April 16, 2024

Watch Out for High Pressure in GM 8L45, 8L90 Valve Bodies

Chris Leach

Hey now! Oh boy, do I have a fun failure to share with you and warn you of today!

Have you encountered a crack in the worm tracks in your GM 8L45 and 8L90 valve bodies (Figure 1)?

Figure 1 – Cracked Valve Body in Worm Tracks
GM 8L45, 8L90 Cracked Valve Body Worm Tracks

Sonnax has, and wow, is it frustrating. Once cracked, there is no way to fix this. Obviously, the valve body is effectively a paper weight at this point. But, is there a way to prevent this from happening? We think there is.

Upon digging into what could be the cause, we discovered that the line pressure is simply too high when at max pressure. Line pressure for these units should peak at around 315 psi, but in our evaluations, we have seen pressures above 415 psi. Over time, with such high pressure in some instances, it could literally crack the valve body casting. It could also shatter the drum or other components.

It is hard to believe, but the entire problem often comes down to a worn pressure regulator valve lineup and a mis-calibrated line pressure blow-off spring. Excessive leakage at that valve will not allow the valve to stroke to the lower pressure position, causing runaway pressure that can be severe. Furthermore, depending on the amount of wear in this bore, a shuttle valve without O-rings may not be sufficient in sealing that leakage. This, coupled with an OE safeguard blow-off spring that is calibrated too high — over 400 psi, in some circumstances — will result in the casting and other components seeing some serious pressures that they are not designed for. Once you have runaway line pressure, it is over (Figure 2).

Figure 2 – Catastrophic Drum Failure Caused By Runaway Line Pressure
Catastrophic Drum Failure Caused By Runaway Line Pressure

So, the next time you are rebuilding an 8L45, 8L90 valve body, it is probably best to replace your pressure regulator lineup and your line pressure blow-off spring every single time. A good safeguard against this happening to one of your vehicles is to do a quick search for any aftermarket product that corrects for the worn valve lineup either through oversizing or with O-rings, like Sonnax pressure regulator and O-ringed valve kit 154740-29K, and a replacement blow-off spring with maximum pressures in the 300–315 psi range, like Sonnax pressure relief ball valve spring 154740-18.

Chris Leach is a Sonnax product line manager. He is a member of the Sonnax TASC Force (Technical Automotive Specialties Committee), a group of recognized industry technical specialists, transmission rebuilders and Sonnax Transmission Company technicians.

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