gone, then paint away. Dry, pull the tape and the lines all work.

.........................................................................................
Well, I'm back working on the Defrost problem. Over the past two months, I had to give attention to a bevy of problems that arose on my wife's 1998. I still have to do her tie rod ends this weekend, but on to the 5.0 Defrost.

Yes. I checked the conductor lines in grid. They all look fine by my eyes.

However, tonight I checked for a bad ground situation as JRichter sugggested. I think I've isolated the problem but I do not know how to solve it.
Here is what I found:

(A). With the hatchback lid fully open, I clipped one end of a thick wire to the black frame of the latch mechanism which is bolted to the hatch. I clipped the other end of the thick wire to a known good chassis ground. I did a continuity test between the hatch lid and chassis with the lid still fully open and the thick wire in place. Good continuity.

(B). Next I flicked the ignition switch to "On" and pushed the Defrost button to "On".

(C). Then I measured the voltage at the LEFT side grid terminal plate (on the window). Saw 11.8 V

(D). Similarly, I measured voltage at the RIGHT side grid terminal plate. Saw 2 V

(E) Next, I disconnected my thick wire (that ran from hatch lid to chassis ground). I measured both sides of the grid again. Now both sides were 11+ volts.

(F) I'm starting to wonder if the ground for the hatch lid are the two vertical bosses on the chassis roof? These bosses are located within each of the two hinge plates, left & right. These chassis bosses look like they were intended to make contact with tabs on the hatch lid.

IS IT POSSIBLE THAT THESE CONTACT POINTS HAVE WORN TO THE POINT THERE IS A GAP & NO GROUND anymore?

The chassis bosses look like they are located by a sliding clip. I tried to shift the chassis bosses with a large screwdriver without any success.

How do we compensate for wear between the chassis bosses & the hatch lid ground tabs?

I'm inclined to think this is my problem for the lack of rear defrost.

Thanks in advance for your patience & suggestions.

Eulers · Aug 5, 2015

7991LXnSHO said:
Tint is a good way to ruin a defrost grid. The tint folks know it and still tint over them. I hope you are lucky with the static cling one.

If you find a break in the lines on the window, it is fixable. I have sold and used a Loctite or 3m product that looks like a bottle of metallic fingernail polish. You clean the window however they say, use two pieces of masking tape to make a new line where the old one grid stripe is gone, then paint away. Dry, pull the tape and the lines all work.

...........................................................................................................
JRichter, Would you take a look at my new findings as I posted in a reply below to 7991LXnSHO? I'd appreciate your comments. Thanks.

Eulers · Aug 5, 2015

jrichker said:
You at least one thing wrong; current is AWAYS measured with the meter in series with the load. Try to do in like you did and you'll blow the fuse in the meter or damage it. Why? because you created a direct short between power and ground. With voltage the meter is a very high resistance, and uses microamps (1/100,0000} of an amp.
With current , the meter is a very low resistance and will use all the current the circuit will provide. The meter is scaled to measure small currents and then larger ones. Put the meter in parallel with the load and you will put all the current the circuit can deliver though the meter. If you are lucky, blows the fuse inside the meter. If you aren't, it damages the meter.

Step 1.) Check the fuse. Fuses will sometimes look good but not pass the rated current.
Diagram courtesy of Tmoss & Stang&2birds

Fuse box layout

Step 2.) One side of the circuit will have voltage when the circuit is energized and one side will be at ground when the wiring is working correctly. Your post said you got 12 volts on both wires. That indicates a bad or missing ground.
The following diagram is one I copied part of from a Ford wiring manual

See the following website for some help from Tmoss (diagram designer) & Stang&2Birds (website host) for help on 88-95 wiring Mustang FAQ - Wiring & Engine Info Everyone should bookmark this site.

94-95 Mustang wiring diagrams
Mustang FAQ - Wiring & Engine Info

Complete computer, actuator & sensor wiring diagram for 91-93 Mass Air Mustangs
http://www.veryuseful.com/mustang/tech/engine/images/91-93_5.0_EEC_Wiring_Diagram.gif

Complete computer, actuator & sensor wiring diagram for 88-91 Mass Air Mustangs
http://www.veryuseful.com/mustang/tech/engine/images/88-91_5.0_EEC_Wiring_Diagram.gif

Ignition switch wiring
http://www.veryuseful.com/mustang/tech/engine/images/IgnitionSwitchWiring.gif

Fuel, alternator, A/C and ignition wiring
http://www.veryuseful.com/mustang/tech/engine/images/fuel-alt-links-ign-ac.gif

O2 sensor wiring harness
http://www.veryuseful.com/mustang/tech/engine/images/mustangO2Harness.gif

Vacuum diagram 89-93 Mustangs
http://www.veryuseful.com/mustang/tech/engine/images/mustangFoxFordVacuumDiagram.jpg

HVAC vacuum diagram
http://www.veryuseful.com/mustang/tech/engine/images/Mustang_AC_heat_vacuum_controls.gif

TFI module differences & pin out
http://www.veryuseful.com/mustang/tech/engine/images/TFI_5.0_comparison.gif

Fuse box layout
http://www.veryuseful.com/mustang/tech/engine/images/MustangFuseBox.gif

87-92 power window wiring
http://www.veryuseful.com/mustang/tech/engine/images/mustang87-92 PowerWindowWiring.gif

93 power window wiring
http://www.veryuseful.com/mustang/tech/engine/images/mustang93PowerWindows.gif

T5 Cutaway showing T5 internal parts
http://www.veryuseful.com/mustang/tech/engine/images/5_Speed_Cutaway_Illustrated.jpg

Visual comparison of the Ford Fuel Injectors, picture by TMoss:
http://www.veryuseful.com/mustang/tech/engine/images/Ford_Injector_Guide.jpg

..........................................................................................................

