GM All Wheel Drive Not Working Check Engine Code C0407 How to Fix

Here we have a 2nd Generation Buick LaCrosse with a service rear axle message on the dash, that's what we're gonna talk about in this video. We scan tested the vehicle, and in the module that controls the rear differential clutch, we found that we had a code C0407 with a symptom 64. What this code means specifically is the all-wheel-drive oil pump is slipping. When you get this code, your all-wheel-drive system becomes disabled, and you get the warning light on the dash. If you only have a two-wheel-drive Buick LaCrosse, you only have these two drive shafts or axle shafts powering the front wheels. When the all-wheel-drive system is disabled, then those are the only tires that are powering the vehicle. In this vehicle, we have the transfer case right here and it goes to the drive shaft that goes to the rear of the vehicle. And then you have the input to the rear differential assembly, there is clutches in here, and then the rear differential right here, and then the axles that power the rear wheels.

There's two different sections in this rear differential. There's the ring gear and pinion that are internal in this section and then the clutches are in this section, and there's a seal in between. Here's where the module is this is the brain that controls whether the rear axle is engaged or disengaged, and it sends a signal to this pump, activates the pump when it wants to compress the clutches, and put power to the rear wheels. For the rear differential side of it, the fill plug is located right here and a drain right there. If you were gonna service this, you wanna take the fill plug out, drain the fluid, put the drain plug back in, and then fill it up. Now, that's separate from where the clutch fluid is upfront and there is a level plug right there. That's not where you fill the fluid, but that's where you check the level. To fill the fluid, you actually have to take one of the hoses off, the vent hoses. Because there's all these valleys and other areas inside here, if you just fill the fluid in there, it's not gonna get to everywhere.

What this code is referring to is when the pump right here is trying to pump fluid to engage those clutches, it's unable to engage those clutches. And now it could be something wrong with the pump, or it could be the fluid level is low. Now, we wanna check the fluid levels, we're gonna start with the rear differential fluid level. Pull this fill plug out and check and see what the level is. If when you pull this fill plug out if the fluid is low, you wanna top off that fluid. If you pull this fill plug out and the fluid level is fine, then you can put it back on and move ahead. If you pull this out and oil comes out, gear oil, then most likely the cause is there's a seal in between where the clutch area is and the rear differential, and fluid has pushed its way through that seal and entered the rear differential. In our case, that's exactly what happened. So, what we did was drain the fluid out, put the drain plug back in, and put fresh fluid in. So, now the fluid level's good in the backside, which realistically has nothing to do with the code. Now we need to check the fluid in the clutch side.

So, we know the fluid level was low in the clutch pack area, so we topped it off with the appropriate fluid, the fluid in the two differentials is different. We're gonna start the engine and let it idle for five minutes in park. And then we're gonna push on the accelerator to wide-open throttle five times. And that's gonna purge the air out of the pump and then we can shut the vehicle off and check the level. Now I'm gonna take this level plug out, just use an 8-millimeter Allen, loosen this up. I'm just gonna have a drain bucket underneath just in case any fluid comes out, and pull that out. Now, we want the fluid right up to that level. So, I need to add a little bit more fluid. So, before I add the fluid, I have to add it to the vent. I am just gonna put this fill plug back in or level plug. Now I'll add some to the vent and then check the level again. And take a piece of rubber hose and just install it where that vent hose goes, that way it's a little bit easier for me to get fluid into there.

Now I'm gonna take a fluid gun, fluid transfer gun, and take the appropriate clutch fluid. Now I'm gonna attach this to that hose. That's pretty good. And this is how you install the fluid. And the reason why you do this is because there's different valleys in here that the fluids not gonna get to if you just install the fluid through the fill hole. So, we're going to do this until the fluid starts coming out the fill hole, right about there. And take the vent hose and reinstall it on the fitting. All right, so I filled it up. Now I'll just pull this level plug out, you can just see, and the fluid's right up to the bottom of that level. So that's exactly where we want it. So, I can put this back in and torque it down and reattach the vent hose. Now our fluids are good in the rear differential and in the clutch area. Now, if the fluid was low in the clutch area, there's only a couple of reasons why the fluid could be low. Either there's an external leak, which we would see, that seal is bad where the fluids going back into the rear diff, or someone serviced this and did not put the appropriate amount of fluid in.

Now, we're gonna drive this car round like this and see how it does. If fluid continues to go into the rear differential, we're gonna have to do something about that seal. But there is 100,000 miles on this car, that fluid could have seeped back there slowly and it might not give us a problem. So, we're gonna drive this vehicle around for a couple of 1,000 miles and recheck that fluid level. We may have to replace that seal or it may be okay. If you enjoyed this video or it helped you out, make sure you subscribe to this channel, ring the bell, turn on all notifications so you don't miss any of our videos.