Re: Allison Transmission Questions The to cooler is the hottest point in the system. That's where you want the temp gauge to tell you the max temp the oil is seeing. Having the gauge in the from cooler port give you a pretty good idea of what temp the trans sump is running, but won't necessarily alert you to oil getting overly hot (300 deg F plus). The trans has a thermistor in the sump which will signal the ECU when the sump is either too cold or hot and the trans will react accordingly. Too cold (-20) and you can't get gear. When the sump starts to get hot (270 I believe) high gear will be restricted.