Tech: Every possible dash switch for GMT400 trucks

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east302

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@Arcane

Did you add rear air to the truck?

I have a two door Tahoe and it's hot as hell in the back.


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Z71Hobbs

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For informational purposes. Here are the two gm fog light switches from early gmt400 (88-94). Black one I snagged today from an 89 sport truck.

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SUBURBAN5

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Remember all the OEM dash switches have backlighting. So, there will always be a dashlight 12v+ and ground for each switch.

Cargo Lamp Wiring Diagram
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Cargo Lamp backlight Diagram
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Is circuit a to just light up the switch and circuit b just ground for the switch?
 

SUBURBAN5

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Also what's the point of a relay? For example if I'm using the switch to turn something on do I need a relay or can I just power it straight?
 

Erik the Awful

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It depends on the current draw. If you're powering a cargo light, the switch is perfect. If you're powering halogen driving lights, run a relay.
 

Supercharged111

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Also what's the point of a relay? For example if I'm using the switch to turn something on do I need a relay or can I just power it straight?

Most square relays are good for 30A, the switches I doubt carry as much. Plus the run of wiring carrying the current can be shorter when a relay is used. The wire carrying the juice to actuate is carrying very little current so can be as long as it needs to be with no worries of increased resistance affecting the performance of the circuit doing the real work.
 

Erik the Awful

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I suppose I could have explained that better. A relay is a high-current switch, but instead of a user-operated toggle, it's controlled by a smaller current. This is typically done to keep the user away from higher current. If you run starter current through a push button, when the button fails it can get red-hot while the user has their finger on the button, or sometimes deliver a nasty shock.

Typically automotive relays have 5 prongs.

Terminals 85 and 86 are your low-current control circuit. It's an electromagnet, so you apply power to one side and a ground to the other (doesn't matter which).

Terminals 87 and 87a are your "normally open" (NO) and "normally closed" (NC) contacts, respectively. When the relay is un-energized, the NO contacts do not conduct power, but the NC contacts do. When you apply power, they flip.

Terminal 30 is a common terminal for the other side of 87 and 87a. Typically power is applied all the time on terminal 30. The load (whatever you're trying to power) plugs straight to 87 if you want it be off unless the relay is energized. If you want the load to be always on unless the relay is energized, put it on 87a.
 
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