Installed the new transmission cooling lines hoping that there was an obstruction that had been causing the fluid to overheat, but that wasn’t the problem - it still overheats. When it gets to about 235-240, the shifting problem becomes active. The engine coolant temp stays relatively steady, around 204-208 degrees. It was 100 degrees here today, so the coolant temp didn’t seem out of line to me. But the tranny temp just kept slowly climbing, same as before. It reached about 244 and did seem to level off at that temp, but the shifting issue popped up around 235 degrees, near as I can tell.

So.....now I guess I’m forced to believe that they did not rebuild my transmission. They removed it, because the engine rear main seal leak was fixed. But I guess they didn’t rebuild it, or didn’t rebuild it completely, or did not replace a key part.

Several questions:

1. Could a defective torque converter cause the fluid to heat up?
2. If so, could that explain why it does not ever seem like the transmission is slipping? Even when the shifting problem is active, it doesn’t seem to be slipping.
3. Is there any scenario that would explain why I have never detected any slip or strange shifting during normal driving, yet would allow the tranny fluid to get too hot?
4. Most Important question: Are all of the XJR transmissions the same? I am thinking about finding a used one before I spend big money AGAIN on a complete rebuild. If I can find a low mileage tranny, it might last me many years. At the very least, it might not overheat the fluid. Would any 98-03 XJR transmission work in my 98?

I’m still contemplating all of this, but I guess I have to believe that they didn’t rebuild my tranny. I will work that angle separately, but I’m not taking it back there for repair. I may take other legal action, but I’m not going to let them touch it again.

Thanks for all the help everyone!

 

They are same in all X308 SC, same part number.

 

Here are some readings from my test drive. Note the input speed RPM vs the output speed RPM. The readings were taken while driving down the highway around 65 mph-ish.

It’s only 2 rpm difference, but does it indicate slippage? Is there any reading I could take that would tell me if there is slippage inside the transmission? I’m just trying to get comfortable with the next step, which is either a used transmission, or a rebuild. As I drive down the highway or interstate, the tranny fluid temp slowly but steadily climbs. I am in 5th gear while the temp is climbing. I don’t know much about transmissions, so I don’t know where the slippage is coming from. Basically what I have to believe is that the tranny is slipping as I’m driving down the road. Right? Or why else would the fluid temp keep rising?




Thanks for any advice you can offer.

 

I’m sure you already looked at this since your transmission was rebuilt but, do you have this fiberglass panel in between your passenger side catalytic converter and transmission?


The above heat shield is what keeps this short wiring harness from getting cooked.


Not on the 722.6 trans but on the 04’+ transmissions in the XJ8/R where the wiring harness couples to the transmission itself. I’ve encountered random codes when oil gets in between those pins where they meet. If you do have oil in there where the pins connect, it’s possible that could be a problem? I know you don’t have codes, but heat (assuming you don’t have that heat shield in place) causes resistance In the harness and could be providing false readings.

Do you know if the conductor plate was replaced when it was rebuilt?

 

Upon second look, there’s something not right about that I/O rpm. If your at roughly 65 mph at 1632 engine rpm, those I/O rpms should be a lot higher... I have an Autologic and I’m going out tomorrow and we’ll compare readings. I’ll post pix tomorrow.

 

I appreciate the input. Yes, the heat shield is in place, and the wiring harness is in tip top shape. I conducted continuity and resistance tests on each of the wires coming from the transmission using the 722.6 manual during a prior bout of diagnostics. There is no trace of oil in the harness. In another bout of diagnostics, after the transmission was rebuilt but did not fix the problem, I pulled the valve body and conductor plate out. There were somewhat recent dates etched into them, which indicated to me that they were likely replaced. Nonetheless, I ordered another rebuilt valve body, solenoids, electrical plug, and conductor plate, thinking maybe the shop etched a date in mine and stuck them back in. No dice. Didn’t fix the problem. I am very confident that I have a fully functional valve body, solenoids, and conductor plate. I ran resistance tests again with the new conductor plate just for fun. All checked out fine, just like they had before. The new plug has new O-rings and everything is clean as a whistle.

Now that I think about it, I don’t exactly remember when I snapped that picture. I was on the highway doing 65, but I may have been pulling into town and likely slowed down to snap the picture. So I guess I don’t know how fast I was going for sure. I can tell you with 100% certainty that I have had transmissions slip in other vehicles, and this does NOT feel like that. This transmission SEEMS perfectly normal to me. It’s not like the transmission lags when you step on the gas. I would call it instant response. Despite that, I did wonder about the input shaft speed vs the output shaft speed when in 5th gear.

