Technical Information

Where is the ETM

Before we can consider the construction of the ETM it would be nice to know where it is in the car. As the throttle is the valve that controls the amount of air entering the engine it is immediately before the intake manifold and after the air filter, the mass airflow meter and the turbo charger (if the vehicle is so equipped). But, that still does not explain the location. Notice the white sticker hidden by the oil dipstick, white indicates an original ETM, if it is yellow it has been replaced.

Note the ETM is buried behind the radiator and under the intake manifold.

Construction of Magneti Marelli ETM

Initial observation:

The Electronic Throttle Module (ETM) (see figure 1) is located on the underside of the intake manifold, on the forward side of the engine, with the inlet air flowing upwards. Removing the fan provides enough working space to remove the 4 bolts holding the ETM to the intake manifold. ( I have not done this so you might want to refer to the VADIS disk) A six conductor electric cable connects it to the engine compartment wiring harness.

Disassembly:

Removing the front cover exposes the circuit board module and connections to the end cap circuits and the external cable. At four places Potting compound covers connections at each end, to the external cable and encapsulates a 470µF 25V capacitor.

Circuit boards:

The cover can be removed by inserting a blade in the glue joint. The circuit boards are made using DuPont ceramic thick film technology with gold, silver and silver-palladium interconnects. An explanation of the materials of construction for this exact board is found at http://www.dupont.com/mcm/applic/H-78296.html .

The advantage of the ceramic is low expansion with temperature extremes. The components are potted with a clear, elastic compound, similar in consistency to gelatin . All components are visible. All connections from the boards to the outside connections are through welded aluminum wires between the board and appropriate feed-thrus. Between the power board and the control circuit board are 11 single wire connections and 3 that are doubled. All components, except resistors, are soldered down using surface mount technology; resistors are screened (painted) on and then trimmed to value by laser. Most passive components (resistors and capacitors) are connected by screened on or photo lithography traces. Welded wire bonds connect the transistors, diodes and integrated circuits. The selection of materials and construction of these circuit boards is excellent and very appropriate for the application.

Mechanical construction of throttle shaft and throttle plate:

As would be expected, there is a throttle shaft that passes though the center of the throttle bore. The throttle butterfly is installed through a slot in the shaft and held in place with two screws that are staked in place. The shaft itself is supported by two precision sealed ball bearings. In this unit, with the throttle plate removed the throttle shaft was free to turn and showed no sign of roughness, wear or binding.

Removal of the end caps:

The connectors to the end caps can be viewed and opened by removing the potting compound. After the connections are exposed they can be unsoldered or breaking the joint open with a small chisel. The end caps serve three functions: First to seal the module from dirt or moisture intrusion, second, to contain the bus wires connecting the circuit boards to the potentiometer and drive motor field and third to contain the throttle position potentiometers.

Throttle Position Feedback Potentiometers:

The function of the potentiometer is to feed back the position of the throttle to the ETM computer. The throttle plate is stamped "86°"which appears to be the total angle that the throttle plate moves from fully closed to fully open. On each end of the throttle shaft is a plastic collar that is keyed to the throttle shaft by a "D" shaped cavity that fits onto the flat of the shaft. To this is fastened the wiper. The wiper is a single piece of conductive spring metal (material unidentified) that is divided into two sections, each further divided into 4 fingers. The wiper tips are bent at right angles so that the tips ride on the resistance film See Figure 4.

Figure 5 shows the position of the resistors in the end caps. The resistors are thick film carbon resistors screened (painted) on a plastic film, 0.006" thick, 0.414" wide and 1.161"long. Note the grooves that are worn into the thick film carbon resistor.

As shown in the schematic in Figure 6, the wiper picks a voltage off of the lower resistor and carries it to the upper resistor.

The theory for the two resistors can be found at  http://www.auto-solve.com/etc.htm. Signals from the two potentiometers are compared to allow the position of the throttle plate to be calculated with great accuracy. Since it is critical to know the throttle position accurately any degradation of the signals from the potentiometer would throw the ETM computer and its program into spasms.

