Actuator 1: Solenoids Solenoids are one of the easiest components to check electrically. In simplest terms, solenoids are electromagnets: They receive power and ground, which energizes the windings and creates a magnetic field. That magnetic field is what causes the solenoid to open or close. Solenoids will usually have either one wire or two. The one-wire solenoids are fairly easy to figure out: The wire always provides the power to energize the solenoid, and they ground through the solenoid housing. To connect your meter or scope, backprobe the connector to the solenoid, and connect your meter s positive lead to the backprobe pin. Then conby Vince Virgilio, EAST Training, Inc. and Steve Bodofsky, Steve Bodofsky Productions In the first two parts of Making the Connection, we looked at easy ways to determine how to connect a meter or scope to a sensor or switch, without checking a schematic. In this, the last issue of the series, we re going to discuss how to check actuators or outputs. These are the components that do the work, once the computer receives and analyzes its Figure 1: Actuators generally fall into one of three basic categories: solenoids, motors or relays. information. In general, actuators or outputs fall into one of three categories: solenoids, motors and relays (figure 1). We ll examine each of these three categories, to see how to check the signals they receive. Figure 2: When testing a one-wire solenoid, connect the positive lead to the feed wire, and the negative lead to a nearby ground. 40 GEARS June/July 2002
Making the Connection Part 3 okay, don t forget to perform a voltage drop test between the solenoid housing and ground. And remember, depending on the component and how it s wired, ground can come from the transmission case, or it may be provided through any number of circuits. In the end, the source of the ground is the battery: A voltage drop of more than 0.1 volts from the negative battery terminal to the component indicates a dirty or burnt connection somewhere in the ground circuit. Figure 3: A simple hatpin is a cheap and functional alternative to a dedicated backprobe pin. nect the negative lead to a good ground surface, preferably close to where the solenoid mounts (figure 2). Don t forget that you can still have a problem in the ground side of the circuit on a single-wire solenoid. If the positive side of the solenoid seems DIAGNOSTIC TIP: A number of suppliers offer specialized backprobe pins that work great for backprobing wiring harnesses. But you can save yourself some money by buying a pack of large hatpins from a local sewing center (figure 3), or wig pins from a beauty supply house. Either of these pins works great for backprobing most electrical connectors. CAUTION: Many techs use piercing probes to connect their meters or scopes instead of backprobing. That works, but it can affect the vehicle s warranty. If you do pierce the wiring insulation a practice we don t really 42 GEARS June/July 2002
recommend make sure you seal the hole when you re finished, to prevent water from wicking down through the harness and damaging electrical components. An easy way to seal the insulation is with a drop of clear nail polish (figure 4). WARNING: Keeping hatpins, wig pins and nail polish in your toolbox might cause someone to get the wrong idea about you! How you check a two-wire solenoid depends on whether both wires go right to the solenoid, or if one provides a common connection for several solenoids. If both wires go directly to the solenoid, connect one meter lead to one wire, and the other lead to the other wire. If your meter reads negative when the solenoid energizes, swap the leads (figure 5). But if one of the wires is a common connection for multiple solenoids, you Figure 4: If you re using a piercing probe, always seal the hole in the wire when you re done. A drop of clear nail polish works great for that. may have a problem determining which lead is power and which is ground. In most cases these connectors feed into the side of the transmission, which makes things more difficult because you can t trace them visually. In that case, you might want to check out a couple books from the ATRA Bookstore: Testing Electronic Transmissions (domestic and import versions). These books provide a lot of information about testing electronic GEARS June/July 2002 43
Making the Connection Part 3 transmissions, including pulling codes and forcing shifts. And they include a picture of the transmission connectors, with each terminal identified. From there, it s easy to determine which wires to check to see if the circuit s working properly. CAUTION: Don t forget, just because a solenoid works okay electrically, that doesn t guarantee the solenoid is good. It can still have a mechanical problem, or be damaged hydraulically. Always check the solenoids completely before reusing or condemning them. Figure 5: When testing a two-wire solenoid, connect one meter lead to one wire, and the other lead to the other wire. If you get a negative reading, swap the leads. Figure 6a: Electric motors, such as this blower motor, create an AC signal when they operate. If the voltage exceeds 500 mvac, you ll need to replace the capacitor or the motor itself. Figure 6b: Don t let the high peak-to-peak voltage fool you: Notice that the RMS voltage is still under 250 mvac. Actuator 2: Motors Just like solenoids, motors will usually have one wire or two. If the motor has one wire, that s the power feed: Ground is supplied through the motor housing. Just remember to perform a voltage drop test between the motor housing and ground. If the motor has two wires, one will be power and the other ground. Just like with the solenoids, connect one meter lead to one wire, and the other meter lead to the other wire. If your meter reads negative, swap the leads. There are some unusual