Noticed yesterday coming over the mountains to Calif that when I used the retarder and then released it (even shut off the enabling switch), that the trans temp shown on the VMS-PC isn't dropping like a stone as it has previously. It just very gradually returns to normal temp (185-190) even when the engine temp is holding at 180. I'm wondering if there is a separate thermostat in the heat exchanger that may be failing. Also noticed that when we head down a hill with a low gear selected (e.g. 3), the trans temp seems to go up even without retarder use, and I can't remember that happening before. One thought I had is that maybe the trans unlocks the torque converter if it thinks you are descending so that more heat can be dissipated.
Anyone have experience with that behavior?
No, haven't experienced that. Did the dash gages also confirm the temperature on the VMS? I think I read somewhere that the transmission will lock out the retarder, if the fluid gets hot enough. Have you checked the transmission fluid for engine coolant and vice versa? Could just be a problem with the VMS. I don't have that fancy stuff. Good luck.
I've noticed this in our 1999 40ft U320. I think it's related to gear selected.
In your case seeing high temps in 3rd, if you'd go up to 4th it might have reduced tran temp, but increased retarder temp. BTW, I'm referring to temps on our VMS-240C
I've done a lot of mountain driving since April and I found cases where to keep temps and down hill speed where I wanted I had to strike a balance between gear selected, retarder, and service brakes.
As an aside, I recently saw a case where VMS had my tran temp at 180 and the dash gauge called it 220! The dash gauge typically reads higher than VMS but I never before saw this big a difference.
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best, paul
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The dash gauges did track the VMSpc reading in this case, and they are usually similar readings, although I trust the VMSpc more. I think my dash gauge in in the trans sump, rather than on the output of the retarder, as seems to be the case for some rigs.
Thanks for the suggestion on higher gear, but it still seems to me that it's behaving differently now. I will try a higher gear on downgrades the next time we're in mountain country.
Dave,
Just a thought as the change in temperature behavior for the transmission seems to be something new. I would look into the transmission cooler to make sure there are no blockages developing from road debris, etc. You may also take a look at the routing of the transmission fluid. Are there possibilities for any blockage, leaks, pump performance failures?
Peter,
Thanks for suggestions, and I haven't crawled under the rig yet to look for explanations, but on this model - maybe on most of them, the transmission gets cooled by a fluid-to-fluid heat exchanger which sends the transmission heat into the engine cooling radiator.
I do need to do the yearly cleaning of the radiator & CAC area, though, and that may help. Not sure exactly what the engine coolant path is, but I'm guessing that the engine water pump keeps circulating through the trans exchanger and radiator even though the engine thermostat is closed when the engine is down to 180.
RE: Dave I think my dash gauge in in the trans sump
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On our non-computerized engine, our dash tranny temp gauge sender is in a 1/4" tapped hole in a big elbow on a hose connecting to the sealed external transmission/coolant heat exchanger. At one time ours was on the return line, now it is on the transmission output line.
If you see a single wire sender on a lower back of a transmission hose elbow, you may assume it is to the dash gauge. Wire disconnection and driving may prove where your gauge is reading temps from.
Hmmm, need to check that when we're back at the rig. Also haven't checked for Allison codes yet. We're house-sitting for some friends in the hills above Palo Alto for a couple of weeks...