Question about 1990 Jeep Wrangler

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4x4 wont engage

Front will drive runs off vacum so does the rear. It's not getting the vacum to engage. Vacum lines lead to two boxes, but not sure where to go from there or which box.

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  • cwpinell Feb 22, 2009

    my four wheel drive is not

    working and the light don't come on when i put it in four wheel drive

  • sniperpan Mar 03, 2009

    my jeep runs and when i engage the 4x4 with the rear driveshaft off the front drive shaft turns but the 4x4 doesn't engage it's vacuum operated so i don't know if thats the problem or not

  • cdaniels424 Mar 16, 2009

    i have the same problem i engage my 1990 yj jeep in four wheel drive and it goes slower like it is engaged but the front wheels are not pulling?



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I installed a Posi Lok on mine years ago... it is awesome, go to
Also, JP Magazine just had an article about these vacuum actuated axles.

Posted on Mar 14, 2009

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Hello, go to this site for an explanation of the locking system. If you need more help just ask. good luck.

Posted on Jan 28, 2009


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1 Answer

4 wheel drive indicator light is on but the front wheels are not engaging

Are they vacum operated or you have to lock it in by hand? If you have an electric over vacuum control switch on the transfer case and 2 electric over vacuum switches, they are on the passenger side front fender well near the rear by the hood hinge, they are either green or blue and made of plastic, they both have vacuum lines and 2 wires connected to them. From there the vacuum lines run down near the front differential where they connect to a slave cylinder. the vacuum routes one way, the slave plunger moves and the differential engages 4WD, the vacuum routes the other way, the slave plunger moves again and the differential disengages 2WD (similar to a 90's model dodge 4x4).

Run this easy test. Label the vacuum lines that come from the top of the vacuum switches on the fender well (front, rear) use masking tape.

Start your truck. (make sure one of those switches has vacuum, put your finger over the nipples where you unplugged the lines, if you feel suction, you got vacuum) trade the vacuum lines, front to rear and rear to front. shift into 4x4, you wont get a 4x4 light, but if you now have 4WD then the problem is in the circuit somewhere between those 2 switches and the master switch on the T-case.

Keep us udated (remember to rate this).

Dec 01, 2010 | 1991 Isuzu Trooper

1 Answer

1987 2.8 S10 pickup; wont engage in 4WD. drive.The vacuum canister dose not move . Dose the vacuum line go right to the shifter? And is there any type of relay involved? Or fuse ,or anything else?

There is a vacum actuator on the right side inner fender or under the battery box. Vacum line in one end, that operates a cable on the other end. This
cable engages and disengages the front axel.
Hope this helps.

Feb 28, 2010 | Chevrolet S 10 Cars & Trucks

1 Answer

4wheel drive will not engage

There are three main things that could cause this. #1: in the passenger side kick panel is a module that controls the 4x4 and it should be clicking as you try to put it in 4x4.#2:There is a vacume activator switch on the transfer case located on the top on the drivers side.(If the module is clicking and trying to put it in 4x4 then I would just go to this activator switch).#3 If you replace the switch and it still dosn't go in to 4x4 there is a vacum diaphram under the battery tray that pulls the front end into 4x4.Check and make sure that there is vacum getting to the diaphram which is controled by the activator on the transfer case.If no vacume at the activator or diaphram check all the vacume hoses going into and out of the activator.If no vacume while vehicle is running suspect bad vacume line.You will need to remove the battery to test the diaphram under the battery tray.You will need to hook up some kind of battery to the cables with the battery out of the car to get it started in order to check for vacume at the diaphram.

Feb 06, 2010 | 2000 Chevrolet Blazer

2 Answers

4 wheel drive hi wont work but 2 wheel and 4 wheel drive lo will in my 03 blazer

first you need to figure out if 4 wheel drive in 4 low is working. i know you said it is but do the front tires actually pull... if it is than you have a transfer case issue... if it does not than you probably have a vacum issue with the front hubs

Jan 30, 2010 | 2003 Chevrolet Blazer

2 Answers

1995 dodge 3500 4wd wont engage have shifter on floor light wont come on or 4wd wont engage

Any time your dealing with a 4x4 system the best start is at you front axle. Its primarily a vacuum leak so check to see if the 2 vac lines are hooked up (its good to check the electric connections.). When your 4 wheel drive is engaged and the motors running you should have vacuum at one or both of thee lines depending on if your in 4 hi or low .
If no vacuum at axle check the Vacum switchs(the plug with 4 vac lines) on top of the transfer case it will have 2 feed lines(grey & green) they conect in to the into the vacum harness on back of motor (passanger side). A vacum schematic is normally located on the raditor core support (right under the hood next to hood release)
Hope this helps

Jan 20, 2010 | 1999 Dodge Ram 2500 4WD

3 Answers

Our Chevy S-10 ZR2 won't go into 4 wheel drive

under battery is a vacum operated actuator that pulls your 4 wheel drive in to front axle.Make sure it has vacum going to it .Sometimes the actuator will get a hole in the rubber housing and 4 wheel will not engage.You could also check your vacum lines going to a canister on drivers side fender as sometimes the will come un-pluged and your actuator under battery will get novacum to it.If not it could be your actuator motor which is on the transfer case and is electric controlled and expencive300-400 dollars.Also could be a vacum line

Jan 09, 2010 | 2001 Chevrolet S-10

1 Answer

When i turn on heat or ac it runs on the defrost setting but wont shift to the vent or floor until the 4 wheel drive knob gets turned on 1999 f250 I heard this was a common problem is it the switch for 4x4...

you have a vacum leak in the 4x4 engagement system. there is a vacum solenoid on the passenger side next to the battery. should have 3 hoses to it I believe. check those hoses for cracks they go all the way to both front wheels. the ones in the wheel wells are usually messed up. if all hoses are good hook up a hand held vacum pump to each wheel they should hold about 15 hg on the vacum guage if not your wheel hub seals are leaking.

Jun 24, 2009 | 2002 Ford F250 Super Duty SuperCab

3 Answers

4 wheel drive won't work

There is a vacum powered actuator on the front differential housing on the passenger side. It gets it's vacum from two valves mounted on the firewall just behind the battery(to left). The white line feeds manifold vacum to the valves. Check for vacum at white line with engine running. If no vacum there is a break in line from manifold. If vacum check the pink and blue lines by applying vacum to each one. The vacum should hold. If not check lines from valves to vacum actuator. Most likely it will be the actuator. If both lines hold vacum check valves. Remove electrical connectors from both valves. Connect a wire from one of the terminals of the valve to ground. Touch a wire from the battery + to the other terminal of the valve. Listen for a clicking sound. If no sound valve is bad. Replace valves(s). If this doesn't lead you to the trouble it's probably a mechanical problem inside the differential. However most likely there would other noises.

Dec 20, 2008 | 1992 Jeep Wrangler

3 Answers

Dodge 4x4 Doesn't Engage

1998 dodge ram quad..front end wont lock in or the lights come on to tell you its in 4x4..***** little

Oct 03, 2008 | 2001 Dodge Ram 2500 Truck

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