Is it the transmission?

Without an engine operating as it was designed to operate, the transmission will not operate and shift smooth at the correct time and give the powertrain that seamless transition of power from start through cruise speed. Just recently I was called to a shop to analyze an automatic shifting problem on a diesel powered vehicle. I was told the transmission wouldn’t shift out of second gear. There were even some diesel fuel injection pump codes stored, along with some codes stored in the automatic transmission module. This one ended up being a fuel delivery problem, all the while, the shop was working on the transmission in an attempt to fix the shifting problem. So, how does a technician go about telling the difference between a transmission shifting problem, or an engine power supply problem?

Many times these kinds of transmission problems can be analyzed by using a scan tool and checking for any information that has been stored in the modules. At the start of any diagnostic process, the technician should be on an information gathering quest. I always like to start with the engine module to see if there are any diagnostic trouble codes stored. If there are, I will make a note of them and move on. The next stop will be to take a peek into the transmission control module. If there is any information stored here, take some notes and start putting a plan together. Problem analysis always starts with gathering information from many sources, then that information is put together in a logical progression and a diagnostic direction is started.

The starting point of the problem analysis is to determine whether the transmission shifting problem lies with the engine or the transmission. Many years ago, I was told, “the art of problem analysis is to get the problem to come to you.” To accomplish this task, I will start by using a scan tool and watching engine data. This data will include watching the loop status, engine coolant temperature, Long Term Fuel Trim (LTFT) and Short Term Fuel Trim (STFT), and oxygen sensor activity from both the front and rear oxygen sensors. The vehicle will be test driven on a route where the vehicle can be accelerated from a slow speed to 60 mph at full throttle, then driven at a steady cruise for about a quarter mile. By driving the vehicle in these conditions the technician can get a feel for and capture data for engine operation in most any driving condition. The technician can also get a feel for the transmission shifting points and get a good seat of the pants feel for the

powertrain operation and with the captured engine data a determination can be made for the next step in the analysis procedure. Keep in mind, if the engine does not operate properly, the automatic transmission will not perform properly, so the engine operation is always the starting point.

To help explain this analysis process a little easier, there is a 1999 Chevrolet K1500 Suburban sitting out in the parking lot. The vehicle is powered with the trusty 5.7 Vin R small block engine, has the power running through a 4L60 automatic transmission and is showing 188,000 miles on the odometer. The transmission had been rebuilt about 12,000 miles ago. The vehicle owner had taken the vehicle back to the transmission shop to have a transmission shifting complaint analyzed. The transmission shop did all they could do and determined the transmission did not shift properly into fourth gear and the torque converter clutch would not lock.

I went out to the vehicle with the ignition keys, a scan tool and a note pad. The scan tool was used to access any diagnostic trouble codes stored, and the only code I found stored was a DTC P1345 (CMP/CKP correlation).  With the engine running, I noticed the idle was not smooth and the distributor offset data showed a value of -25 degrees.  Engine data was captured for about seven minutes with the engine operated at several different engine speeds. The scan tool data shows the engine did not go into closed loop. Looking at the engine coolant temperature data found one reason for the open loop condition - the engine coolant temperature was reporting around 60 degrees engine coolant temperature. Closed loop strategy on this vehicle needs to see the engine coolant temperature somewhere above 130 degrees for closed loop operation. Without closed loop operation, the torque converter clutch will not go to its lockup state. Hey, this is getting easy, but wait, I think a little more testing needs to be done before the vehicle is brought into the shop.

The vehicle was taken out on the highway for a quick test drive to monitor the misfire data. The engine ran a little rough and the transmission would not shift into fourth gear unless the vehicle was on level ground, or going down a slight downgrade. Capturing misfire data turned out to be a waste of time, since an engine running in open loop does not run its misfire diagnostics very well. There is another way to find out what is causing the misfires. I could get out the ignition scope and hook up to the secondary ignition system and spend some time there, but with the distributor offset of -25 degrees, I first need to start with setting the distributor to its proper adjustment of zero degrees offset. I also know the engine coolant temperature sensor is only reporting around 60 degrees engine temperature. A quick shot at the engine with an infrared temperature gun verified the engine was running about 180 degrees, which tells me there is either a misreporting engine coolant temperature sensor or a problem in the engine coolant temperature sensor circuit. A quick test with an ohm meter of the engine coolant temperature sensor verified the sensor was not reporting properly. A new engine coolant temperature sensor was installed and the vehicle test driven.

With a new engine coolant temperature sensor installed, the engine coolant temperature data showed correct on the scan tool and the engine went to closed loop a few seconds after startup. The distributor was also removed and installed properly and the distributor offset adjusted to zero degrees. Now its time to get on the road and see what the misfire counters will do. By watching the fuel trim data and misfire counters, the technician can do a pretty good job of determining the cause of the misfire and where the next step of the problem analysis will take them.

Out on the road, the scan tool misfire data shows a lot of misfires on cylinders two, five and seven, with most of the misfires on cylinder five. By watching the misfire data, I can rule out a few things. Since the misfires are on both engine banks and misfires happen at all engine speeds and loads, an exhaust restriction on one cylinder bank can be ruled out. Some of the misfiring cylinders will only misfire with the engine under load. This directs me to a secondary ignition problem. In a case like this, a quick way to get the problem to come to you is to fire up an aqua scope (spray bottle of soapy water) and give the secondary ignition wires and distributor a quick bath. This quick test found two ignition wires lying close enough to the exhaust manifold to have some charred insulation.  With a new set of ignition wires installed, the engine runs nice and smooth. Now that the misfires are gone and the engine runs in closed loop, the vehicle can be road tested to see if the repairs to the engine have fixed the transmission shifting complaint, or to see if a real transmission problem will pop up. 

Even at full throttle, the engine ran smooth, with no misfires logged on the scan tool. Why should the technician take the time to use a scan tool to verify a fixed problem, instead just taking a test drive and using the seat of your pants to verify the fix? How many times have you seen misfire codes on a vehicle and you can’t feel any misfire? We all know that a misfire is easier to feel on a four-cylinder engine than on a V8 or a V10, but just because you can’t feel the misfires doesn’t mean the scanner can’t. We all know of cases where misfires were being logged and there were no misfires, and in a case like this there is a chance the powertrain control module will not allow the transmission to shift properly. I call this being thorough in your testing. After all, this is what we as technicians get paid for - fixing the cars right the first time. 

In this case, the misfire problem was fixed with a set of new spark plug wires and by setting the distributor to its correct position. The engine coolant temperature sensor has fixed the engine temperature problem, so the engine is now running in closed loop. With these conditions met, the transmission now shifts properly and by using the scan tool to log both the engine data and the transmission data, I can verify this problem is fixed. The scan data shows the transmission shifting nice and crisp, the torque converter clutch locking nice and smooth and the torque converter clutch not slipping when the engine is put under load. By using both the seat of the pants and the scan tool to verify the repair, I know this vehicle will not be back with this transmission shifting complaint.