Torque Converter Tech Tips
- Subject: Converter shudder after radiator replacement and transmission overhaul
- Unit: RE5R05A
- Vehicle Application: 2000 Nissan Pathfinder
- Essential Reading: Rebuilder, Diagnostician
- Author: Gary Carne
This vehicle is a 108,261-mile 2000 Nissan Pathfinder with pink ATF. Sound familiar? Yep, the radiator had failed. We had ATF in the radiator and anti-freeze in the ATF – typical Nissan repair these days. Seems like 20 years ago we had the same issues.
Initial repair: Replaced radiator and overhauled the transmission with new frictions, new plastic, new valve body with TCM and a new converter, hot-flushed the external cooler and lines, filled it with Nissan ATF and went to Nissan to have it flashed. The initial test drive was flawless. We should have known something was up at this point. We gave the vehicle back to the customer, confident that everything was done properly.
Repair #2: Customer brought the vehicle back about two weeks later with the complaint that the vehicle would shake while being driven around town. During the test drive, we noticed a severe torque-converter shudder after about four to five miles – I mean, a really bad shudder at about 40 mph up to 70 mph. Release the throttle and it would stop, reapply the throttle and the shudder would return just as severely. This has got to be a bad converter, right? I mean, we put the whole ball of wax into this unit. Cooler flow checked perfect with the Hot Flush. So we replaced the converter and the problem went away. No shudder at all. It’s FIXED!
Repair #3: Customer brought the vehicle back about two weeks later with the complaint that it was still shaking while being driven around town, but now it would shake on the highway, too. Oh, boy, here we go again! Another test drive, and again we noticed a severe torque-converter shudder after about four to five miles – just like before! No way!
OK, let’s get it fixed. Check the cooler flow: two quarts in about 16 seconds; OK, good. Replace the torque converter and the valve body with TCM? This has got to be the problem, right? So again we replaced the torque converter, but this time we replaced the valve body with the TCM. Go on a test drive; this time we drove for about 50 miles. No problems. No shudder! It’s got to be fixed this time – I mean, we drove the stuffing out of this Nissan AND we took it back to the dealer to be flashed again. Yeah, it’s fixed.
Repair #4: Customer brought the vehicle back about 10 days later with the complaint that it was still shaking while being driven around town and on the highway. OK, now this really does not make any sense at all. Three converters and two valve bodies/TCMs, Nissan fluid every time. Cooler flow was fine.
So my question to the guys at this shop was: “Be specific, guys; what does it do? Is this an engine problem or something else?”
“No, we are sure this is a converter shudder,” was the response.
OK! So I went back up to this shop and drove the car; sure enough, it was a torque-converter shudder. No question about it! So we added a bottle of converter-shudder cure to the vehicle. “It’s fixed,” was the response, but no one at the shop was really convinced! Everybody had that sheepish grin on their faces like: “I sure hope it’s fixed …” Back to the customer after another 50-mile test drive with no problems at all.
Repair #5: Customer brought the vehicle back about five days later with the complaint that the vehicle was still shaking while being driven both around town and on the highway. “Can you fix my car?” was the lady’s question. “It worked for about two days, then it came back again. Why?”
So, guys, how do you answer this? Wow! Were we stumped. Again, I traveled to this shop to see whether I could help. This is a really good shop with few if any problems; they do things the right way, they pay attention and they REALLY try to get things fixed. BUT, this one wasn’t fixed.
The Real FIX – Repair #6: The first thing we did was replace all the fluid using a different type of synthetic fluid to see whether maybe we had some bad fluid from Nissan. No luck; the vehicle started to shudder after about 35 miles. After running several tests, including line pressure at all available ports, cooler flow and cooler pressure, I made several calls and e-mailed the guys at Sonnax for help.
During my discussions with Ed Lee and Bob Warnke, Ed suggested we bypass the radiator cooler.
“It’s brand new,” I said.
“Simply bypass it to see if there was any difference,” was Ed’s response.
So simple and so easy! The results: 90% of the shudder that had existed only two hours earlier was gone. It had a shudder only at about 45 mph, and only a very slight shudder at that. Two different people drove this to verify. “Sure, sure, it’ll be fixed for about five days, and then it’ll be back,” was the resounding chorus! So, this time we did the logical repair based on this test: We bypassed the radiator cooler for the transmission and installed a large external cooler from Tru-Cool. No radiator, no air-to-oil cooler from Nissan, just a simple external cooler. YES, it IS fixed!
Five days later we talked with the lady and she said, “It works fine. No problems.” Fifteen days later we talked with the lady again and she said, “It works great! THANKS!”
Yes, we finally fixed this problem and this customer is happy. What a great day! Moral of the story: Never assume! Yeah, I can see all of you out there smiling at this one. Never assume that a new part is ALWAYS good. In this case, this brand-new radiator caused a whole bunch of heartache for this shop.
Why did the radiator cause this?
This is a three-path converter. The converter clutch is a fixed-cover multiplate, similar to the Mercedes 722.6, in which application pressure is fed through the turbine shaft. It functions similarly to an input clutch on a transmission.
The other paths are converter in and out. The out passes over the converter lubrication valve and onto the clutch-control valve. The clutch-control valve is stroked into the apply position by the TCC solenoid. The speed at which the valve strokes is controlled by the varied spool diameters and the amount of pressure that is applied to the spools. When the cooler circuit becomes restricted, the pressure in the circuit rises. Because of the variance in spool area and the rise in pressure on this area, the travel of either valve may be affected. One way to isolate this valve movement is to check cooler flow. If either valve is cycling or pressure has pushed the valve into a stall position, flow will cycle or shut down. In this instance, the new radiator/cooler provided enough backpressure to affect valve travel and converter application.
Special thanks to Stuart Mays, Jamie Wise and Todd Edwards for their hard work and contribution to this article.
Gary Carne is technical director of Coleman-Taylor Transmissions, Memphis, Tenn.