Question about 2001 Isuzu Trooper

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2001 Trooper: Front axle will not engage when I press dash 4x4 switch

48,000 miles, shift on the fly 4x4 (does NOT have T.O.D.). Beginning just before last winter, the front axle does not engage (in other words, it does not go into 4x4 mode when I press the dash switch). Everything else is perfect, but when I press the dash switch for four wheel drive, the dash 4x4 icon blinks but the front axle will not engage. At idle I hear a 'ca-clunk' noise when I press the switch, so it's obviously trying to engage something.

SOMETIMES, if I press the 4x4 switch BEFORE starting the vehicle after it has been sitting all night, the front axle WILL engage, and the dash 4x4 icon will stay lit, and the vehicle will operate in four wheel drive mode just fine...but when I turn the 4x4 switch off, then on again while I drive, the dash light will just flash and it will not go back into 4x4. HELP PLEASE! A Canadian winter is on its way!!

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  • Tanner78 Dec 04, 2008

    I have the same problem (2001 Trooper 118K mi). In the past when this happen (3 X) the dealer replaced the Vacuum Sensor Valves (solinoid valves). These are mounted on the front axle under a skid plate. T avoid paying the dealer $400 again (parts and labor) I found the part on line for $150 shipping and handeling. It took about 1/2 hr to replace. It worked fine for a bout 2 weeks and again will not engage. I'm at a loss as to what else might be causing this issue. It may be one of the vacuum lines, the acctuator or the vacuum tank. I don't want to start shotgunning the rest of the system.

  • Anonymous Dec 20, 2008

    4x4 not engage

  • emailaak Dec 28, 2008

    Tanner78, where did you purchase the vaccum swicthing valve for $150.00?

  • emailaak Dec 28, 2008

    Tanner78, where di you purchase the vacuum switching vavle for $150.00?

  • WesleyByron Jan 10, 2009


    YES, as Tanner 78 has said, it was the Vacuum Switch Valves (VSV) (well, T78 said 'vacuum sensor valve' but I''m sure he's talking about the same thing)

    The VSV's are tiny little things, one grey and one blue. They are made of plastic and are each about the size of a Bic lighter. They're mounted together on a thin rectangular metal plate which is then mounted onto the front axle, and there is a skid plate that protects them.

    Each VSV has a solenoid inside it...a solenoid is an electrically operated device that moves a little internal plunger back and forth. In this case, the plunger sends 'vacuum' to either the RIGHT side or to the LEFT side of the vacuum device that "engages" or "disengages" your front axle.

    In my case (as it turned out) the little plungers inside the solenoid of the VSV's must have been plugged up with something...gunk... oil...I don't know, but they were obviously not doing the job.

    HERE IS HOW I FIXED MY TROOPER FOR FREE: I cleaned out the VSVs. Simple as that.


    1) Remove the VSV's from the front axle. EASY DOES IT! There are three little black vacuum hoses running to each VSV...they are a pain to remove, and BE CAREFUL because the body of the VSV is PLASTIC and if you break buy a new one. The hoses just slip over the little have to pry them off gently with a straight blade screwdriver. However you do it, remember you're dealing with a tiny plastic part. ALSO, be sure to note where the hoses came off! LABEL THEM!

    2) ELECTRICAL: You also have to remove a little electrical wire from each
    VSV...the wire connects with a colored plastic plug (one grey, the other blue). There is
    a TINY brass wire that locks this plug onto each VSV. You have to
    GENTLY pry this wire up to let it clear the little locking lug. It's a pain...make sure you have good light. And again, it's plastic, so take it easy.

    3) When you have the hoses and wires off, remove the little nuts that hold the thin metal plate to the axle. The whole assembly...which is about the size and weight of a pack of cigarettes...will come right off.

    4) Remove each VSV from the metal mounting plate.

    5) Where the electrical wire connects to the VSV, you will see there are two 'spades', or little brass ears where the plug went on. You need to get two little jumper wires with tiny alligator clips on each end, because you have to apply battery power to these VSVs to test them.

    6) Hook up the jumper wires to the spades on the VSV, then place one clip on the negative post of your battery and one on the positive. If the VSV is working, you should hear a very tiny CLICK from inside the VSV. You might have to hold it right against your ear.

    7) If it makes a click, it's working OK. Test it a few times.

    8) If you get NO click, the plunger inside is plugged, and you need to clean it out.

    9) Use a SOLVENT BASED electrical cleaner and spray it into end nozzle of the VSV. CAUTION: Do NOT use a greasy spray like WD40 or anything like that, or you will gum up the VSV and it will be useless. Make sure you use a solvent based spray that completely disappears when it dries. Spray some into your hand and let it dry...there should be NO residue or oil on your hand at all.

    10) Now it's a matter of 'spray and shake' and hooking up the wires to the battery to listen for the CLICK. Spray the cleaner into each of the generous with it... If you have some compressed air, shoot that in too...just get the gunk out. You might not see a huge pile of **** come flying out...we're talking about a small device here.

    11) If you get to the point where each VSV makes a nice, clear CLICK every time you apply the battery power, you're done. Reinstall everything and it should work. MINE DID! First try...the front axle engaged, and it works fine months later.


  • captdon Jan 19, 2009

    I've got a 2002 Trooper with the same problem. Th VSV's been replaced once before, while it was still under warranty. I had it off a week or two ago and worked on it for two hours with no luck. The blue one seems to work, and I can hear a click from the gary one -but not the same "click" like the blue one-, but I can't get air to flow through and shift ports like the blue side does. I used Gumout carb cleaner, contact cleaner - no go.

    Before I drop $150 for a new VSV, what's the probability of success? Could it be something else?

  • Tanner78 May 11, 2010

    I purchased the part at The Part number is 8971018581 (OEM Part Number).



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Hi guys i would check all the small vacume pipes that conect the sol valves toi the vacume actuators having just replaced the engen in mine i found that these do not have any clips holiding them on tight and can get oil on them and become oversized and leak you can test the diaframes in the actuators by putting a length of pipe on one port and sucking by mouth you can hear the actuator move and then find you can put toung on end of pipe and it will hold a vacume if it does not then chances are the actuator diafram id u/s hope this helps bill

Posted on Jan 09, 2009

  • billburgess4 Jan 12, 2009

    Hi Good fix just wonderd did you manage to work out how the dirt is getting in them ?

    could we fit inline filter someware to stop dirt getting into vacume system ?



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I had the SAME problem. I replaced the VACUUM SOLENOID. It is easy to replace but expensive part. After replacing it, everything worked. People don't know that the grease in the solenoid does need "exercise" throughout the year. Engaging it and disengaging it once a week all year round driving it at least 3 miles while exercising it.

Posted on May 01, 2014


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