A Fresh Take. on Chrysler s Pressure. Figure 1

Transcription

1 A Fresh Take on Chrysler s Pressure Boost System BEYOND THE BASICS A Fresh Take on Chrysler s Pressure by Dennis Madden members.atra.com Boost System Boost Valve Exhaust Boost Tube Figure 1 Ifind some of the most interesting tech isn t from new transmissions or technolo gies but from some of the units we ve had around for years. And where they become interesting is with systems that are unknown or misunderstood. Sometimes we ll go years without learning the whole truth about how something works. Not just a little on how it works but how it really works. For example: the pressure boost circuit in the Chrysler 42 through 48RE series transmissions. These transmissions have been around for about 20 years if you consider the introduction of the A500 in 1993, which was an early derivative. We can go back even earlier if we include the basic three-speed models from the early 60s. Since then, Chrysler has taken that early three-speed and adapted it for modern applications, first adding a lockup torque converter in 1978, then an overdrive section in 1993, and finally additional computer controls in Even within a single model 36 GEARS October/November 2014

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3 A Fresh Take on Chrysler s Pressure Boost System Figure 2 The system schematic shows an orifice that feeds an area of the pressure regulator valve. That feed orifice is located in the separator plate. Most Models 48RE you ll find modifications to adapt it to newer applications; an old system with a twist. And the pressure boost circuit is just that. My first look at this system was about 18 years ago. The circuit was pretty simple: It creates line pressure boost whenever lockup is applied or when the transmission s in 4 th gear. I did some live testing and found that it boosted line pressure around 6 PSI. Got it next. For the most part I kept this as one of those knows ; something we all Figure 3 38 GEARS October/November 2014

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5 A Fresh Take on Chrysler s Pressure Boost System Figure 4 know about. But recently the topic came up on the GEARS Magazine fixit forum Although the discussion wasn t centered on the size of the boost valve exhaust orifice (figure 1), there were some interesting discussions, including a comment where someone had seen the orifice drilled to about ¼ (0.250 ). What s basically understood about this feature is that the larger you drill the exhaust orifice, the higher the boost. But there s a limit to how much you can drill it, and when you reach that size, boost won t go any higher. So what is the limit? At what point does boost pressure no longer rise and how much line pressure can you get from it? Can it get too high? If people are drilling the orifice to ¼ (0.250 ) they must not know the answers to these questions. So let s dig in and see what we find. The system schematic (figure 2) shows an orifice that feeds an area of the pressure regulator valve. That feed orifice is located in the separator plate (figure 3). You can find the orifice in one of two places, depending on the transmission: There s the standard location and a location that s only used on the 48RE. The orifice is That feed oil then runs through a tube (figure 1) that leads to the boost valve. Figure 5 When the transmission shifts to 4 th gear or applies the converter clutch, the boost valve exhausts oil from the circuit, reducing balance pressure from the pressure regulator valve. 40 GEARS October/November 2014

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7 A Fresh Take on Chrysler s Pressure Boost System Keep in mind, the exhaust orifice is larger than the feed orifice, yet it still caused pressure to drop to 30 PSI. Figure 6 Figure 7 This creates line rise (figure 4). All of which begs the question: How much pressure does the boost circuit exhaust based on the size of the exhaust orifice in the plate? You can t test it directly on the valve body, so to answer that question I set up a test on an Answermatic valve body tester. The feed for the test circuit had a orifice that fed a tube about a foot long, which led to an exhaust orifice, just like the valve body. Here s what I found: The system was running at 75 PSI, so the boost circuit would have 75 PSI with the exhaust orifice sealed off (figure 5). When I used an exhaust orifice of (the factory original size) the pressure dropped to 30 PSI (figure 6). Keep in mind, the exhaust orifice is larger than the feed orifice, yet it still caused pressure to drop to 30 PSI. Next I tried a exhaust orifice, which is factory for later models and considered an upgrade. This still resulted in substantial pressure; 12 PSI (figure 7). Okay, let s keep going: There s tech data out there that suggests drilling the orifice to This resulted in 9 PSI. Still other information mentions 0.096, which resulted in 4 PSI. Keep in mind that a exhaust orifice has almost five times the area of the feed orifice. This was the surprising part to me: that you could have an exhaust that large and still maintain pressure in the system. At 0.110, my next test, the system went to zero; I ve included a graph of these findings (figure 8). There are a couple takeaways from this: First, Chrysler wants pressure in the boost system as part of the design and uses it as part of the line pressure control system. That is, it isn t intended to drop to zero. The other thing is that once you get to zero (roughly ) drilling the orifice larger doesn t do anything. So the next time you see one drilled to ¼ (0.250 ), you know that the technician who worked on it either doesn t understand the system or has a limited variety of drill bits. Okay, so now we know what effect the orifice size has on boost pressure. But how does this affect line pressure? For that I created a chart of what to expect for minimum line pressure for Figure 8 the various orifices tested (figure 9). Keep in mind, however, that this is based on the tested pressure regulator spring; your results may vary, depending on how you set the adjustment. There s another factor to keep in mind, too: We ve seen aftermarket separator plates that use a orifice instead of This means the boost pressure will be lower than the results we observed for each exhaust orifice tested; that is, until it gets to zero; then it doesn t matter. This smaller orifice allows for higher line pressure without having to enlarge the boost exhaust orifice. So if you always drill the boost orifice to and you install one of these plates, you ll exhaust all the pressure in the boost system when it goes into lockup or 4 th gear. 42 GEARS October/November 2014

8 Finally, you can get line pressure too high, either by oversizing the boost orifice or overadjusting the pressure regulator setting (the Allen screw adjustment for the pressure regulator spring). Two things can happen if line pressure rises too high: First, you may wind up with a P1762 code: governor pressure offset. Here s why: When the vehicle reaches about 50 MPH, the computer turns the governor solenoid off. At that point, governor pressure equals line pressure. If line pressure is too high, the transducer signal will trigger the code. The other problem with line pressure being too high is that these transmissions don t have a TV limit system. At wide-open throttle, TV pressure equals line pressure. So, if line pressure is too high, you ll wind up with full-throttle shift timing problems, like an oversensitive kickdown (3 rd to 1 st instead of 3 rd to 2 nd ). Problems tend to start creeping in once boosted line pressure reaches 75 PSI or so at light throttle. So it might be time to dust off the old pressure gauge to make the final adjustment. All in all, the boost circuit is a good refinement; it helps keep the transmission alive while grandpa s drivin his 15,000 pound 5 th wheel up a 6% grade in overdrive. And while modifications to the boost circuit are commonly recommended and worth making, there are limits to what you should do to it. How do you choose those limits? Figure 9 In nearly every case you need to understand how the system works and what those modifications are doing to it. Without a clear understanding of what you re doing and why you re just rolling the dice with your rebuilds; never a great way to build a dependable, durable transmission. GEARS October/November