Cooler Bypass - Transmission Digest

Cooler Bypass

External cooler bypass valves are becoming more and more popular, it seems. There are also many different variations of these bypass valves. It’s important that you check the cooler lines and cooler closely to see whether the vehicle you’re working on has one of these bypass valves. In most instances they can be removed and cleaned.

Cooler Bypass

R&R Tech

Subject: External cooler bypass valves
Unit: Ford 4R100
Essential Reading: Rebuilder, Diagnostician, R & R
Author: Mike Steen

R&R Tech

  • Subject: External cooler bypass valves
  • Unit: Ford 4R100
  • Essential Reading: Rebuilder, Diagnostician, R & R
  • Author: Mike Steen

External cooler bypass valves are becoming more and more popular, it seems. There are also many different variations of these bypass valves. It’s important that you check the cooler lines and cooler closely to see whether the vehicle you’re working on has one of these bypass valves. In most instances they can be removed and cleaned.

Most all automatic transmissions incorporate some sort of bypass in case the cooler becomes restricted, but they are typically found inside the unit.

One example of an external bypass would be on a 4R100 (Figure 1). This is a mechanical bypass that is in the tube that bolts to the case.

Another example would be a thermal mechanical bypass, like the one shown in Figure 2 or the one in Figure 3.

In the case of the 4R100 it’s not a problem when it comes to flushing the cooler lines and cooler, because it bolts to the transmission and is separate from the lines. Where it does become more of a challenge is when the bypass is in the cooler or lines, as with the ones in figures 2 and 3. In either instance if the thermal bypass valve is not heated during flushing, to fully stop bypassing the cooler, you will get little if any flushing of the cooler.

Here’s another thing to consider. If the cooler is plugged and there is a mechanical or thermal mechanical bypass in the lines you’re flushing, it will bypass the cooler. In that case there would still be flow but not sufficient cooling. If your cooler flusher has a pressure gauge, it should indicate higher-than-normal cooler pressure if the cooler is restricted.

The best way to ensure a proper flush is to flush around the bypass if possible. Or you might have to get creative and make a plug like the one in Figure 4 that you can install in place of the bypass valve. That way you guarantee you’re flushing the cooler.

The following chart shows the flow rate at different temperatures of the thermal mechanical bypass in Figure 2.

Mike Steen is the manager of Certified Transmission’s Technical Department.

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