Small but mighty

Jan. 1, 2020
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.
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.

Color is Important
You will find quite an assortment of different colored connectors and part numbers available for these switches. Other than that, they essentially look the same and can be easily mixed up (see Figure 5). The different colored connectors are there for identification. They identify an internal mechanical difference between each type of pressure switch. Each of the different color coded pressure switches are designed to close and open at different pressures. This is done via a small internal metallic dome that is designed to move at a specific pressure.

FIgure 4

We can’t say this enough, timing is everything. That little metallic dome inside is a moving part. It can wear out and cause the pressure parameters to change inside the switch. In turn, the computer will start telling the transmission to shift at the wrong time and you will start to feel those drivability symptoms we talked about earlier. We have also seen the dome get stuck due to fatigue or debris. It is important to take note that the computer will not always give you a pressure switch code when the switch has gone out of its pressure parameters because the switch is still functioning. It will usually give you a code if the switch is stuck.

Vehicle wiring and connection problems can also cause big issues with these pressure switches. We brought a 2000 Accord V6 into our shop with a flare on the 1-2 shift. The 2nd clutch pressure switch connector looked OK. But upon taking a closer look, we noticed that the connector clip was not fully seated. We gave it an extra push until the clip seated, and went out on a test drive. Problem solved.

Figure 5

Excessive resistance in the connection was caused by the clip not being fully seated. This is actually a highly common occurrence out in the field after a fresh install job. We have fixed many of these issues on our tech calls just by asking the mechanic to unplug the connector and plug it back in. We also have seen other wiring issues such as pinched wires, inferior butt connections and the spade connector not properly seated in the connector housing. Paying attention to these important details can save you an unnecessary R&R job and get your customer in and out of your shop as quickly as possible.

The pressure switch might not always be your problem, but it is important to not overlook it in your diagnostic procedures. You may find out that you had to do a lot less work than you thought. 

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