I mean, it HAS to be slipping, right?? At least a tiny bit? Otherwise what would cause it to generate heat like that? Hmm. I think we’re narrowing it down, but I still have a little way to go in my mind because it SEEMS to shift and act perfectly fine.

Edited to add: pay attention to the relationship between the input shaft and output shaft speed in different gears. I did not pay specific attention to that, and only really noticed my readings later. I do know that I was in 5th gear because if I wan’t going 65, I was “coasting” down FROM 65, and was still in 5th.

 

Obviously, something is very wrong with the transmission rpm-s. The input speed should be the same as the engine speed (1637) if the locked torque converter is not slipping. The output speed in 5th gear (0.83 ratio) should be 1637/0.83 = 1972.

 

OK. So would “coasting” have any effect on those numbers? I bet I had let off the pedal and was coasting as I fumbled with my phone for the picture. Based on the numbers you show, something would be terribly wrong, yet there does not seem to be any detectable problem. If I push the accelerator, it takes off. If I push it harder, it downshifts and takes off. In sport mode, the downshifts are almost instant when I press the gas. At no time have I ever detected what I would consider “slipping”, where the engine revs without a downshift or acceleration from the car.

 

Also, at 1637 rpm in 5th gear, the speed should be 50 mph, calculated using the 0.83 gear ratio, 3.06 diff ratio and 2.09 metres tyre circumference. It appears that MaxiCheck is unable to read the speed sensors correctly.

 

Sorry about the awful picture quality. I was flying solo on a busy highway trying to take pix while driving safely.

Anyways, I was driving anywhere between 65-72 with sport mode off and torque converter locked in OD maintaining as best I could within about 2-3 mph for each picture.



As you can tell with the TC locked,
400-500 rpm variance from input to output is about average.

One way you can tell if your transmission is slipping is when you first get it on the highway when the ATF is relatively cool and it has a 400-500 rpm difference from I/O speeds and as your fluid is getting really hot like we saw at 240* the other day... what is the I/O rpm correlation while in OD at 240*?

Also, excluding the ATF temperature, what was the other issue your transmission was showing again?

 

The main issue is that the transmission doesn’t upshift after it reaches 235-240 degrees. At 3500 rpm, it will finally upshift, but it doesn’t “coast” when you let off the throttle. So if you’d normally coast up to a stop light, it stays in whatever gear the car is in at the time instead of coasting. Kind of acts like a Jake brake on a semi.

Thank you for posting the I/O numbers. I’m not sure what to make of your numbers, other than my reader either is not capable of reading the I/O numbers, or I have a huge problem. Since the transmission isn’t noticeably slipping, I tend to believe the former. It is also possible that I have not figured out a setting or something since the reader is new to me.

Thanks for the PM the other day. I am going to do as you suggested, which is get a “second opinion” of the actual tranny fluid temp by putting a temperature probe down the dipstick tube. I had not thought of that. That will tell me if I truly have hot fluid, or if the car just thinks I do. Although I did run continuity and resistance tests on the wires coming from the transmission during a previous round of diagnostics, the car was not hot when I did it. I suppose it is possible that the temperature wire is somehow compromised, and when it gets hot the resistance changes, and it sends false information to the computer.

I will cross that bridge first. If the fluid is truly “hot”, then I’ll know my transmission is defective. If it’s not hot, then I’ll know I have a bad wire, and I will go down that path. I don’t think it would be impossible to run a new wire from the electrical plug up to the TCM if that ends up being the problem.

 

I suspect the cooling side of things is not working correctly. The easiest/ cheapest option may be to buy an oil cooler and plumb it directly to your new hoses, just to see what happens. Doesn't have to be a permanent fixture.

 

I hope to have time this weekend, and I plan to stick a temperature probe down the dipstick tube to verify the actual fluid temp. If the fluid is truly hot, then I will know it's a mechanical problem, and I might try your oil cooler idea before I rip into the tranny itself. It is possible that the internal cooler is partially plugged. Maybe it's supposed to flow through 3 coils, and 2 of them are plugged - or something. No way to know for sure, since I haven't been able to determine what the flow rate through the cooling system should be. What we did was put the return hose in a milk jug, start the car for a few seconds, and verify that it was pumping fluid - but again, I don't know how much fluid it should pump in a given amount of time. Plumbing the fluid through a supplemental cooler would be a logical step before I replace or rebuild the tranny. I like that idea, thank you. I hope to know more this weekend.

