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Back in the 90’s, ZF’s design philosophy was to reduce the number of sprags (one-way clutches) in their transmissions. The 5HP24 transmission had just one sprag clutch and that dropped to zero with the introduction of the 6-speed.
GM’s transmission designers, on the other hand, were massive fans of these “mechanical diodes” and the 5L40-E transmission uses no less than 4 of them.
These are great for automatically synchronising gearshifts but they mean that, during the overrun condition, the driveline disconnects from the engine and the vehicle freewheels. If the driver wants some engine braking he/she has to select Manual mode and then each sprag has a clutch which connects to either side of the sprag which is then engaged to by-pass the sprag to connect the engine to the wheels.
So, more specifically,
For 3rd gear the Overdrive Clutch is engaged to bypass the Intermediate Sprag
For 2nd gear the Second Coast Clutch is engaged to bypass the Second Sprag
For 1st gear the Low/Reverse Clutch is engaged to bypass the Low Sprag
Although you don’t explicitly state in which gears you don’t get overrun braking when in Manual mode, you seem to be saying that it’s all of them? The Coast Clutch is engaged to bypass the Forward Sprag in all gears other than Reverse and 5th. If you’re unable to achieve engine braking in 1st, 2nd, 3rd & 4th gears I would therefore suspect a Coast Clutch issue.
Phil
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22nd Oct 2025 1:44pm
