We're the original owners...and each winter we see a minor drop in mileage but we've been cold here for months...just in the past two weeks the mileage has suddenly dropped dramatically from the usual 16-17mpg around town to barely getting 10 mpg. Temp is doing fine...there doesn't seem to be any problem there... temp comes up and rides at 195 or so which is what the new thermostat is when the engine was installed 6K miles ago... Thanks.

 

Reference vacuum transfer/actuator switch...I thought this year (1997) had an electric transfer??!! I'll check on this...great post.

 

You're correct, 1997 has electric shift. When you select 4WD, the Transfer Case Control Module, (TCCM) located behind the passenger side kick panel, commands the encoder motor, (driver side of transfer case) to shift. As the linkage inside the transfer case moves, it activates the vacuum switch, also on the driver side of the transfer case. The vacuum switch supplies vacuum to the axle actuator, (under the battery tray). The actuator pulls a cable and engages the right front axle with the front differential. When the axle is fully engaged, a switch in the axle tube is activated and sends a signal to the TCCM telling it the axle is engaged, the TCCM turns on the 4WD light and you have 4WD.

The problem with the vacuum switch is that it gets stuck between 2WD & 4WD which causes it to leak vacuum past the seal and axle actuator engages the axle. The transfer case remains in 2WD, and the 4WD light will not come on, but the gears turning in the front differential adds drag and will decrease fuel mileage.

The vacuum switch can also leak vacuum into the transfer case. When that happens, the fluid is under negative pressure, (vacuum) and will suck transmission fluid past the transmission rear seal and the input shaft seal of the transfer case. This causes the transmission fluid level to decrease and the transfer case fluid level to increase. Eventually the fluid gets sucked into the 4WD vacuum lines and the axle actuator. 4WD & HVAC vacuum lines are shared from the same source. All of the vacuum lines, vacuum reservoir, check valve, blend doors, actuators and the mode switch can be contaminated with fluid and must be cleaned out. You might get lucky and not have to change the transmission and transfer case seals after the root cause is repaired, (the vacuum switch) but everything else must be cleaned out regardless.

 

Interesting. So how would you know if this is happening? I guess only if you start getting 4WD mileage suddenly? The few times I had to use 4WD last winter, I was really surprised at how much gas it sucked down. Makes sense and all, but it was surprising. My sisters 93 Bravada with full time AWD seems to get about the same 17mpg no matter what. I sort of wish I had her drivetrain. That thing goes like a mountain goat through anything.

 

If the spring loaded ball on the vacuum switch gets stuck in, it will constantly apply vacuum to the axle actuator engaging the front axle at all times. It will not actually be in 4WD due to the transfer case is not supplying power to the front drive shaft. Fuel mileage will not suffer as bad as it does in 4WD. If vacuum leaks past the internal seal of the switch, it will suck fluid into the vacuum lines.


An easy test is to install a vacuum Tee in the axle actuator vacuum line. Connect a vacuum guage to the Tee.* Start the engine and check for the presence of vacuum on the guage. When 2WD is selected, there should be zero vacuum. When 4WD is selected, manifold vacuum should immediately be present. Select 2WD and vacuum should immediately drop to zero. If changes in vacuum are slow, the switch ball could be binding, there could be a kink or restriction in one or more of the lines, or there's fluid in the lines from a leaky vacuum switch.


*The axle actuator must be connected during the test. If it's not connected, the axle engagement switch will not get tripped when 4WD is selected, and the TCCM will shift the transfer case back to 2WD making your test results worthless.


The 1993 Bravada uses a different 4WD system. The transfer case is a single speed design, (no low range) and there's a clutch pack that engages the front differential when slip is detected at the rear wheels. The system is not equipped with an axle actuator, the front axles are engaged at all times.

 

Excellent post...I'll check this out and really appreciate your thoroughness!!

Incidentally, I revisited the EGR valve issue because I discovered it was NOT replaced when they put in the new engine...I took it out and noted it was loaded with carbon and the pintel was sluggish... After cleaning it and the corresponding manifold ports the mileage has jumped back from 10 mpg to 15 - 16 mpg...which is good but on the highway we used to 20-22mpg... So seeing as a cleaning the EGR mad so much difference, I installed a new EGR...time will tell if that makes even more difference. After cleaning the old EGR, the pintel was still sluggish as compared to the new one.

Next I will follow up on the 4wd issue because I suspect this still is something that needs attention...slipping into partial engagement can not be good so even if it isn't the root cause of the poor fuel economy it will obviously result in damage and if it does engage, even partially, any part of the drive line then as you indicated it will impact mpg. Again...many thanks.

 

Most auto parts stores carry a replacement gasket for the EGR valve that has a screen built into it. The carbon chunks are broken into small pieces, (powder in some cases) as they pass through the screen, before they reach the actual valve, (pintle). If you install one of these, your carbon chunk issues are a thing of the past.



Let us know what happens with the 4WD.

 

unfortunately installing the new EGR vavle did not further improve the MPG...in fact it continues to flucuate between 13-15mpg when it used to and should be getting much better.
I'm now jacking all four wheels off the ground and going after the 4x4 test as well as revisiting all the vacuum lines and actuators...

Where is the best place to get access to the vacuum line to test for 0 vacuum in 2x4 and manifold vacuum in 4x4 mode?

 

So...as I start into investigating whether the 4x4 has anything to do with the sudden loss of mpg...I now have the truck fully on jackstands... Initially when started and put into drive...only the rear wheels turned...now after engaging and disengaging the 4x4 several times, and stopping / starting the wheels between shifts, the front wheels engage all the time whether the vehicle is in 2 or 4 wheel drive...hence I think I might be onto something...???

Still want to check the actuator vacuum line and the actuator valve and will now go looking for them.

 

The vacuum line for the actuator gets vacuum from one of the T's near the driver hood hinge/master cylinder area. IIRC on '97, it's a black plastic line, ~3/16" in diameter, with rubber adapters on the ends. I don't recall the exact route it takes, and it really doesn't matter where you install a T for checking, as long as it's in the line to the actuator. From your initial tests, it sounds like the vacuum is not being vented to atmosphere when you switch back to 2WD. It should vent immediately when you switch from 4WD to 2WD, and apply vacuum immediately when you switch from 2WD to 4WD. If the vacuum switch sticks or binds, or if transmission fluid has migrated into the vacuum system, there could be a delay, or it may not switch the vacuum at all.