Venting the 5-speed MAU5 Transmission

The short version of the transmission (trannie) venting issue is that ISUZU did not install breathers on the MUA-5 trannie installed on a number of the 1989-1991 Troopers. They did, however install a breather on top of the transfer (x-fer) case, go figure.

The process causing the venting issue is that as the trannie heats up while driving, pressure builds up. With nowhere for the pressure to go, oil seeps through the oil seals to the x-fer case. This causes the x-fer case to overfill and thus oil will seep out of the x-fer case breather.

I could always tell when this happened because invariably after a road trip, the back of the Trooper would be covered in oil, along with the underside of the truck.

ISUZU published a TSB that called for replacing the x-fer case seals.
Provided by Lonesome Amigo (Nick) on the 4x4Wire:
BULLETIN NUMBER SB92-05-005
ISSUE ATE September 24, 1992
GROUP TRANSMISSION
MUA TRANSMISSION TRANSFER CASE OIL SEALS
AFFECTED VEHICLES Trooper (KT), Rodeo (UC), Pickup (TF) and Amigo (TC) 4-wheel drive manual transmission models produced before July 1991, equipped with V6 or 2.6L 4ZE1 engine.
SERVICE INFORMATION Excessive oil in the transfer case and low oil level in the main transmission case indicates that oil is seeping past the transfer case oil seals (Figure 1). If this condition is present, follow the procedures in this Service Bulletin for transmission diagnosis/repair. As mentioned in the Service Policy and Procedures ManuaI (section 6.1-7), replace individual components rather than complete assemblies whenever possible.
SERVICE PROCEDURE
1. If there is excessive oil in the transfer case and low oil level in the main transmission case, Proceed to Step 2.
2. If necessary, bring the transmission and transfer case oil levels to their proper quantities.
OiI Quantity Specifications:
Transmission OiI - 3.1 quarts (2.95 Iiters)
Transfer Case OiI - 1.5 quarts (1.45 Iiters)
3. Road test the vehicle, checking for normal transmission operation.
4. a. If road test reveals symptoms of transmission damage, perform transmission repairs as necessary, following the procedures in the appropriate Workshop ManuaI. Ensure that new transfer case oil seals are installed during the repair.
NOTE Use transfer case oil seals listed in the "Parts lnformation" section of this Service Bulletin.
b. If road test reveals normal transmission operation, remove the transmission assembly from
the vehicle and replace the transfer case off seals as described in the appropriate Workshop Manual.
IMPORTANT The new transfer case oil seals must be installed squarely into the transmission or leakage will occur. Before reinstalling the transfer case rear cover, install a layer of black electrician's tape around the front output shaft, from the splines to the end of the shaft. This will protect the oil seals from becoming damaged. Remove this tape after reinstalling the transfer case rear cover.
PARTS INFORMATION
8-94422-392-0 Transfer Case Oil Seal Quantity required 2
FLAT RATE MANUAL UPDATE INFORMATION

This is a bit more than I was willing to bite off. MLClark a moderator on the 4x4Wire suggested installing a breather. I liked that idea a whole lot more so I did. (I've only been dealing with this oil problem for 10 years, no I don't procrastinate)

Of course, when you go to the auto parts store and attempt to explain what your trying to do, only one thing will happen - empty stares and lotsa head scratchin'. They (Car Quest) told me to go ask this Honda dude about what I would need for this. The old guy didn't laugh me off the lot, but I could tell he thought I was psychotic. Anyway, he gave me a plastic vent cap from an old Honda trannie and told me what parts would work.

So, to minimize the pain: Ask for: approximately 18" of 1/4 ID heater hose and a 1/4 NPT pipe thread fitting with a barbed or smooth nipple for the hose along with a hose clamp for the hose.

The procedures is this:

Pull the rubber shifter boot up from the floor. Using a 12mm socket (a universal will help as well) remove the four (4) bolts that hold the shifter assembly to the transmission. You can remove the shifter first (three bolts) but it is not necessary.

