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copleydad 03-18-2010 11:22 AM

4wd light stays on WITH a new switch
Okay. Replaced the vacuum switch on the T-case, hooked her back up and the dash light still stays on.

Under the hood I traced the vac line to the blue plastic line going to the switch. When I unplug this from the blue line's diaphragm thing between the blue line and black vac hose, the light goes out and the engine RPMs kick up a bit. I still have 4wd hi and low with this line unplugged.

When I shift into 4 H or L with the line unplugged, the light acts as it should - comes on when engaged and goes out when disengaged.

With the line connected to the blue line, RPMs drop, 4 H and L still work fine, but the damn light won't go off.

Here's another thing. The last couple times I put it into 4wd during the snow, I noticed that the light woiuld stay on until I shut off the engine. Upon restart, the light would be off.

Since this started I've noticed a slight drop in MPG so thinking maybe the fork is stuck (?). Dunno.

Remember, the 4wd works fine except for that damn light. Could it be the diaphragm thingy?(someone please help me with a name for it).

Thanks guys............Bobby

kik 03-18-2010 03:02 PM

Is it possible that any of the lines were switched during the install.

copleydad 03-18-2010 04:23 PM


Originally Posted by kik (Post 575156)
Is it possible that any of the lines were switched during the install.

Nope. She's a low mileage almost virgin Jeep. No broken lines, no vac leaks, and the cap that goes on the switch is keyed so only goes on one way.

Wondering if that diaphragm/space ship/vacuum hoo daddy thing is the culprit.

This is what I disconnected to check and when I did the light went out and the RPMs got higher.

Phone was acting up before so couldn't post this pic

kik 03-18-2010 05:21 PM

I have an 89 4.2 5spd. I never had a problem with the 4wd, but from what you are describing I bet the vac. diaphragm is leaking or some other way malfunctioning.

NAVYCHIEF 03-18-2010 05:55 PM

maybe the diaphram is installed backwards.

copleydad 03-18-2010 06:27 PM

Chief,I know it is def NOT installed backwards. It hasn't been touched since the guy in Canada hooked it up. Didn't have a problem for 20 years until a storm in January.

So, it's called a vacuum diaphragm? Well, I was close I guess. I guess I'll try and find one at Napa or AZ and swap it out tomorrow.

Weird that everything works fine without being connected and the light acts the way it's supposed to. Any other thoughts, let me know.

NAVYCHIEF 03-18-2010 08:03 PM

it should flow one way and not the other. is it clogged or got a hole in it?

copleydad 03-19-2010 04:02 PM

Okay. It's officially called a vacuum check valve. Guy at the dealership said it is probably stuck in the closed position. Didn't have any, only in Detroit. When I asked how much I nearly dropped!!!


Needless to say I ordered 5. They only had 38 in stock. If this fixes it I'm gonna order the rest - sell them on Ebay for $10 a whack or have a stash for my WF brethren.

kik 03-19-2010 05:12 PM

I'll reserve one now.

copleydad 03-19-2010 05:45 PM


Originally Posted by kik (Post 576174)
I'll reserve one now.


Figure it's better looking at the than looking for them.

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