You Can't Check Volvo Controls Without Proper Scan EquipmentPosted 9/18/2005
By David Tidaback
Does it seem car manufacturers are all going in separate directions with their control systems? Each car company has its own proprietary systems that won't talk to your scan tool, and will talk only marginally well to onboard diagnostics (OBD-II). To make things worse, trying to translate from OBD-II to the manufacturer's own code scheme is challenging. Both manufacturer-defined codes are not in the standard OBD-II lists and the notion of what should be standardized OBD-II codes having definitions - according to car companies - only approximate what our OBD-II lists say they should be.
You ask yourself: How could things possibly get worse? Let me give you one example that shows just how much worse they can be. I take calls every day on late-model Volvos. By "late model," I mean the 1999 model year and later - when Volvo radically changed the control systems on their cars and the communications among those systems.
Recently, I got a call on a 2000 Volvo V70 with the low-beam headlights not working. The tech had determined there was no power to those lights, but there was power to the high beams. He wanted to test the relay, fuses, etc. Fuses 1 and 2, which are the low-beam fuses, were getting no power at any time, so evidently the relay was not being actuated or was faulty. I gave him the locations of the low and high-beam relays on the central electric module (CEM) and suggested he exchange them, since they are the same part, and the high-beams worked. No change; the high beams still worked, and the low beams still did not. Apparently the CEM was not switching the low-beam relay - but why?
The logical thing at this point was to see if the switch output from the high/low-beam dimmer switch was correct. The headlight switch had to be working because the high beams came on every time, so it seemed logical to the tech to test the dimmer switch. This is where the real difference between Volvo and more conventional cars becomes apparent. The high-low beam flasher switch is incorporated in what Volvo calls the steering wheel module (SWM). This module manages signals for high and low-beam headlights, turn signals, wipers/washers, radio volume/station selection, cruise control and more.
Checking the wiring diagram for the headlights and their controls shows seven relevant wires at the SWM. One is battery (constant) 12-volt power, two are grounds, and four are not labeled and do not show direct connection to any other modules but are simply cut off on the diagram. Other control modules on the page - such as the driver's information module (DIM), which incorporates the instruments, and the CEM - show similar four-wire configurations. These wires are part of the controller area network (CAN) bus system. The CAN bus transfers digital data signals between connected control units. There are actually two CAN bus systems on these Volvos - a high-speed bus used primarily for power train control, and a low-speed bus used for body and accessories.
The SWM is on the low-speed bus. Two of the four CAN bus wires are for communication between the supplemental restraint system (SRS), which deals with the air bags and seat belts; and two of them communicate with the CEM, sending and receiving signals for the systems listed above. How, then, can we tell whether the SWM is sending high/low beam switching signals to the CEM, so that it will control the relays for the headlights? Without the Volvo factory scan tool and diagnostic system (VADIS), we can't. Simple voltage tests on the bus are meaningless, since the signals are digital, not analog. With an oscilloscope, we might see pulsed square-wave signals on the bus, but have no way of interpreting them. This means that there could be a fault in the SWM or CEM or a problem with coding of one of these units. Without the proper scan equipment, we have no way of telling.
The same problem crops up in dealing with the engine systems. We may see a fault code referring to a loss of vehicle speed signal, for example, or for a throttle position sensor (TPS) fault. In each of these situations, the high-speed CAN bus system is involved. The vehicle speed signal is sent from the ABS as a digital CAN signal, and the throttle position information is sent from the electronic throttle module to the PCM in the same way. This means that those signals are impossible to test with conventional means, such as a digital volt/ohm meter (DVOM). In most such cases, we have no choice but to recommend that the tech refer the car to a dealer for further testing. Replacement of components on the CAN bus is also problematic, since they almost invariably require programming at installation, which again is generally possible only with the factory scan tool.
It seems a pretty bleak outlook in dealing with CAN-related faults on these late Volvos, but scan tool software development is progressing. Also, there are scanners on the market that are CAN-capable, so signals can often be tested, although programming capabilities are still almost nonexistent. Still, it is often helpful to know in advance which systems on a given vehicle you can and cannot deal with, so that those problems can be referred to a dealer without wasting too much time on them, leaving you free to do profitable work.
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