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Small but mighty

The Honda pressure switch has a lot of say in what goes on in today's transmissions.
Wednesday, May 29, 2013 - 13:56
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The Honda pressure switch has a lot of say in what goes on in today's transmissions.
Figure 1

The Honda automatic transmission pressure switch might be small in size, but it can pack a mighty punch, causing drivability symptoms if it falls out of its acceptable parameters. Here at H&A Transmissions, Honda and Acura transmissions are our lifeblood. As a wholesaler, we focus on nothing but Honda and Acura transmissions all day, every day. We have done extensive research on these mighty little switches working on vehicles in our shop and working with other shops in our local community. In addition, we perform daily diagnostic work over the phone through our technical assistance department. We have seen and heard of many pressure switch problems out there causing flares, slips, harsh upshifts, harsh down shifts, late shifts, early shifts, you name it. With all the problems these little pressure switches can cause, there is good news. There is very little labor involved in replacing them as they are an external part and are able to be replaced without having to R&R the transmission. That’s always good news, right?

Figure 2

You will find these pressure switches on most Honda and Acura automatic transmission 4 cylinder and V6 applications, with the exception of a few models. Usually, you will find two pressure switches on one transmission. On most 4-speed applications, there will be one second clutch pressure switch and one third clutch pressure switch (see Figure 1). On most 5-speed applications, there will be one third clutch pressure switch and one fourth clutch pressure switch (see Figure 2). But on some of the later V6 models, you might find up to three pressure switches on one transmission. For example, the 2007 and newer Odysseys will have a 2nd, 3rd, and 4th clutch pressure switch. This is just more reason to have a good understanding on how these pressure switches work and the problems they can cause out there in the field.

 We have spent a lot of time here at H&A learning everything there is to know about the Honda/Acura pressure switch. We have tested thousands of these pressure switches, using a solenoid test machine that we have modified and customized for internal research (see Figure 3). We use this test machine exclusively for pressure switch testing on a daily basis. The pressure switches are tested on an aluminum test block with hot pressurized ATF (see Figure 4) and we monitor them electrically with an ohm meter as they open and close with the pressure that is applied to them. The pressure is applied via a manually operated variable pressure regulator valve.

Figure 3

So how do they work? The Honda/Acura pressure switch is a single wire switch. The metal body of the switch grounds itself on the aluminum transmission case. The computer monitors the switch through the signal wire as the switch closes and opens. The switch closes when pressure is available in the designated hydraulic clutch circuit. It opens when the pressure is released. So when the computer sees the switch close, it will make the designated upshift or downshift at the correct time. When the computer sees the switch open, it will continue on with its chain of commands for the next shift depending on what gear you are in and what model application is involved.

 A good working pressure switch will electrically read 0 to 1.0 Ω on an ohm meter when the switch is closed at its designated pressure. It will hold steady as the pressure drops, until it reaches its designated opening pressure. When the switch opens, it will read as a fully open circuit on the ohm meter. There is a little hydraulic, mechanical, electrical ballet going on here. In this instance, timing is everything.

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