Temps

[Don Bradner]

Assuming you are talking an Allison HT755 with transmission retarder, the max
temp for the transmission retarder is 330.

I do not have a separate fan for the trans cooler; assuming you are talking
about the main fan, the standard thermostat will cycle it on at 195 and off at
190, with an effective coolant range of 189 to 197 (because of overshoot).

I now have accumulated a modest amount of experience with downhill driving, and
some techniques for keeping speed under control. It is very common to find long
5 and 6% grades in the west (many miles long) and usually shorter segments of 7%
or rarely 8%.

I find that, at 6%, I could put it in 4th with the retarder on and run down at
constant speed (around 60) without a problem EXCEPT it would overheat fairly
soon. So, I use 3rd, and take it slower. In third gear the bus will decelerate
with the retarder on, and accelerate with the retarder off. I use the "enabled"
switch position so that I'm turning the retarder on with slight brake pedal
application. I apply the brake/retarder at 2100RPM and release at 1800RPM,
roughly 49 and 42MPH respectively.

In this process I turn the fan override on, as it would otherwise typically not
come on going downhill. In 100 degree ambient temperatures, the retarder reaches
roughly 300 at the point where I release it, and drops back to about 260. If you
get a stretch where the road has less grade and you aren't using the retarder
its temp drops very rapidly.

I've run 6% grades of 4 miles or more several times with this technique
(example: I-17 northbound into Camp Verde). I think the hill could be infinitely
long and the technique would hold up indefinitely.

Don Bradner
90 PT40 "Blue Thunder"
Eureka, CA

On 6/17/2008 at 3:12 AM Mike McMahan wrote:

>I'm fixing to take my first hot weather trip(to Denver) in our 96 42'
>Wanderlodge series 60 six speed towing my 28' race car trailer. My 89
>PT 40 always stayed in normal operating temps on this trip. My owners
>manual says normal engine operating temp 190 to 210. I can't find any
>temps listed as high for the trans or retarder only a reference to a
>high temp light that will come on at some point. Does anyone know what
>is the high temp spec for these. Also at what temp does the cooling
>fans on the trans cooler activate? Thanks, Mike McMahan 96 42'
>Wanderlodge