Keep Those Trannys Rolling - November - 2022

Hyundai’s A4CF2 No Forward Engagement

Late-model vehicles can present unique problems. With electronics controlling so many functions inside transmissions, we are forced to verify solenoids, sensors, and other seemingly unrelated systems to ensure we don’t miss a serviceable component or an external source for a transmission problem. In this issue of “Keep Those Trannys Rolling,” we will look at a 2009 Hyundai Elantra with a no-forward engagement concern.

THE PROBLEM

The vehicle came to the shop on a tow truck because the customer experienced a no-forward engagement condition. However, reverse was okay. After verifying that the fluid level and condition were good, we contacted the customer to discuss their concern further. He said the no-forward engagement problem started a couple of weeks ago, but he could get it to engage by moving the shifter lever around. However, this time, it would not engage forward, but reverse was always okay. With that added information, it was time to look at this little Elantra.

INITIAL INSPECTION

This 2009 Hyundai Elantra is equipped with an A4CF2 transmission behind a 2.0L engine. Our initial vehicle inspection started with a general walk-around, noting any lighting concerns and possible vehicle damage repair areas. All checked out well. Next, we checked under the hood, inspecting the battery, wiring harnesses, connections, and all fluid levels. After finding no issues, we pushed the vehicle into the shop and raised it on the rack so we could inspect the underside of the car. Again, we verified the vehicle did not have any apparent areas to check for mechanical or electrical issues. Now, it was time to dig in and diagnose this concern.

DUPLICATE TO DIAGNOSE

Usually, this would be when you would test-drive the vehicle and duplicate the customer’s concern. But in this case, the problem was obvious. We connected our scan tool and checked for diagnostic trouble codes in all modules but found none. Next, we started the vehicle and immediately noticed that the reverse indicator in the PRNDL display was lit. Next, we monitored the transmission PIDs to see if anything was out of the ordinary. While monitoring the transmission range switch (TRS) data, we observed that the PID always indicated the reverse position no matter what was selected. With the transmission range switch operation in question, it was time to focus on the circuit and everything associated with it.

TRANSMISSION RANGE SWITCH CIRCUIT DIAGNOSIS

Now we knew that we had a Transmission Range Switch-related problem to solve:

  1. We started with a physical inspection of the harness, connectors, and switch. We performed a resistance test on the transmission range switch in each position (figure 1) and confirmed that it was working properly.
  2. We load-tested each transmission range switch to TCM circuits and verified that they could carry the signal to the TCM.
  3. The last diagnostic routine step was checking the TCM powers and grounds.

If they showed good, the OEM diagnostic tree recommended replacing the TCM.

During the TCM power and ground checks, we noticed that the TCM reverse input circuit (TCM pin #7) always had 12 volts. So, we disconnected the transmission range switch while monitoring the reverse input circuit at the TCM. The reverse input circuit continued to have 12 volts on the circuit. We disconnected the TCM connector and rechecked for voltage at the TCM connector pin #7 terminal. There were still 12 volts on the circuit, indicating that the reverse input circuit was shorted to power somewhere within the vehicle harness. After identifying a short-to-power concern, it was time to take a closer look at the TCM’s reverse input circuit.

A CLOSER LOOK AT THE TCM’S REVERSE INPUT CIRCUIT

With a possible short to power in the TCM reverse input circuit, it was time to take a closer look at a wiring schematic. After reviewing the transmission system wiring schematic (figure 2), it was apparent that the reverse input circuit was also tied into the backup lights circuit (similar to Toyota). After verifying that the TCM reverse input and backup lighting systems share the same circuits, it was time to check the backup lighting system.

CHECKING THE BACKUP LIGHTING CIRCUIT

We started our inspection with the backup lighting system wiring schematic in hand (figure 3). A visual check of the backup lights indicated they were working correctly. First, however, we needed to verify by backprobing the circuits with a voltmeter. We looked for 0.1 volts or less at each connection using the voltage drop method.

We opened the rear hatch and removed the interior combination lamp covers, and checked the backup lamp ground (GF51), located in the left C pillar. We checked the backup lamp sockets and bulbs for possible concerns. Everything looked good. During the backup lighting circuit inspection, we found a lone wire soldered, and heat shrunk into the right backup light circuit. According to the backup lighting circuit wiring schematic, there was not supposed to be anything tied into the right backup lamp circuit. Now, this was a mystery!

MYSTERY AND PROBLEM SOLVED

We traced the lone wire that was tied into the right backup light circuit to an after-market rear-view camera system installed on the rear license plate (figure 4). At this point, the module associated with the rear-view camera malfunctioned, producing a 12- volt feed directly into the harness.

We disconnected the rear-view camera system and rechecked the PRNDL display on the dash. Now everything was working correctly, and the “R” indicator was only lit when the shifter was in reverse. Next, we rechecked the transmission operation; lo and behold; we had forward engagement!

We contacted the customer and informed him of his rear-view camera concern. He said he didn’t even know he had a rear-view camera, so it was left disconnected.

Well, there you have it. With a bit of understanding of the Hyundai reverse input circuitry and a couple of wiring schematics, you, too, should have no problem keeping those trannys rolling down the road!