Figure 7 are photos of the thick film resistor. The top is backlit to show the absence of the resistance material. Dusting with powder enhances the visibility of the wear grooves in the lower view.

Left End cap:

At this end is the drive motor (also referred to as a servomotor) for moving the throttle shaft shown in Figure 8. With the throttle plate removed the throttle shaft can be removed. The armature is a strong circular magnet 1.25" in diameter by 1" long. The field winding is a toroid with copper wound around a cylinder 1.5" inside diameter, 2" outside diameter and 1" long.

The armature magnet is polarized N/S, perpendicular to the shaft. Thus by controlling the current in the toroid, the field is shifted so that the throttle shaft follows. Essentially it is a DC (direct current) motor but instead of the magnet continually chasing the rotating magnetic field around a circle, it follows the magnetic field as it moves back and forth through 86°.

Right end of the casting:

At the right end of the ETM, keyed to the throttle shaft by the D flat, is a 2 diameter disk. There are two adjustable, but locked and sealed pins, one on each side of the disk. The inside pin (the one behind the disk) aligns with a stop that adjusts the no power position of the throttle. Under no power conditions the edge of the throttle plate is about 0.040 inch open. This stop is spring loaded so the throttle plate can be driven fully closed against this spring by the servomotor.

The outside stop (the one on the near side of the plate) is about ¼ " away from its outside pin under no power conditions. When the disk is pushed against the outside pin a thin crescent of light can be seen around the periphery of the throttle plate. The clearance is about 0.001 inch as measured with paper shims of varying thickness. When moving the disk against this pin there is a significant spring resistance from the backside stop. This acts as a stop effectively holding the throttle open under no power condition.

Normal idle is less than 20 mph; which means the throttle servomotor is bi-directional and can drive the throttle more open for higher speed but also more closed for idle and creep.

Summary:

Other than the selection and design of the throttle feedback potentiometers the construction is excellent. The casting is smooth and free of voids. All internal surfaces were clean with no traces of carbon or oil contamination. The internal machined surfaces are mirror bright. The throttle bore had only a slight bit of carbon type buildup. The circuit boards and construction are very appropriate for the application. Visual examination shows no signs of thermal damage. The throttle plate position stops were of a reinforced thermoset plastic. They were free of any damage and their movement was smooth with no binding. The movement of the throttle shaft was smooth and when against the full closed position there was no sign of binding or sticking that would require throttle bore cleaning

ETM Failure Analysis

Summary:

The final failure mode for the Volvo Electronic Throttle Module, made by Magneti Marelli in Italy and used on 1999, 2000 and 2001, 70 and 80 series Volvos, is the wearing out of the resistor film on the two throttle butterfly position potentiometers.

The time to failure is determined by the repetitive movement of the potentiometer wiper across the softer thick film resistor. Thus, in-town stop and go driving with many throttle movements per mile will wear out the potentiometer film faster than long distance driving with few throttle movements per mile.

Construction:

See the construction Details of Volvo Electronic Throttle Module and Failure Mechanism report for nomenclature and parts location.

Observations:

As the wiper moves across the resistance element, it, first polishes the tracks where it contacts the resistance element then carves grooves in the thick film resistor. The more it cycles, the deeper the grooves. In one area the grooves are deep enough that the resistance film is completely removed and light can be seen through the backing film. This failure was only observed with one potentiometer. The other end is of similar construction, though the resistance material has not yet been scraped completely away, though there are clear tracks and an accumulation of resistance material at the end of the track.

Without this signal the ETM shuts itself down, and preventing the ETM from possibly going into a runaway engine mode. When the computer shuts down springs and stops in the end cap move the throttle to the limp-home mode, about 20 mph.

Notice the wear areas on the thick film resistor below. You can see where the grooves are deep enough to expose the plastic backing. In the lower element, the wear is not as extensive but there is sufficient wear to give an open circuit signal to the computer. In only this one specific throttle position would the driveability problems occur. The wear corresponds to the position the wiper fingers would rest on while at idle and slow speed operation.

