Snippage follows:
On 4/27/06, jon@dakota-truck.net <jon@dakota-truck.net> wrote:
> This is one of those "quasi-religious" issues ;-)
Quite true. I'm not an expert - just applying my limited college
thermodynamics education plus those FSM tranny diagrams. Anyway, 95%
of the time, you'll know within a minute or so that you're burning
your transmission up, no matter where you put the sensor. Hell, follow
your nose and you'll probably know within a few minutes without a
gauge. But to the bunch of us for whom that's not good enough... well,
you're reading this :)
> It has been my experience that the "generally accepted" spot to
> put a tranny temp sender is in the pan.
My experience has been otherwise. So there :-p
> The reason for this is that
> the line running to the tranny cooler will give you the temp of the
> fluid immediately after it has exited the trans, usually right after
> being manhandled by the torque converter. This makes the fluid very
> hot, but this does *not* nescessarily mean that the tranny is that
> hot. This is one of the main purposes of the fluid after all; to
> get the heat out of there.
Well yes that is a main purpose, but as such, the fluid can't be
hotter than the hottest part of the transmission unless it's coming
INTO the transmission with that heat (ie. with a cold tranny & warm
engine - it would pick up some heat in the radiator and bring it to
the tranny).
> Any heat which is in the fluid is heat
> that is not able to be absorbed by the transmission. The fluid which is
> in the pan is going to be much closer to the temp that your transmission
> is actually running at.
With a hot transmission, the fluid is absorbing heat from the
transmission, not visa versa. The fluid in the pan has exited the
transmission, run through the radiator (and aux cooler if you have
one) to dispense heat, returned to the pan, sat sloshing around and
cooling further for some time, and awaits pickup. When it gets picked
up, it better be significantly cooler than a hot transmission -
otherwise it ain't doing one of it's main purposes: cool the
transmission!
> If you are monitoring the "hot line", you are
> going to see spikes in the fluid level, depending on how you are driving.
> Pulling a heavy load up a hill, it might not be unusual to see a reading
> there over 300 degrees.
Yes, you'll see spikes. Hopefully not to 300 degrees though! If your
fluid is reaching 300 degrees -anywhere-, you're burning it... or damn
close. If thats happening, I want to know ASAP, which would put my
ideal sensor location on the fluid exit line.
> This is not a long term situation for the fluid
> though; its about to go through the tranny cooler and get mixed in the
> pan with much cooler fluid, so the brief spike is much less harmful
> to the fluid than an "oh crap!" glance at a tranny temp gauge reading
> 320 degrees might seem to indicate. :-)
If you're burning your fluid (even just briefly), that's a long-term
problem. If it reaches 320 degrees even for a few seconds, you've cut
the life of the fluid in half or worse. Lucky for you, it'll cool
quickly through the radiator and then mix with other cool unburnt
fluid (if this is a short-term problem) when it returns to the pan...
but some fluid damage is already done. You'll be non-the-wiser with a
sensor in the pan. Now, fresh fluid has already followed your newly
burnt fluid, and if the problem has stopped (you crest the hill), the
fluid will soon stop burning and continue cooling your previously
overheated transmission. If the problem doesn't stop (clutches are
slipping, you've broken something, or the hill is extra long), then
you keep burning fluid and finally your pan fluid temps are rising to
dangerous levels. At this point, you better STOP, unhook your load,
and get to a shop to have your fluid flushed... cause it's burnt and
now it's a very poor lubricant and is cooling much slower now.
> As an aside, I'm not sure what you mean by saying that the action
> of the fan or thermostat will affect your trans temp readings if you
> put the sender in the pan - can you clarify that?
When you start the car, your engine will warm up first, warming the
coolant almost as fast. Your tranny fluid is being pumped through that
warming coolant and at first, it's actually picking up heat in the
radiator. There are too many variables involved to interpret these
heat transfers by looking at your gauges. Your tranny will heat the
fluid slowly when idling, quickly when driving... if you've idled a
while and your coolant is at full temp, your tranny may still be
catching up, with the help of the hotter coolant. On the other hand, I
live at the bottom of a hill, and I rarely let my vehicles idle long
before driving. The tranny heats up quick, and transfers heat to the
cold coolant, which actually makes my heater core warm up
ever-so-slightly faster... I appreciate that in the winter :)
Then again... I say tomato, you say tomato. I'm terribly sick of
Chryco automatics anyway. I'm frantically looking for a cheap donor
Dak to swap in a manual trans for my rapidly failing automatic.
:)
Don
89 Dakota Convertible 4x4
74 Dart Sport 340 (yeah, get me started on the temp dynamics of this car!)
CT
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