Watch out for high pressure in GM 8L45, 8L90 valve bodies - Transmission Digest

Watch out for high pressure in GM 8L45, 8L90 valve bodies

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

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

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.

After digging into what the cause could be, 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, such high pressure 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 (as seen in Figure 2).

Figure 2.
Figure 2.

So, the next time you are rebuilding an 8L45/90 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 and a replacement blow-off spring with maximum pressures in the 300–315 PSI range.

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

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