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Jeep Wrangler YJ Resources
YJ How-to Guides
How to retain 4WD light with passenger solid axle shaft upgrade
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<blockquote data-quote="machoheadgames" data-source="post: 438771" data-attributes="member: 18789"><p>It’s pretty simple. The 4WD light is operated by ground. The gauge cluster feeds the bulb ignition power, and the ground is switched closed or open depending on the vacuum 4WD. From the factory, an 87-90 YJ has the little blue vacuum switch on the firewall with two wires connected to it, as you’ve shown. One of those wires comes from the bulb, one goes to ground. When in 4wd, that thing gets vacuum and closes the circuit, lighting the bulb. </p><p></p><p>The 91-95s are different. They have a simple 2-wire switch mounted straight on the CAD cover. The fork slides over and presses the button on the switch, which lights the light.</p><p></p><p>When folks install a posi-lok, they receive a switch that threads into the new posi-lok CAD housing. The switch has those two screws as you’ve indicated. This is really more geared towards the 91-95 YJ, which has the 2-wire switch. If a 91-95 owner wired theirs up as the kit intended, then their light will work like factory, which is only when the fork moves over. </p><p></p><p>The TJ transfer case switch referenced in this thread is for telling you about the transfer case, which is really more intended to use that switch when you have a solid front right axle shaft. In that case, the switch tells you the tcase was shifted, boom, done. With the posi lock it’s weird - do you want to know if the tcase shifted properly, or that you moved the CAD fork properly? Neither alone will give you 4WD, so you have to decide which you’d rather have operate the light. </p><p></p><p>However you want to do it, the two wires at the blue vacuum switch are where you’ll start. Those need to be extended to the CAD or the tcase depending on which one you want to operate the light. </p><p></p><p>If you wanted to be creative and make sure the light only comes on once the transfer case is shifted AND the CAD fork is moved over, you could do so by extending the wire for the gauge to the CAD (one of the screws on the posi lock switch), then attach a wire from the second screw and run it to the transfer case using the TJ switch, then you’d run the second TJ switch wire to ground. This way the bulb would not light up until both the transfer case and front axle were shifted into 4wd mode. </p><p></p><p>I think the reason the factory chose to run the light off of the CAD was for 2 reasons: 1) because it’s the last part in the sequence and 2), if the tcase didn’t shift, you CAD probably wouldn’t engage. With the CAD disengaged, the left axle shaft turns the spider gears and rotates the center axle shaft backwards, while the right axle shaft still spins forwards. That means if you’re going 50 mph, the difference between the right outer shaft and the intermediate shaft are spinning opposite each other by about 100 mph. When the tcase shifts, the driveshaft spins the diff up to vehicle speed which should then synchronize the intermediate shaft up to the right speed and the right direction so that the CAD can engage. So I would imagine that’s why the switch is on the CAD, if the tcase for some reason didn’t actually engage 4wd and start spinning the driveshaft, I would imagine the CAD would have a hard time or possibly impossible time getting the fork to move over with only a simple vacuum suction force. </p><p></p><p>Hope that helps.</p></blockquote><p></p>
[QUOTE="machoheadgames, post: 438771, member: 18789"] It’s pretty simple. The 4WD light is operated by ground. The gauge cluster feeds the bulb ignition power, and the ground is switched closed or open depending on the vacuum 4WD. From the factory, an 87-90 YJ has the little blue vacuum switch on the firewall with two wires connected to it, as you’ve shown. One of those wires comes from the bulb, one goes to ground. When in 4wd, that thing gets vacuum and closes the circuit, lighting the bulb. The 91-95s are different. They have a simple 2-wire switch mounted straight on the CAD cover. The fork slides over and presses the button on the switch, which lights the light. When folks install a posi-lok, they receive a switch that threads into the new posi-lok CAD housing. The switch has those two screws as you’ve indicated. This is really more geared towards the 91-95 YJ, which has the 2-wire switch. If a 91-95 owner wired theirs up as the kit intended, then their light will work like factory, which is only when the fork moves over. The TJ transfer case switch referenced in this thread is for telling you about the transfer case, which is really more intended to use that switch when you have a solid front right axle shaft. In that case, the switch tells you the tcase was shifted, boom, done. With the posi lock it’s weird - do you want to know if the tcase shifted properly, or that you moved the CAD fork properly? Neither alone will give you 4WD, so you have to decide which you’d rather have operate the light. However you want to do it, the two wires at the blue vacuum switch are where you’ll start. Those need to be extended to the CAD or the tcase depending on which one you want to operate the light. If you wanted to be creative and make sure the light only comes on once the transfer case is shifted AND the CAD fork is moved over, you could do so by extending the wire for the gauge to the CAD (one of the screws on the posi lock switch), then attach a wire from the second screw and run it to the transfer case using the TJ switch, then you’d run the second TJ switch wire to ground. This way the bulb would not light up until both the transfer case and front axle were shifted into 4wd mode. I think the reason the factory chose to run the light off of the CAD was for 2 reasons: 1) because it’s the last part in the sequence and 2), if the tcase didn’t shift, you CAD probably wouldn’t engage. With the CAD disengaged, the left axle shaft turns the spider gears and rotates the center axle shaft backwards, while the right axle shaft still spins forwards. That means if you’re going 50 mph, the difference between the right outer shaft and the intermediate shaft are spinning opposite each other by about 100 mph. When the tcase shifts, the driveshaft spins the diff up to vehicle speed which should then synchronize the intermediate shaft up to the right speed and the right direction so that the CAD can engage. So I would imagine that’s why the switch is on the CAD, if the tcase for some reason didn’t actually engage 4wd and start spinning the driveshaft, I would imagine the CAD would have a hard time or possibly impossible time getting the fork to move over with only a simple vacuum suction force. Hope that helps. [/QUOTE]
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How to retain 4WD light with passenger solid axle shaft upgrade
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