95 Yukon rear defrost relay - where is it?

[RCGuy666]

Hello. Does anyone know where the relay for rear defrost is located?
1995 yukon gt 2 door.
Fuses are good. Found purple wire in convenience center - by parking brake - looks like it got pretty hot and started melting the connector. Can't get power to NAT terminal when pressing defrost switch on control panel.
There must be a relay somewhere??

 

[Schurkey]

Download the service manual set for your vehicle from the links in the sticky section of the Engine forum.

 

[Road Trip]

Hello. Does anyone know where the relay for rear defrost is located?
1995 yukon gt 2 door.

Hello RCguy,

First, that's one sharp ride - Nice!

To help locate the answer to your question, I opened up the factory '95 Wiring Diagram manual,
did the "Ctrl F" (Find function) for Rear Defogger...and after a few uninteresting stops, page
1116 popped up, and has the entire circuit on 1 page:

You must be registered for see images attach

So, the relay in question is physically located in the HVAC Control Module on the dash. And
the rest of the circuit from there is pretty straightforward.

You may find the rest of the Rear Defogger section to be helpful too:

P. 1117 - Component Locations
P. 1118 - Diagnosis (Troubleshooting flowcharts)
P. 1119 - Circuit operation

Note: As Schurkey mentioned, you really want to download the '95 factory service manuals, which
gives you a big advantage while troubleshooting something like this. Having your own copy will
give you the ability to zoom in/tailor it to your device/easier to read, etc. (See attached.)

https://www.gmt400.com/threads/88-98-service-manuals.43575/

Best of luck chasing down the open in this circuit!

 

[RCGuy666]

Hey thanks for this. I was going through the 1000+ pages of the service manual but not really finding much. It makes sense that I can hear a faint 'click' in the HVAC module when I press the defog button. Not the typical relay sound - I'll remove it and see what I can find. Currently, pressing the button doesn't get me any voltage at the terminal in the footwell courtesy panel to the Purple NAT wire location. You've given me a new path! Thanks

 

[RCGuy666]

Hello RCguy,

First, that's one sharp ride - Nice!

To help locate the answer to your question, I opened up the factory '95 Wiring Diagram manual,
did the "Ctrl F" (Find function) for Rear Defogger...and after a few uninteresting stops, page
1116 popped up, and has the entire circuit on 1 page:

You must be registered for see images attach

So, the relay in question is physically located in the HVAC Control Module on the dash. And
the rest of the circuit from there is pretty straightforward.

You may find the rest of the Rear Defogger section to be helpful too:

P. 1117 - Component Locations
P. 1118 - Diagnosis (Troubleshooting flowcharts)
P. 1119 - Circuit operation

Note: As Schurkey mentioned, you really want to download the '95 factory service manuals, which
gives you a big advantage while troubleshooting something like this. Having your own copy will
give you the ability to zoom in/tailor it to your device/easier to read, etc. (See attached.)

https://www.gmt400.com/threads/88-98-service-manuals.43575/

Best of luck chasing down the open in this circuit!

Hello RCguy,

First, that's one sharp ride - Nice!

To help locate the answer to your question, I opened up the factory '95 Wiring Diagram manual,
did the "Ctrl F" (Find function) for Rear Defogger...and after a few uninteresting stops, page
1116 popped up, and has the entire circuit on 1 page:

You must be registered for see images attach

So, the relay in question is physically located in the HVAC Control Module on the dash. And
the rest of the circuit from there is pretty straightforward.

You may find the rest of the Rear Defogger section to be helpful too:

P. 1117 - Component Locations
P. 1118 - Diagnosis (Troubleshooting flowcharts)
P. 1119 - Circuit operation

Note: As Schurkey mentioned, you really want to download the '95 factory service manuals, which
gives you a big advantage while troubleshooting something like this. Having your own copy will
give you the ability to zoom in/tailor it to your device/easier to read, etc. (See attached.)

https://www.gmt400.com/threads/88-98-service-manuals.43575/

Best of luck chasing down the open in this circuit!

https://www.yps-usa.com/cdn/shop/products/z_5819_7_0_c514977a-2708-4c16-a187-1eb3dabc0ab9_2048x.jpg?v=1652305893

Think this is what I will find when I pull the hvac controls this evening...

 

[Road Trip]

Hey thanks for this. I was going through the 1000+ pages of the service manual but not really finding much. It makes sense that I can hear a faint 'click' in the HVAC module when I press the defog button. Not the typical relay sound - I'll remove it and see what I can find. Currently, pressing the button doesn't get me any voltage at the terminal in the footwell courtesy panel to the Purple NAT wire location. You've given me a new path! Thanks

Glad you find that helpful!

Reading around, it seems that the '95 has some uniqueness to it? Not only is it different that the '88-'94,
but at the same time the '96+ is integrated into the newly-introduced VCM? Anyway, there is discussion that if
you end up going down the road of replacing the HVAC control module that you either have to get the exact
duplicate OR there's some sort of jumper that sorts out the '95<>'96 differences for you? And that this used
to be included, but now is separate? ('95 Gurus with firsthand experience with all this please jump in here.)

****

Anyway, the point I'm trying to make is that let's be absolutely sure where the problem is before we swap out
a $$-$$$ module. (!) Given the observations that you have made down at the convenience center in the driver's
footwell, I would be really interested in what voltage you see at Pin G shown below?

1) IF there's power on & off at Pin G when you are using the Rear Defogger switch, then this would lead me to
believe that there's an open in the wiring between the HVAC panel and where you have been measuring? Possibly
in the vicinity of the signs of excess heat?

2) If there is no power at Pin G, then how about the power feed at Pin H? If the associated fuse is good, but
power is missing, then we've narrowed it down to an open Orange wire? But if the power is good, then now
we're down to either the Rear Defogger switch is worn out, or a component failure in the SS Time Delay Relay?

You must be registered for see images attach

Not trying to belabor the point, but we want to verify that the HVAC control module is the perpetrator of
the problem, instead of it being a victim of a associated wiring issue either upstream or downstream of it.

And that hot spot you discovered down at the 'Convenience Center' in the driver's footwell is worthy of further
investigation. Especially if the +12v output at Pin G is working as advertised.

Happy Hunting -- and let us know what you discover.

 

[RCGuy666]

Again, great info and much appreciated. I'll start digging tonight. Agreed, the signs of high-current leading to brittle connector and small amount of melted insulation on the purple wire lead me to believe there is/was a problem somewhere down the line. I'll start tracing tonight and look at the connector at the hatch. Ohm check on the purple wire back to rear of vehicle was 50ish ohms I believe (connected to defroster in glass).

 

[AK49BWL]

Yeah 95 was a weird year for these lol, updated interior but still has the TBI. HVAC panel is one-year but you can make adapters to use the later models, someone here was selling new harnesses for a while but I don't think they come with new units and the 95 unit is no longer made by anyone.