E4OD Transmission Cooler, new and ready to install. Fits the E4OD. Ships fast from the USA.
If your Ford Super Duty, Excursion or E-Series van with the E4OD or 4R100 runs hot when it tows, the converter clutch feels soft or slips once the fluid is warm, and you keep seeing an overtemp or TCC code even after the cooler and lines check out, the small cooler bypass valve on the cooler line is worth a hard look. This listing is filed under transmission cooler, but it is not a radiator or a finned cooler. As the photo shows, it is the Sonnax cooler bypass valve kit: a hardened T-valve, a new spring, a hardened checkball and seven O-rings that let you rebuild the bypass assembly and keep the factory fittings and tube.
The bypass lives in the banjo fitting at the return end of the cooler bypass line. Under normal running it stays shut, so all of the converter release oil is forced out through the cooler, back into the case and on to the lube circuit. If the cooler or a line gets blocked, pressure builds until the checkball lifts against its spring and lets oil skip the cooler, so the geartrain is never starved of lube. Sonnax sets the new spring to crack open at 70-75 psi. The trouble starts when the factory parts wear: a relaxed spring, a sticking T-valve or a poorly seated ball lets the bypass open early. That oil short-cuts the cooler, so you lose TCC apply pressure, you lose cooler flow to the overdrive section, and you lose the radiator flow that actually pulls heat out of the fluid.
Overheating under towing is one of the three failures these units are best known for, and a weak bypass makes it worse because the fluid stops going through the cooler exactly when it needs to.
Rule out the easy stuff first: correct fluid level, a clear cooler, and no kinked, crushed or weeping lines. Our E4OD cooler line replacement guide covers how to separate a line problem from an internal valve problem with a flow check. If flow at the return is low with a clean cooler and good lines, pull the banjo fitting and look at the ball seat, the T-valve and the spring. A ball that does not seal against its seat or a spring that has visibly shortened is your answer. TCC codes can also come from the EPC circuit, so if pressure is low everywhere, read our E4OD EPC solenoid symptoms page before you condemn the bypass.
Sonnax lists this kit, part number 36605-02K, for Ford 4R100 and E4OD units from 1997 on, with either style of outboard cooler fitting. That covers the late E4OD and the 4R100 that replaced it in 1998 and ran into the mid-2000s behind the 7.3L Power Stroke, 5.4L and 6.8L V10 in F-250 through F-450 Super Duty, F-53, Excursion and E-Series vans. If you have a 1989-96 E4OD, take the fitting off and compare it to the photo before ordering, or send us your year and VIN and we will check it. Everything else for these units is on our E4OD and 4R100 parts page, and our 4R100 vs E4OD guide explains how to tell which one is in your truck.
If you tow heavy, pair the rebuilt bypass with an auxiliary 4R100 transmission cooler kit. If the unit already shows TCC damage or burnt clutches, the bypass is only half the fix; a 4R100 master rebuild kit covers the internal soft parts.
| Fits | E4OD |
| Part type | Transmission Cooler |
| Condition | New |
| Brand | Sonnax |
| Ships | From the USA |
| Warranty | 1-year |
Doing this repair right usually means replacing these at the same time. Same transmission, ships together.
No. It is the Sonnax 36605-02K cooler bypass valve kit: a T-valve, spring, checkball and seven O-rings that rebuild the bypass valve in the cooler line banjo fitting. It reuses your factory fittings and tube. If you need more cooling capacity, add an auxiliary cooler as well.
Sonnax lists it for 1997-later Ford E4OD and 4R100 units with either style of outboard cooler fitting. On a 1989-96 E4OD, compare your fitting to the photo first or send us your year and VIN.
Soft TCC apply or converter slip, overheating while towing, low cooler flow with a clean cooler, and codes 1783, 1741 or 1744. A bypass that opens too early lets oil skip the cooler and robs TCC apply pressure.
The new Sonnax spring is set to crack the bypass open at 70-75 psi, so oil only bypasses the cooler when it is truly blocked or restricted.
Yes. Reform the ball seat in the return fitting by striking the old checkball two or three times with a punch, then discard it. A worn seat will leak even with new parts.
Refill with Mercon V ATF, and back-flush the cooler before reconnecting the lines so debris does not go straight back into the new valve.
