Let’s complicate a transmission diagnosis by adding a few extra codes on a 2010 Jeep Wrangler Sport with a 42RLE automatic transmission stuck in 2nd gear. Very briefly, the 2010 Wrangler’s transmission solenoids are powered through its totally integrated power module (TIPM), which is a fuse and relay box with a logic board that controls most body electrical functions. The TIPM communicates with various switches and body control modules via CAN/bus and other bus communications systems. With that said, most Chrysler TIPMs vary considerably in complexity, operating strategy and structure. For example, some TIPMs have built-in, non-servciable relays, while our 2010 Jeep Wrangler Sport’s TIPM has many replaceable relays.
A FEW EXTRA PIECES
My first step on any CAN/bus vehicle is to scan the complete powertrain and body control system for codes. A diagnostic link connector (DLC) torn from its mount and left dangling by the wires told me that somebody had been working pretty hard to solve the transmission problem. I retrieved four powertrain, three body control, and two ABS codes, but focused on the P0882 (low voltage, transmission control (TC) relay circuit), P0750 (fault, low/reverse solenoid circuit), and P0706 (gear position rationalization failure). I cleared all codes because, due to previous diagnostic activity, any circuit disconnect with the ignition on can create a false code.
P0882 ENABLE CRITERIA
Key-on, the PCM grounds the transmission control (TC) relay by sending a 12-volt signal to the TIPM via the green C4 connector, pin #18. Voltage to the shift solenoids is supplied by TIPM fuse #J18. The TC relay was mistakenly labeled “PCM relay” on the fuse box cover, which added a few extra pieces to my diagnostic jigsaw puzzle. An aftermarket diagram ascertained that the “PCM relay” is indeed the TC relay. The transmission control module (TCM) is integrated with the PCM. The TCM senses TC voltage supplied by the relay through C4 pins #28 and #38. If the TCM fails to detect voltage at either pin, it sets the P0882 code.
P0750 ENABLE CRITERIA
Pressure switches located in the solenoid assembly monitor low/reverse, 2/4 clutch and overdrive oil circuit engagement. Another part of the service information indicates that the PCM also looks for a voltage spike from each of shift solenoid circuits as they’re activated to confirm engagement. Either or both enable criteria could be accurate, but the S.I. appears a little fuzzy on that point.
If a P0750 is set, first gear and torque converter clutch application will be inhibited. According to service information, The vehicle will launch in 2nd gear and shift normally through the gears without allowing torque converter lockup. If the set condition is no longer present during the same key start, the transmission will return to normal operation.
THE MISSING CODE
But there are some missing pieces to our diagnostic puzzle: “Limp-in will NOT occur unless DTC P0841 is accompanied by a code P0706.” The malfunction indicator light (MIL) will then illuminate after five minutes of “substituted” operation. The problem with the above service information is that I was missing code P0841. In brief, P0706 sets when the gear position indicated by the gearshift PRNDL doesn’t match the gear position relayed to the TCM by the transmission’s gear range sensor. This P0706 occured randomly. My scan tool contains a P0706 rationality test, which the gearshift PRNDL passed. So, at this moment, I wasn’t too concerned about P0706.
THE DIAGNOSTIC DILEMMA
Let’s begin assembling this Jeep puzzle by starting at the corners and working toward the center: 1) P0882 and P0750 are always set after the codes are cleared and the Jeep Wrangler is driven; 2) I’ve never found a combination of the P0841 and P0706 codes in the current or history codes. Although not completely relevant, the service information doesn’t explain why the Jeep stayed in the limp-in mode in the absence of the P0841 code. So, now I’m confused by poorly written or incomplete service information.
TESTING THE TIPM
Is the P0882 code (low voltage to the low/reverse solenoid) caused by a bad TIPM or not? The TIPM can be tested by supplying battery voltage to C4 pin #16, which commands the TIPM to close the TC relay. The TC relay should then supply voltage to the solenoid assembly and to C4 pins #28 and #38. The TIPM passed this test several times, which led me to believe that the low-reverse shift solenoid was developing a shorted circuit and setting the P0750 DTC at some point during the drive cycle.
One way to get a handle on a P0882 low-voltage problem is to test for shorted/open solenoids. At 55º F, the low/reverse (L/R) solenoid measured 2.65 ohms, while the remaining three solenoids measured 1.50 ohms. The L/R resistance looked suspiciously high, but within the specified 1.0 to 3.0 ohm range. Unlike the inexpensive 2/3 solenoid used in the Chevy S-10 transmission, my diagnostic strategy was affected by the Jeep solenoid assembly costing ten times as much and requiring at least three more hours to replace.
Long story short, I had to release the Jeep Wrangler for the Christmas holidays because it could still be driven on short trips in the limp-in mode. Little did I know that the Jeep went to another shop for diagnosis. I didn’t realize this until it returned in the middle of January with a fresh fluid change and a glob of silicone dielectric grease on each PCM/TCM and the transmission connector, not to mention that several fuses and relays had been left loose in the TIPM. So, with the assembled pieces of my jigsaw puzzle tossed to the wind, I began from scratch.
When the Jeep returned in January, I was completely surprised when all four solenoids tested 1.34 ohms, which left me with a bad TIPM. But wait a minute. Let’s bring the transmission up to operating temperature and THEN test the L/R solenoid resistance. That being done, the L/R solenoid resistance tested in the meg-ohm range, which is essentially an open circuit. Surprise, surprise.
THE DIAGNOSTIC SOLUTION
Since this is the dead of winter, my initial resistance readings were taken with the transmission oil temperature somewhere between 0º F and 35º F. As the oil temperature increased, the low/reverse solenoid resistance exceeded the 3.0-ohm limit, so the P0750 solenoid DTC was set, and the transmission went into limp-in mode. As an afterthought, I noticed the P0750 code appeared to set one drive cycle before the P0882, which confirmed that the shift solenoid assembly, rather than the TIPM, was at fault.
In this month’s Diagnostic Dilemma, the OEM service information regarding the P0841/P0750 codes wasn’t perfect. But the P0882 and P0706 codes self-corrected as soon as the new solenoid assembly was installed. The P0882 indicating low voltage at the low-reverse shift valve evidently set as a result of the PCM/TCM disabling the transmission relay, so it was irrelevant to the final outcome. Like assembling a jigsaw puzzle, solving the mystery of the P0706, P0750, P0841 and P0882 code combinations took several attempts before I could make all the pieces fit. In this case, the P0796 and P0841 were extra pieces that I could well afford to ignore.
Article courtesy Underhood Service.