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Diagnosing and replacing a Chrysler 42LE transmission

The first quarter of 2014 proved to be a rather challenging time for the original Allpar 300M. Though it made it to 144,000 miles sans any major component failure, in February the transmission went, followed a couple weeks later by the radiator. Both of these things comprise two crucial systems that will undoubtedly turn any car into an expensive lawn ornament if not dealt with properly.

chrysler 300M

Even amidst the hours of reading and searching through the dozens of forum posts and websites that have accrued from the years of other people encountering similar issues, I still found that I had a lot of questions. I spent two months learning much about the 42LE transmission and I hope that through this article, I can share the things that I’ve learned and hopefully make things a little clearer for anyone who may find themselves facing a transmission transplant.

What happened

I was in Loveland, Colorado visiting my friend James, who had recently acquired a 1999 Chrysler Concorde LXi. Remembering how pivotal Craig Campfield’s help was when I acquired my first LH car, I wanted to “pay it forward” and help James out (to the best of my ability) getting his Concorde up to snuff and running properly — helping make it “look like a club member owns it” as Bob Day says. While running an errand to pick up supplies, my 300M went to shift into third gear and instead, the car began to coast and the tachometer shot up. Moments after the consequential “Oh [expletive deleted]!” there went the warning chime, the check-engine lamp lit — up and I knew I had a transmission problem.

flatbed

I was able to drive the car in first gear and it would and it would shift fine into second. Then, once the tachometer reached the 1,900-2,000 RPM mark OR 18-23 MPH, the engine would break-free and the car would coast as if in neutral. Once vehicle speed dropped back into the second gear range (sub-20mph) the transmission would re-engage.

Given the car would still downshift into first upon deceleration as well as shift up into second gear upon acceleration, it clearly was not stuck in “limp-home” mode. I later discovered that the transmission needed to “break loose” two or three times before the computer would register a problem and engage limp-home mode. Once in limp-home mode, I was able to drive the car up to 25-30mph in second gear. Without limp-home mode, it would continue to try and shift into 3rd and break free into neutral instead, limiting my speed to 18-20mph.

A diagnostic by Best Western Transmission of Fort Collins, Colorado would later find that a snap ring broke in third gear, preventing the transmission from being able to shift into third gear at all.

What I learned

gauge with p0700 error codeThe speed sensors used on the 41TE/42LE Chrysler transmissions are known for often being the culprits behind the “limp-home” mode condition. There have been many references to people who have had cars stuck in limp-home mode easily-fixed with a new pair of sensors, OEM (dealer) sensors at that. (The general consensus is that after-market speed sensors have been met with little luck.) [Editor’s note: as the previous owner of this car, I can attest to that — the speed sensor did indeed fail and resulted in a P0700 code. Teterboro Chrysler-Jeep immediately found and fixed the problem for less than the warranty deductible.]

Since sensor replacement is cheaper than transmission replacement, I went through the ordeal of changing the sensors with new OEM units bought directly from Fort Collins Chrysler. Unfortunately, this did nothing to change the transmission’s state. I finally took it to Best Western Transmission in Fort Collins, who ran a battery of tests. They discovered that the clutch-volume-index readings were tremendously off-spec and when combined with the shifting behavior and P0733 (Incorrect 3rd Gear Ratio) code, indicated a broken snap ring in third gear.

A broken snap ring is repairable in these transmissions, but only through a complete teardown and rebuild of the entire unit. When a snap ring fails, it will most likely take other components with it when the failure occurs, so the safest/most practical option is to do a transmission service and full-rebuild if a total transmission replacement isn’t an option.

What is important

wheelThe second-generation LH platform cars used four different engines (2.7L, 3.2L, 3.5L “Magnum”, 3.5L High-Output/Special 3.5L HO) across the model line-up, but all used the same Chrysler 42LE 4-speed, front-wheel-drive automatic transmission. In vehicles with the 2.7L V6 (Intrepid and Concorde) and the 300M Special, the final drive gear ratio is 3.89:1. Vehicles with the 3.2L, 3.5L “Magnum” and the non-Special 3.5L High-Output have a final drive gear ratio of 3.66:1.

Owners with 3.66:1 final drive ratio cars can upgrade their gearing to the 3.89:1 gears used in the 2.7L and “Special” applications for a bit of a performance boost. Swapping the gears is fairly straight forward and a fun mod to do. It’s worth noting, however, that when changing the final drive gear ratio of your vehicle from the stock setup —the speedometer will not read accurately unless it is recalibrated with a DRBIII scan tool. An alternative to this is swapping the transmission control module for one out of a vehicle that came with your new gearing to correct the wrong readout.

When it comes to the TCM, or transmission control module (the computer “brain” of the unit), earlier model second-generation LH (1998-2001) cars had a stand-alone TCM unit in addition to a separate powertrain control module (PCM) unit. In later second-generation LH cars (2002-2004), Chrysler combined the TCM and PCM into one single unit.

What I did

Since I had to replace the transmission in my 300M regardless, I decided to pick up a low-mileage (21,000 miles) transmission from a 2002 Chrysler Concorde that had the 2.7L motor. That way, I could upgrade to the 3.89:1 final drive gear ratio without having to do any extra work, the new transmission already had them installed. All that was left was to install a TCM out of a 1998-2001 car that had the 2.7L motor in order to correct the consequential speedometer inaccuracy. Since my car is a 2000 with the PCM and TCM as separate modules, a combined TCM/PCM unit out of a 2002-2004 model would not work. [As noted earlier, a dealer could have reprogrammed the existing module if no unit was available.]

transmissionAs far as any differences between the transmissions used on the early models (1998-2001) and the later models (2002-2004), there was some discussion that Chrysler changed the plug interface between them, so that a wiring harness change might be necessary. Further research indicated that the plug change actually occurred prior to 1998 and therefore does not impact any of the second-generation LH cars. Indeed, I was able to put a later 2002 model transmission in my earlier model car without the need to bring over the harness from the donor vehicle.

I bought three gallons of Mopar ATF+4 transmission fluid from Fort Collins Chrysler, along with a new reusable transmission pan gasket Chrysler released as an alternative to the Mopar RTV liquid gasket that is generally used during a pan drop service. I also picked up brand-new clutch plate bolts from the dealer that come pre-coated with blue Loctite along with a new transmission oil filter pack from Mopar.

With the exception of one of my transmission mounts having disintegrated, Best Western Transmission was able to install the 2002 transmission with the upgraded gearing into my 300M with no problems. They drained and replaced the differential fluid with non-synthetic Mopar Hypoid Gear Lubricant (SAE 75W-90).

I’ve put about 1,500 miles on the 300M since the repairs were completed and all seems to be well. I’m still not entirely comfortable using AutoStick, but the few times I’ve used it since, the shifts have been much more forgiving than previous. After all this, I’m convinced that the transmission had a hard mechanical failure that was slowly occurring over time until the snap ring finally gave way.

If you’ve got a P0700 code, it is worth making sure you don’t have a valve body/solenoid pack or speed sensor issue before going ahead with a full replacement. A transmission diagnostic from a reputable shop [one which has the ability to see proprietary, Chrysler-specific codes] is always helpful.

Many thanks to the members of Allpar.com, The Chrysler 300M Enthusiasts Club and LH Owners of America for their help in educating me through this process.

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