How to Solve Volvo S80 Wiring Issues

Posted 6/1/2008
By Adam Garberg

If you were to lay out all the wire used in a 1998 Volvo S/V 70, it would reach nearly one mile, with almost 2,000 terminal connectors. With the demands for higher safety, comfort and government mandates, you would think the amount of wiring would skyrocket with the newer models.

In 1999, Volvo introduced the S80. This vehicle had many new features such as dynamic stability and traction control (DSTC), immobilizer, available telephone, navigation and speed-sensitive steering. Typical logic would indicate this would equal more wires and more headaches for the technician working on the vehicle. But in fact the number of wires, connectors, junction points – and problems – are going down. On the S80, for example, 11 wires go through the driver’s door jamb, while on the 1998 S/V 70, the wires number 24.

"... in fact the number of wires, connectors, junction points – and problems – are going down. On the S80, for example, 11 wires go through the driver’s door jamb, while on the 1998 S/V 70, the wires number 24.

The S80 was the first “fully networked” vehicle from the Swedish automaker. Basically, networking means that a group of computers talk to each other on a standardized communication protocol named the controller area network (CAN). Instead of multiple wires between modules communicating one message per wire, the CAN has two wires that link all of the modules together to carry numerous messages and information. This system and network also appeared on the 2001 S60 and V70 models.

These wires are referred to as CAN H (high) and CAN L (low). The colors are always green and white. An easy way to remember this is Green = Low (think of grass) and White = High (think of clouds). Keep in mind that the wires are a twisted pair, to reduce interference. The same information is transmitted at the same time on each wire, but the voltages are slightly different. CAN L operates from 1.5 to 2.5 volts and CAN H operates from 2.5 to 3.5 volts. If you are using an average voltohmeter, expect to see about 2.2 volts on CAN L and 2.8 volts on CAN H.

There are two separate networks on these vehicles, high speed (HS) and low speed (LS). The HS network operates at 250 KB/sec and consists of powertrain-related modules (ABS, transmission, engine, suspension, differential). The LS network operates at 125 KB/sec and consists of interior-related modules (power seat, instrument cluster, radio, climate control, windows, etc.). These separate networks are connected by the central electronic module (CEM). The CEM is also the gateway for scan tool diagnostics.

Checking network resistance is also a helpful diagnostic tool when dealing with permanent faults. Two terminating resistors, 120 ohms (placed in parallel), are built into each network. So, with the battery disconnected, if an ohmmeter is placed between the green and white wires anywhere on either network, the resistance will be 60 ohms. The resistors are located integral to the electronic throttle module (ETM) and the brake control module (BCM) on the HS network. For the LS network, the upper electronic module (UEM) and rear electronic module (REM) hold the terminating resistors. A wiring problem that affects one or all of the modules on the network will almost always show up in the resistance check.

Now, let’s put what we have learned into the real world. A 2001 S80 shows up at your shop with the passenger window, lock, side mirror and the ambient temp indicator in the dash not working. After reviewing the wiring diagrams, you find that all of these functions are controlled by the passenger door module (PDM).

Checking the CEM for codes shows a CEM-1A53 for faulty communication with the PDM, but no other pertinent codes. The passenger door panel is taken off to access the wiring. First, the basics (power and ground) must be checked and if OK, the network can be checked. Going to that module, you will see the familiar twisted pair with a green wire and a white wire. Now that you know what should be seen on those wires, you can check them using a scope or a voltohmeter, or by checking resistance – or all three. If you see a problem with the wiring, check the door jamb for wiring problems.

If no problems are found with the network wiring, disconnect and reconnect the PDM and recheck its operation. If it is still not working, it will need to be replaced. This same basic principle can be used when diagnosing communication faults with other modules as well.

Change is not always bad.

After you are accustomed to the layout and diagnosis of network-related components, you will see that even with new technology, diagnosing and repairing these systems will become second nature. When looking back at the old cars, you may just wonder ... what were all those wires used for?

Adam Garberg is a Volvo technician with ASE master and L1 certification.

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