The debris picked up from the resistance element is shown below

Wear tracks, 60X original magnification

This clearly illustrates the wear tracks on the throttle position sensor potentiometer film. The width of the wear area, shown by arrow, is just over 1 millimeter. The orange area is the plastic substrate. By the location of the wear area it is possible to tell if the vehicle was mostly an in city, short trip driver or a freeway commuter that drove consistently at highway speeds. There is NO way that this wear can be prevented by a change in the software, though the rate of wear may be reduced.

Conclusions:

The construction of this potentiometer is obviously not suitable for the application. Considering that in the life of the car there will be multiple millions of cycles of the throttle, the action of the hard metallic finger tips riding on the soft thick film carbon resistor element is a designed-in shortcoming that will result in an estimated 100% or more, failure rate in the life of the Volvo automobile.

How to do your own failure analysis

So you have a failed ETM, what now? You are convinced that the ETM is failing because there is erratic idle, like the computer is trying to decide what to do, or the car has gone into a limp home mode or a full stall. You may or may not have tried the throttle body cleaning, replaced oxygen sensors or blamed the problem on bad gas.

You can perform this in about 30 minutes or less. You can see for yourself the problem then if you would send the films to Don Willson.

Copy down the year, model, mileage and even the VIN number, (You can read it through the windshield at the left front of the dash.) Copy down all of the information on the label. Copy the stamped information on the black electrical connector Get the date code off of the casting, numbers in two circles on the casting.

Also, what is data on the label of the new unit? What is the part number on the service ticket? What is the color of the label?

The easiest way to get the films out is: Unscrew the end caps; it takes a #20 Torx bit. With a screwdriver wedge the end cap up from the side opposite the label. Lift it up and the solder joints at the front (label side) will break off. Carefully lift out the circular metal retainer. Lift out the wiper assembly, that is the black plastic sleeve with fingers on it. At the side near the external connectors is a metal spring contact; it looks like 6 flat metal fingers pointing toward you. Lift it out with needle nose pliers. Now, at the area where the spring connector was carefully lift out the resistance film, it is reddish brown. While touching only the ends tape them to a stiff piece of paper putting tape only on the ends..

Hold a flashlight behind the resistance films and you should see the worn through areas. If you are a technician in a garage, show the parts around the around the shop so the mechanics will have a better idea of the failure.

You may be able to make a Xerox copy by holding a flashlight behind the resistance film as the copy is being made with the lid open.

On Board Diagnostic (OBD) Codes

Check out http://www.obdii.com/ It confirms what I posted below. There are two categories of DTC's that apply to OBD II. They are listed below with Type A being the more severe.

Type A
      These are items that would cause damage to the engine or car if the car is continued to be driven. A couple of items are loss of oil pressure or coolant temperature too high (loss of coolant.).

1. Emissions related.

2. Requests illumination of the MIL after one failed driving cycle.

3. Stores a freeze frame DTC after one failed driving cycle.

Type B
     This time fault is a transient or less serious problem. A common one is a loose gas cap. However in the case of the ETM it can be just an erroneous or momentary loss of signal from the throttle position sensor to the ETM computer. Turning off the engine and restarting often clears the pending code so no signal is present when you go into the dealer. Say you drive home from work 20 miles away and you get a fault at highway speeds. The next morning you drive to the dealer without experiencing the fault. This, no fault driving cycle clears the pending code so there is none for the dealer to read. They tell you it is nothing to worry about and send you on your way.
     So if you get a message that indicates a problem, pull off to a safe area, stop the car, turn off the engine. Then restart the car and continue on you way, driving at the same speed and manner and hope you can get the code again.

1. Emissions related.

2. Sets a Pending Trouble Code after one failed driving cycle.

3. Clears a Pending Trouble Code after one successful driving cycle.

4. Turns on the MIL after two consecutive failed driving cycles.

5. Stores a freeze frame after two consecutive failed driving cycles.

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