Line-to-Lube/Converter: A Balancing Act — PR Valve Position is Key

The function of a main pressure regulator (PR) valve is to regulate line pressure according to the vehicle operating conditions and often to control flow to the torque converter, lube and cooler circuit. Typical forces acting on the PR valve are throttle/EPC fluid pressure, spring force that establishes baseline pressure and balance line pressure. When the PR valve is optimally positioned, it is regulating pump pressure for the transmission at the required level AND providing feed to the converter circuit (Figure 1). But trouble can occur when the PR valve is positioned in the bore such that the converter circuit feed is greatly diminished or completely starved (Figure 2).

Figure 1 – RFE PR Valve Optimally Positioned

Figure 2 – RFE PR Valve Positioned with Minimal Converter/Lube Flow

There are times when the PR valve should be positioned as in Figure 2 for short amounts of time to raise line pressure. But when positioned like this for an extended period, converters and transmissions will overheat, TCC linings will fail, codes will set and engine surge or stall at idle can occur. One common culprit for errant positioning is wear in the PR casting bore that is preventing proper valve stroke. The simple remedy for this is to refurbish the bore through reaming and installing an oversized valve.

Another cause could be the OE design combined with driving conditions, as is the case with the Honda 4‐ and 5‐speed PR valve (Figure 3). The Honda transmission has low pump capacity, and when a vehicle is towing a trailer, overloaded and/or climbing hills, the PR will cut off converter and lube flow long enough to blue the converter and overheat the transmission.

Tuning or chipping an RFE transmission to have higher line pressure for various performance reasons will typically raise idle line pressure from the factory 60–70 psi range to a 100–105 psi range. It is this change that results in a mispositioned PR valve at idle, resulting in low cooler flow, overheating, converter complaints and engine stall.

Prevent Comebacks & Practice Safe Performance Tuning with an Improved Sonnax Valve

Sonnax has developed and patented numerous PR valves that correct these issues by incorporating an internal feed passage that allows line to feed the lube/converter circuit even when the valve is in a TCC/lube cut-off position (Figure 4). For the RFE crowd, this is like having your cake and eating it too, as tuning or chipping the transmission won’t result in a meltdown.

The internal features of the Sonnax valve also provide a sealing of the passage during engine off. This provides added insurance against the negative side effects of drilling a passage in the casting wall that allows converter drain back and delayed engagements, and results in overworking the pump to account for the full‐time additional flow requirement.

Figure 4 – Sonnax Lube Regulated PR Valve for RFE
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Here is a complete list of the various Sonnax line‐to‐lube valves available to strengthen your rebuilds: