Why is my transmission suddenly overheating? In this case the dodge truck is a 2008, 6.7 R3500 with a 68RFE. That should be perfectly fine to tow a small bass fishing boat. So why is my transmission suddenly overheating?
Here are the top six reasons why your transmission could be overheating are:
- Plugged cooler Lines
- Plugged cooler
- Plugged Filter in the transmission
- Overloading the transmission
- Engine overheating
- Engine Fan Clutch
Let’s take a closer look at these potential causes of transmission failure due to heat.
Cooler Lines
Plugged or restricted cooler lines can be diagnosed using a flow meter or by checking flow into a bucket. Visually inspect the cooler lines for pinches, kinks or road damage such has dents and crush marks. Flow should be at least a gallon per minute.
Transmission Cooler
A plugged transmission cooler can also be checked using a flow meter or checking for flow at the ret urn side of the cooler. Before installing the transmission, the cooler is flushed and monitored for restriction. In some cases you might have auxiliary coolers or external coolers taped into the main radiator or cooler system. Make sure you trace the entire cooling system taking care not to miss anything.
Filters
Internal and external filters can be checked be removing them and visually inspecting them. They can also be checked by using a flow meter from the output side of the transmission cooler line location as we did, previously.
Vehicle Overload
This is probably one of the most common issues. Always check the owner’s manual to verify the tow capacity. Many times when you ask the customer if they tow they’ll say something like, “yeah but the trailer’s only 14,000lbs”. Not realizing the capacity is only 11,000lbs.
Engine Overheating
If the cooler is in the radiator tank, and the engine is running too hot, it will also prevent it from cooling the transmission. High engine or transmission temperature is by far the automatic transmission’s worst enemy. Heat destroys more transmissions than everything else put together. So here is a fact you should remember: the ideal maximum transmission temperature is 200°F. For every 20 degrees you go above 200 you reduce the expected life span of the transmission by a factor of 2. Simply put, at 220 degrees you can expect ½ the life span, at 240 a ¼ and at 260 it is reduced to 1/8th.
Engine Fan Clutch
You could have an issue where the engine is running normal and the transmission is overheating. This can be caused by the electronic fan clutch. There have been reports that the fan clutch operates but not to specification, essentially only cooling part of the system.
Diagnosing this is pretty simple. Make sure the engine is at normal operating temperature. Set the parking brake and verify the transmission is in park or neutral.
From here set air conditioner (if equipped) and blower fan to the OFF position, connect the scan tool and select appropriate model year and engine option. Check for and correct existing DTC’s.
Using special Tool 6801, back probe pin 1 of the electronically controlled viscous fan drive connector to ground. Using the scan tool, verify that DTC 0480 set. If it is not set, verify that a good ground is available.
Start the engine and raise engine speed to 2000 RPM. Go to the SENSOR screen on your scan tool and observe the fan speed. Run the engine at 2000 RPM until the fan speed increases to 1850 RPM or more for 30 seconds. This may be a slow process and may take up to 15 minutes before the speed increases. The fan speed should be in accordance to the Speed Chart (Figure 1).
If fan speed doesn’t increase, make sure the jumper wire has a good connection. If so replace the electronically control viscous fan drive. If the fan speed does increase and there’s still a concern suspect the cooler thermostatic bypass valve.
Thermostatic By-Pass Valve
If you get a vehicle that’s overheating after a major repair or just before a major repair and you’ve verified the cooler lines aren’t plugged or pinched, the cooler’s not plugged, the cooling for the engine has been checked including the fan and the operating temperature is 195-200 degrees. You could be dealing with a faulty cooler thermostatic bypass valve (Figure 2).
The purpose of this bypass valve is to bypass fluid during diverse conditions. In a cold weather condition it brings the transmission up to operating temperature faster. Conversely in hot weather conditions it should be wide open allowing normal operations.
Problematic Location
In the case of the this 2008, 6.7 R3500 with a 68RFE, bypass location is difficult to get to. To gain access to the valve you’ll need to remove the grill, intercooler, and air conditioning condenser just to see the transmission cooler (Figure 3). The valve is located on the top right hand side if you’re looking at it from the front of the vehicle. If the bypass valve is bad, the only way to fix it is to replace the cooler. If this valve is stuck open, a flush machine will not clean the cooler at all.
Repair or Replace?
Some of the blogs and forums have you block the system so the transmission fluid is forced through the cooler. We have done this on other units and have dropped the transmission temperature up to 80°C while towing over a grade. But there can be some diverse affects. This could cause drain back issues.
With so many Dodge trucks that are out there, look for the aftermarket to come out with a drop-in valve.
In the mean time I would inspect the cooler bypass valve if you have any question of its condition replace the cooler as an assembly. And that’s not just smart, THAT’S STREET SMART!
We’d like to thank our suppliers Superior, TransGo and Sonnax for their input on this situation.