 

Important update:

It’s definitely a mechanical problem, not electrical. As per Addicted2Boost’s suggestion, I put a thermocouple down the dipstick tube to confirm (or refute) the fluid temperature being reported by the car to the diagnostic tool. The fluid is definitely running too hot. The thermocouple reported within a few degrees of what the car was reporting...that the fluid is WAY too hot.

I really wish I knew how much fluid the tranny should pump through the cooling system should pump. It is possible that there is an obstruction in the cooling coil(s) inside the radiator, and I hate to replace the tranny, only to find that the fluid is still getting hot. I think my next step will be to plumb in a supplemental transmission cooler temporarily. If the fluid doesn’t overheat, then I can assume that there was a blockage. If it still overheats, then I can probably assume that the transmission was not properly rebuilt, and that there is a defect that is causing it to overheat the fluid. I’ll cross that bridge at that point.

Anyone have any thoughts or advice from this point? Thanks again. It feels like we’re getting closer!

 

Good data there. What I would do next is completely bypass the existing trans oil cooler and install a good size (not the smallest nor the largest) B&M trans oil cooler out in front. If you drive it around double the amount of time as before and it didn't even come close to what you were getting, I'd call it fixed. If your not excited to having a source of heat in front of the SC heat exchanger, then get another radiator for it. But the B&M is SOOOO much cheaper for testing purposes than buying another radiator first unless you get a used one.

 

I’m not real excited about it on a permanent basis, but I’m ok with it temporarily to determine if I have a transmission problem or a cooler problem. I’d like to find a fitting to convert the existing cooling lines to a hose barb fitting to run hoses to/from the new cooler. I will study it more tomorrow to figure out what kind of fitting I would need. Probably going to just end up pushing a hose over the end and using a hose clamp. I obviously don’t want to destroy the brand new cooling lines that I spent two weeks replacing!

 

Did we fix it??

I’ve been disappointed too many times, so I will wait a while to make the final call, but I have good news for a change!! NO SHIFTING PROBLEM!

I took the afternoon off today to install a transmission cooler and give it a good hard test drive. I took fredd60 and Addicted2Boost’s suggestion and bypassed the existing cooler completely, plumbed in a supplemental cooler, and mounted it in front of the radiators. It wasn’t too bad of a job and actually went well.

I added some fluid, and then it was test drive time. The air temp today was 88 degrees, which normally is plenty warm to cause the issue to surface in 45 minutes or less. You may recall that my fluid temp was running in the 240-250 range, which was WAY too hot. So today, with the new cooler, I drove for two hours and the temp stayed in the low 170’s range. I could force the temp to climb a little by making the car unnecessarily downshift while passing, and forcing it to downshift while climbing a hill, etc. I had to work to get the temp up to 182 today, but it always fell back quickly once I resumed normal cruising. When I pulled back into town after the test drive, the air temp had fallen to 84 degrees, and the tranny temp dropped to 167 as I was ambling down the street to my house. The engine coolant temp never got above 200, and the high speed fans never came on.

I plan to continue testing it for a while each time I drive it. If everything stays cool, I think for the time being I am going to leave the new cooler in place and not replace the radiator. I did a pretty good job plumbing the lines, and I will zip tie them nicely and put the grill back together. I’ll drive it that way until I decide whether to replace the radiator.

Wow - what a journey this has been! I hesitate to declare victory, but we’re closer than we’ve ever been. I really appreciate all the advice that finally got me on the right path!

 

Fingers and toes crossed that it’s finally fixed. With that temp gap, I’d feel comfortable to say it is fixed.

 

Just thinking ahead: How concerned would you be about having the tranny cooler in front of the other radiators? Do you think it would matter much?

Edited to add: I bought a Hayden model 404. It’s a tube and fin design, so not the most efficient, but it has a lot of clearance for airflow. I’m thinking that having that cooler sitting there won’t appreciably affect the other radiators, but I don’t really know. I do know that the engine coolant hit a high of 199 today, and most of the time it ran around 195. That would obviously go up if it was 110 degrees outside, but still pretty good.

 

199F is still well below the high speed fan trigger temp of 207F so that temp is certainly ok. You shouldn't worry seeing engine temp up to 210F in certain conditions, as long as it does not continue higher but comes down after some time (while the fans at high speed control it).