Hi All,

As some of you may have seen, I had a long standing issue with my transmission "appearing" to overheat and going into pre-limp mode (refusing to upshift), you can read the boring details here

In a nutshell, a month or so after replacing my radiator and fans with new, genuine parts I began experiencing the above and my live data tool was showing the transmission oil temp way above normal (115*C/240*F). I followed a similar path to @aquifer who offered some good suggestions.

The problem is resolved, albeit unsatisfactorily. It turned out to be the new radiator, however, I don't know exactly what was wrong with it because I decided to bypass the transmission oil cooler and put in an external cooler. My [extremely seasoned] jaguar mechanic said that the transmission oil flow through the radiator was perfectly fine so it was a mystery to him why the heat exchange was so poor and mused over the possibility that the radiator design had changed, but, of course you can't see what's happening inside that section of the radiator.

I could have requested a replacement radiator at no cost, but I was concerned that a) there would be the same problem b) it would cost me significantly more to swap it out again rather than just put in an external cooler.

Some more background, when I place thermo sensors on both the inlet and outlet transmission oil cooler hoses I could see that the most reduction in temperate was 3*C / 5*F, so it just slowly got hotter and hotter over a 30-40min period in traffic.

 

Looking on Jag classic parts https://parts.jaguarlandroverclassic.../brand/jaguar/ there's only two part numbers for the rad, I'd suspect but have no proof that the only change occurred at the point the AJ26 was replaced by the AJ27.

 

There are many things can reduce the effectiveness of a radiators cooling
of the transmission fluid flow through. The pressure of the flow may be reduced
by several things as well as there could be some sort of restriction in the lines.
Reduced flow through will impact the temperature of the overall transmission
as circulation is reduced from the targeted design.

 

A cheap aftermarket radiator likely isn’t going to have the best internal TOC. I’d still highly suggest a bar and plate style external TOC vs the old school tube and fin. A better (perhaps) solution would be a good used oem radiator on eBay. Trust me, I know what it’s like chasing your tail to resolve an issue. You got this, you got that and it should be enough to remedy the situation. 5*F isn’t much of an improvement at all.

 

The radiator is a genuine Jaguar radiator, the new external TOC is a bar/plate cooler. While I'm no longer measuring the in/out temperature change, the transmission oil temperature is much cooler.

Yep, I've tried a few things, however, the flow through the radiator or lines wasn't restricted, and as soon as the external TOC bypassed the radiator the problem resolved.

Saying all of the above, it's now nearly winter here in Sydney - the ambient temperature has dropped since I last had this problem from 25*C/77*F to 18*C/64.

 

My apologies, I don’t know why I had one of those 100% all aluminum China radiators on my mind. Since you have the external mount cooler, you may just want to add an electric fan on it.

 

Hmm, I'm not if I'm misunderstanding you or we're both confused :-) The problem has been fixed by adding the external cooler. My real question was:

1. Old OEM brass radiator (which sprung leaks all the time) -> No transmission oil overheating
2. Replaced with a new genuine radiator -> Transmission oil overheating
3. Bypass the new genuine radiator and add external TOC -> No transmission oil overheating

So, how could a new, genuine radiator that was checked for flow issues (it had none) through the TOC not be cooling the transmission oil unless there has been an internal change in the radiator.

Does that make sense?

 

You are saying the transmission fluid flow through was checked in the new one?
Or was it only the coolant flow through that was checked.

 

Hi Randy, the transmission oil flow was checked both a) through the new radiator (transmission oil cooler) and b) the flow from the transmission oil cooler pipe (into the radiator).

 

I’m glad the external cooler worked for you. I’ve had zero issues since installing one. Curious - how did you end up adapting the line fittings to work for the external cooler?

 

I just cut the lines, rather, my mechanic did. As much as I'd like to keep my car standard, I'm not too worried. I'm looking at buying two new lines while they're still available, but, to be honest, I can't see myself replacing the radiator again - after all this, it did make me wonder about the wisdom of purchasing a genuine radiator as opposed to an OEM from China. Next time I think I'll just get the cheap OEM.

 

Not to beat this to death, but maybe useful for someone who finds this thread down the road. Did the mechanic cut the hoses and use hose barbs to attach hoses for the external cooler? Or did he cut the aluminum pipe and use hose clamps to secure the hoses for the external cooler? A picture of the solution might be helpful.

 

Hi there, sorry about the delay, it's been a busy autumn for me here in Sydney.

The mechanic was going to put an inline cooler in place, just after the return from the radiator, but we agreed to bypass the radiator entirely because the initial problem is caused by the radiator itself (not due to flow though).

The both the in and return pipes were cut and hose clamps were used to secure the hoses to the external cooler (as you suggested)


This is the return transmission oil cooler line. Just in case you're wondering there is a thermocouple attached to the pipe to get a temperature reading.

This is the in transmission oil cooler line.


I wanted to get a photo of the installed cooler too, however I can't remove the grill to get access and a good phot at the moment (but will do when I can).

 

oem x308 radiators are too expensive for anyone to do this research. they were always made by denso marston and probably out of production for years

 

Jaguar still sell "genuine" radiators, I purchased one late last year.