A Blast from the Past - Transmission Digest

A Blast from the Past

Today, we have a similar problem with welds cracking in General Motors’ 4T40-E oil-feed-tube assembly (see Figure 1). Lube failure to the forward-clutch-support assembly, slipping or no reverse, loss of engine braking in manual low and slipping or no forward are all possible with weld failure of the oil-feed-tube assembly.

A Blast from the Past

Technically Speaking

Authror: Wayne Colonna, Technical Editor

Technically Speaking

  • Authror: Wayne Colonna, Technical Editor

For those of you who remember the Slim Jim (Roto 10) years, the weld on the tube going to the fourth coupling would crack. Depending on the severity of the crack, the transmission would either slip or neutralize going into fourth gear. That was then.

Today, we have a similar problem with welds cracking in General Motors’ 4T40-E oil-feed-tube assembly (see Figure 1). Lube failure to the forward-clutch-support assembly, slipping or no reverse, loss of engine braking in manual low and slipping or no forward are all possible with weld failure of the oil-feed-tube assembly.

If you encounter a 4T40-E that has lost forward, you will need to be careful not to pull the unit right away. What I mean by that is being a good diagnostician, you may place a gauge on the line-pressure tap only to discover that line pressure is high. A P0730 pending code (gear-ratio error) may be set. With line pressure being more than sufficient to apply the forward clutch, it may lead you to believe that the input sprag or low roller clutch is not holding. So you pull the unit only to find that both are OK. This is upsetting when all along it was a forward-clutch leak at the pipe weld (see Figure 2), which is accessible for repair by just dropping the bottom pan.

The fluid going through the pipe to the forward clutch first must pass through a 0.090-inch orifice in the spacer plate (see Figure 3). Line pressure feeding this orifice comes from the manual valve, and the pressure tap for line pressure is in the circuit before the manual valve. The feed pressure on the “tap” side of the orifice via the manual valve was not affected by the pressure drop on the “pipe” side of the orifice. And this is why a line-pressure check could mislead you into thinking there is enough pressure to apply the forward clutch.

A quick way to inspect for leaks at the welds in the oil-feed-tube assembly for forward or reverse problems is to drop the pan and filter. Then blow air into the main line tap (see Figure 4) with the selector lever in Reverse and/or Drive. You should not hear or feel air coming from either end of the pipes where they weld to the flanges.
Another blast from the past.

Many thanks to Louie Zabala at WiWi’s Transmissions in Miami for the oil-feed-tube assembly.

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