JRichker, Would you take a look at my new findings as I posted in a reply below to 7991LXnSHO? I'd appreciate your comments. Thanks.
..............................................................................................................

Mustang5L5 · Aug 7, 2015

The tint prob brok the lines

Measure continuity across the grid lines from one side of glass to the other. I bet they are open

Eulers · Aug 9, 2015

Mustang5L5 said:
The tint prob brok the lines

Measure continuity across the grid lines from one side of glass to the other. I bet they are open

......................................................................
I understood that I should measure continuity on each grid line using a probe on the far left of each and a probe on the far right of same.

Well, starting from the top of the window & moving on down, I did it for half of the grid lines. Those all had continuity (i.e. My DVM beeped at me each time).

What should I try next?
Thanks again.
.......................................................................

Mustang5L5 · Aug 10, 2015

IIRC, there are two tab connections at the bottom of the glass for the grid. Can you remove the wire lead and measure continuity there?

Eulers · Aug 10, 2015

Yes, I can do that Tuesday evening (tomorrow). I will post results.

Eulers · Aug 10, 2015

Mustang5L5 said:
IIRC, there are two tab connections at the bottom of the glass for the grid. Can you remove the wire lead and measure continuity there?

..............................................................................
I pulled the tab connectors off each side.
I performed a continuity test between the left tab & the right tab, both of which are located at the bottom of the hatch window.
RESULTS: Good continuity (The DVM beeped).

Where is the hatch lid grounded to the chassis?
I'm starting to think I have a bad ground.

Thanks.
...............................................................................

Mustang5L5 · Aug 10, 2015

Pull the plastic hatch trim off. Iirc it grounds to the hatch right under it. I could be wrong though

Sent from my big ass iPhone 6 using Tapatalk

Eulers · Aug 11, 2015

I imagine that you mean the black exterior window trim. But I want to double-check. Do you mean the exterior trim or the interior panel trim in the hatch?
Thanks again.

Mustang5L5 · Aug 11, 2015

Inside the hatch. Pop the hatch and there is a trim panel underneath. 4 feet wide by 18" tall or so?

I think the defrost grounds to the hatch surface under there. I could be wrong though as I'm going off memory.

Sent from my big ass iPhone 6 using Tapatalk

Mustang5L5 · Aug 11, 2015

Looks like passenger side under the trim is the ground point. If you have 12v on opposite side and a good ground, it should work.

ImageUploadedByTapatalk1439330414.441813.jpg

Sent from my big ass iPhone 6 using Tapatalk

Eulers · Aug 12, 2015

Good. Thanks. Yes, I took my panel off as you show in the picture. I did some continuity tests:

Continuity - Grid to Hatch:
- FROM the passenger-side grid tab on the glass
- TO the passenger-side defrost grid's "ground" bolt which is located underneath the hatch trim panel as you showed.
- RESULT: Good continuity (with hatch lid OPEN)
Continuity - Two Points on Hatch:
- FROM passenger-side defrost grid "ground" bolt underneath the hatch trim panel
- TO the passenger-side unpainted tab on hatch lid that is located just above the hinge.
- (The 1/8" thick bare metal tabs are triangular being about 1/2" X 3/4". There is one tab at each of the hatch's two hinges. There are part of the hatch metal).
- RESULT: Good continuity (with hatch lid OPEN)
Continuity - Two Points on Chassis:
- FROM the passenger-side bare metal boss which is located inside the hatch's hinge.
- TO a passenger-side chassis ground .
- RESULT: Good continuity (with hatch lid OPEN)
- At this point, I figured that when the hatch lid closes, then the hatch's bare metal tabs must contact the chassis hinges' bosses to complete the ground circuit. That's a guess b/c I cannot physically see the gap close down to contact when I close the hatch lid. So I did one last test.
Continuity - Hatch to Chassis with hatch lid CLOSED:
- FROM the defrost grid's "ground" bolt underneath the hatch trim panel
- TO the same good passenger-side chassis ground as used above.
- RESULT: BAD. There was NO continuity (with hatch lid CLOSED).

CONCLUSION: The hatch lid is not getting grounded to the chassis. I believe the gap/contact of the bare metal tabs-to-bosses is at fault. Decades of opening and closing the 1987 hatch lid has worn the tabs and bosses so that they no longer contact, IMO. I have not devised a test to see what the gap is. However, to get them closer, I tried to pry the boss and the tab to no avail. They won't budge.

Now I'm scratching my head. What to do?
Thanks.

jrichker · Aug 12, 2015

Add a ground wire between the hatch and body. Be clever and you can find a way to hide it in the hinge area.
OR
The GT models used a ground wire for the 3'rd tail light and with some effort you probably can find it is the wiring bundle that supplies power to the window defroster. It should still be present on the LX since they used the same body wiring harness.

Eulers · Aug 15, 2015

Yes, it looks like I will have to conceive my own way of grounding the hatch to the chassis. I'm thinking of devising a gap/contact mechanism inside the latch. That way it is hidden.

I'm still curious what the ground path was in the factory design.

I concluded it is not the boss & tab at the hinge like I previously thought. I put paper & cardboard shims of various thicknesses between the boss & tab and I never got an impression. So the gap is so big that it seems they were never intended to be a gap/contact grounding point. They're probably some factory assembly aid.

At about the same time the Defrost stopped working, I had replaced the gas struts on the hatch. The OEM struts had metal ball sockets. The replacements have plastic ball sockets. I wonder if the the hatch was being grounded through struts. Likely it'd be a shock hazard with the lid open so I doubt it.

Eulers · Aug 18, 2015

Thanks again you guys.

I'm switching to body work mode while the weather is still warm.

Electrical Rear Defrost - '87 - Volts, Amps: Yes. Deice: No.

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