C6 Diagnostic Tips

In last month’s issue we discussed the origin of the C6 transmission and how this unit found its way from the automotive world into the ground support equipment industry. We talked about how versatile the C6 transmission is and how it can be adapted for use in many different applications. Now let’s talk about diagnosis.

The only thing worse than removing a failed transmission from a vehicle, is installing a different one, only to find out that it has the same problem as the transmission just pulled out. This is always a hard one to explain, especially to the person paying the bills. Luckily, most C6 transmission concerns can be easily diagnosed with basic equipment, such as a pressure gauge, vacuum gauge and a handheld vacuum pump. All of these are available at reasonable prices at most local parts houses. The following are some of the common complaints with transmission issues and how to diagnose them.

Common problems dealt with on a daily basis include:

• The vehicle feels like it has a lack power on take off from a stop.
• A transmission will not shift out of low gear.
• The transmission will shift through the gears, but will not downshift back to low gear when coming to a stop.
• It skips a gear.
• There is a delayed transmission engagement or it will not engage forward or in reverse.
• The transmission slips in forward and reverse and a whining noise comes from the front pump or torque converter area.

Most industrial transmissions are set up with a parking brake attached to the back of the unit. These brake assemblies have been known to come out of adjustment which can cause the brake to drag. We have seen transmissions removed from the vehicle with the complaint that the trans engages well, but pulls hard on acceleration, as if it were pulling a heavy load but there was no load hooked to the back of the vehicle. Once again, the transmission is inspected and no problem is found that would cause this condition. Upon further review, we find the parking brake has been dragging so much it turned the brake drum blue from heat. So check the brake adjustment.

That’s how simple it can be. Now let’s stop for a moment and talk briefly about C6 101. Before we can dive further into diagnosis, we need to know a little bit about how the system works.

The first thing to be aware of is the type of engine powering the vehicle. This will determine what type of vacuum and/or mechanical device is being used to control modulation. Recalling the previous article the C6 transmission uses engine intake manifold vacuum, which on a diesel is usually a cable operated mechanism to tell the transmission what type of load or how much throttle is being used. There is no need to get into the specifics of the hydraulic circuits on this unit and how engine vacuum and the cable mechanism affect these hydraulic circuits, but it is important to know the basics.

Here are the two important things to remember: No. 1, the higher the engine vacuum, the lower the line or modulation pressure will be and conversely, the lower the engine vacuum, the higher the line or modulation pressure will be. No. 2, As most technicians already know, when an engine is at idle (no load), the intake manifold vacuum is high, which on an engine in good running condition will be around 15 to 18 hg (inches) of vacuum. Conversely, when into the throttle or under a load, the vacuum is low.

Concerns to keep in mind
How does this tie into the transmission? To fully understand this, it’s helpful to know some basic hydraulic and hydraulic valve theories and what has to take place to make the transmission shift. In the hydraulic diagram (see figure No.1) there are two shift valves, 1-2 shift and 2-3 shift, as well as color-coded oil circuits. The red and orange oil represent main line pressure and modulation oil, which are each going to shift valves. This oil is keeping these valves from stroking. In order for these valves to stroke, the blue oil circuit, which is governor feed oil, has to overcome the mainline and modulation pressure to make the valve stroke. The governor feed oil is produced by road speed or the vehicle moving. When governor feed oil gets high enough, it will override the opposing oil and the 1-2 shift valves will stroke, thus making the transmission shift from first to second. As road speed increases, so does governor feed pressure.