motors in use, such as some electric window motors, which switch polarity to switch direction. But they re the rarity, and they probably won t affect you while working on a transmission. One thing you should remember to do when checking an electric motor is to check it again with your meter set to AC volts. Motors actually generate an alternating current while operating. That AC signal can cause a serious disruption to the computer system. That s why most manufacturers now use some type of capacitor on the motor feed: To filter out the AC created as the motor operates. Any more than about 500 millivolts AC indicates a problem in the circuit (figures 6a and b). Excessive AC voltage can trigger the computer or ignition system, and cause the engine to run rough, stall intermittently, or cause other difficult-to-find driveability problems. Too much AC can indicate a bad capacitor or even a bad motor. In either case, you should take care of that problem before returning the car to the customer. Actuator 3: Relays A relay is a device that allows a small current to control a second device that 44 GEARS June/July 2002
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Making the Connection Part 3 One set of contacts is normally closed; energizing the solenoid opens these contacts. The other set of contacts is normally open; energizing the solenoid closes these contacts. Figure 7: A relay allows a low-current signal to operate a high-current device. Figure 8: At low speeds, the current for the motor passes through the normally-closed contacts. requires a large amount of current. Commonly used with cooling and blower motors, relays are even being used to provide power to the computer system or transmission on some cars. Chrysler, for example, uses the EATX relay to provide power for the transaxle solenoids. We ll talk about relays as if they re two separate parts: The low side and the high side. The low side is the part of the solenoid that the computer or switch energizes. The high side is the part that provides a high-current signal to the motor or device it controls. Virtually all relays use the same basic internal schematic (figure 7). The low side (in blue) is an electromagnet. When energized, the magnet pulls against an arm that controls two sets of contacts for the high side of the solenoid (in red). While not every application uses both sets of contacts, most relays have both sets available. A common example of using both sets is a standard blower relay, found on most GM vehicles (figure 8). The normally-closed contacts provide the circuit connection for the low speeds to the blower motor. The current travels through a series of resistors, through the normally-closed contacts, and to the blower motor. When you select high speed, the blower switch energizes the relay (figure 9). This opens the normally-closed contacts, and closes the normally-open contacts. The normally-open contacts now provide full system voltage to the blower motor, which allows the blower motor to operate at full speed. So how do we use that information to check the relay circuit? Start by identifying which wires are low side and which are high side. Many Bosch relays actually have a schematic molded right into the housing, making it easy to identify the two halves (figure 10). If you don t have a schematic, check the wire size: Remember, a solenoid allows a low current to operate a high current. In most cases, the highcurrent side will use much larger wires than the low-current side; or in some cases they ll use double wires for the high-current side (figure 11). 46 GEARS June/July 2002
Figure 9: When you move the switch to high, it energizes the relay. This opens the normally-closed contacts and closes the normally-open contacts, to provide full system voltage to the blower motor. Figure 11: The size of the wires can give you a strong indication of which wires deliver the low-current signal (green arrows), and which provide power to the highcurrent side (orange arrows). From there, you ll need to know what operates the relay. For example, on a blower relay, the blower switch operates the relay. On the Chrysler EATX relay, the vehicle computer supplies the power, while the low side is normally grounded. Check the small wires to see if you have power and ground. If so, try disconnecting the wires, and listen for the relay to click. If the relay doesn t click when you energize and de-energize it, Figure 10: Many Bosch relays have their schematic molded right into the relay case. it s bad. If the relay clicks, next check whether you have power to the high side of the relay. If not, take care of that problem first. Most relay high side circuits are protected by either a fuse or fusible link. If you have power to the high side, check whether you have power to the relay output when it s energized. If not, the contacts are damaged or burnt. But if you have power coming out of the relay, the relay itself is fine: Any problem would have to be in the circuit it s controlling, such as a bad motor or broken wire to the motor. So, whether you re checking a sensor or solenoid, input or output, taillight or computer circuit, one thing remains the same: The trick to diagnosing the circuit is understanding how it s supposed to work. From there, figuring out which wire to check is the easy part. Lifts and holds Allison 1000 & 2000/400lbs. Built for rebuilders by rebuilders. Used by the Pros, Atra, Transonline, ATSG Loads directly from transmission jack Capable of mounting most front and rear wheel drive transmissions Complete work bench when using parts tray and tool trays Adapts to bench mount fixtures An approved vendor in the General Motors Dealer Equipment Program. Toll Free 1.866.767.5438 www.rnrebuilder.com NOLIFT TRANSMISSION REBUILDING SYSTEM UNIVERSAL WITH ADD A PLATE MOUNTING. GEARS June/July 2002 47