Be exceptionally careful during these procedures as debris can fall down into the transmission and this would truly ruin the day. There is most likely a ton of goopy sludge all around this area so be ready.

Ease the shifter assembly up through the floor. On the front of the shifter assembly is the switch for the reverse light. Turn the switch out being mindful of the wires.

Most likely, on the right rear shifter assembly bolt, there might be a wire keeper, don't lose it. I almost did and had a heart attack, worried it fell into the trannie.........whew!

Directly below the main shifter boot is a smaller 'dust boot' covering the shifter and flange. If this boot (or the main one is trashed, this is the time to replace it. St. Charles carries replacements). Don't let anyone tell you different, the dust boot will slide down over the shifter, just use some grease or WD40. You'll just have to remove the main boot first.

This picture shows the new 'dust boot' installed. The stamped steel flange that holds the shifter to the housing is directly below the boot.

Looking at the front of the shifter assembly, to the right of the reverse-light switch opening, is a nylon plug. Remove the plug with a pair of pliers (it's simply pushed in, friction fitting) and it should come out fairly easily. Inside the tunnel is a spring and a ball bearing. Take these out and set aside.

Using the appropriate size drill (size escapes me now, but at lease one to two sizes less than the threads on the pipe threaded nipple) drill out the hole you just opened. It's already almost the exact size, it just needs a quick run down.

Looking at the housing from this angle, the nylon plug is to the left of the reverse-light switch. The huge hole to the left is my half-baked attempt at fixing the pressure problem earlier......it didn't work so don't try this at home

Once drilled out, you'll need to tap the opening for the nipple using a pipe thread tap. I bought mine at Lowe's and it was spendy, much more than a standard tap.

The inner part of the opening is offset because of the ball bearing. Tap down until it bottoms out at this offset (or until you start to get nervous your going too deep, hehehe). USE CAUTION HERE - tap the opening from the bottom to let the shavings fall out to avoid debris getting into the shifter assembly.

A vise/clamp is very handy during this operation.

If you look at the bottom of the assembly, you'll see that the shifter stem pushes against the 'pistons' as you shift gears. These pistions push against springs that give the shifter its tension. You don't want to get shavings in these piston tunnels.

The next step involves the meat of the matter. Using at least a 3/16 drill, you'll need to drill into the tunnel from the bottom side of the assembly where you just tapped. This will be the actual breather hole. On the bottom side of the shifter assembly, you can see the form of the tunnel.

Near the front end and butted next to the tunnel is a stamp mark that looks like a plug. This is where I drilled at a slight angle into the tunnel. Same thing applies, drill so that any shavings fall out and away from the assembly.

Once this hole is drilled and entry into the tunnel verified (blowing air through the tunnel so you can feel the air coming out of the new hole, time for reassembly.

Once all drilling and tapping is done, that the spring and bearing previously set aside should be reinstalled before installing the new pipe threaded nipple.

Install the pipe threaded nipple, install the hose and clamp and your good to go. I used some teflon tape on the pipe thread to help keep it from leaking and also to keep it from working loose. Reinstall the assembly in reverse order, except that you may want to use locktite on the bolts. The assembly will go back in with the new vent/line attached, it does take some fishing though.

My brother-in-law, who helped me with this fix, ran the vent line up along the trannie bell housing and attached it to the firewall to one of the other wire wraps already there, just towards the middle next to the brake master cylinder. I used the vent cap the Honda dude gave me so you'll need to either cap the line or bend it over to keep debris from getting into the line.

Since this fix, I've no longer had any issues with oil overflow and am able to take on road trips with a lot more confidence. (Sounds like a TV commercial ;-) )

I have only a couple of pics from an earlier fix of the assembly, I'll post 'em on the forum.

Hope this helps.

Cheers,
Don B. (LTC Don)