CAN BUS Diagnosis, Step by Step

Failures in the CANBUS system are one of the problems that most often intimidate technicians, dismissing in many cases the option to attempt to diagnose and repair the fault, before even starting any testing. In my opinion it can be one of the easiest faults to identify, diagnose and repair, especially if it is a wiring problem on the CAN line.

With these 8 steps you will be able to diagnose a fault in the CAN BUS or discard it as a possible cause, without invest too much time:

1. Electrical diagram.
2. Easy access point.
3. All connected 60 Ω.
4. Internal 120 Ω.
5. Only one Terminal Resistance 120 Ω.
6. Another terminal resistance 120 Ω.
7. Shorted to Ground.
8. Sectors discarded.

To make it so simple, it is required to understand a couple of key concepts which we are going to handle.

What is CAN-BUS?

CAN communication bus is a serial bus used for communication between all control units in the vehicle.

With this communication network, the unit that receives the vehicle speed information, for example, can share the information with all other units, a signal that can be used by the radio to adjust the volume according to speed and aerodynamic noise , by the sunroof to disable its opening and closing from a certain speed if necessary, by the electronic steering to adjust steering assistance to speed, etc.

In this way the vehicle information can be shared between all the units using only two cables, greatly reducing the meters of cable needed, costs, vehicle weight and probability of breakdowns.

You also have this CAN BUS Diagnosis online Training available.

Components and operation.

The CAN BUS has two lines, CAN High and CAN Low (CAN-H and CAN-L) and the control units that communicate through these lines.

There may be different types of CAN in a vehicle, depending on its transmission speed. The high-speed CAN has two terminating resistors in two of their units, these resistors are located between CAN-H and CAN-L and they will make the task of finding a fault easier. The location of these terminal resistances varies depending on the manufacturer, but they are usually located in the engine control unit, instrument cluster or body control unit.

This type of CAN is normally used for propulsion and safety systems among others, so in addition to being the easiest to diagnose, it is the most likely to show a failure.

The two terminal resistors are 120 ohms each, they are placed in parallel, as shown below:

The equivalent resistance of two equal resistors placed in parallel is half of them, in this case 60 ohms, so, if we measure the resistance value between CAN-H and CAN-L it should be approximately 60.

In the event that we disconnect one of the terminal resistances (control unit that contains it), the resistance value between both lines must be 120 ohms.

With these simple crystal clear concepts, we should be able to check the CAN BUS and diagnose a fault whose cause could be any control unit or wiring problem, you will even be able to determine the exact area of the wiring where the failure takes place.

Keep in mind that there are different types of systems, in a basic system, the CAN lines will go directly to the DLC connector (OBD), but there are other types of systems in which a control unit called GATEWAY acts as a link between the CAN of different speeds, in these cases, you probably will not be able to perform any type of measurement from the DLC (OBD).

CAN without Gateway:

CAN with Gateway:

It is very important to have the specific electrical diagram of the vehicle which we are working, if the resistance between CAN-H and CAN-L is measured from the DLC connector (OBD) and it is a system with Gateway, we will spend a lot of time looking for false faults because the measurement is not correct, make sure that the resistance measurement is done directly connecting to the CAN lines.

How to identify a fault in the CAN?

CAN faults always present fault codes of the type; CAN time-out, signal error or CAN OFF, among others. In addition, all these faults will be accompanied by the corresponding fault light on the cluster.

If you have a case with communication problems between all or most of the units, easily recognizable by the large number of fault lights on, you are probably facing a shorted that causes a total drop in the CAN.

In the event that the communication problems are between a few units, the problem may be caused by an open circuit . You can guess where the open circuit is by analyzing which units can communicate with each other and which cannot, using the wiring diagram, but below we will explain a better technique.

It may also be the case that the cause of the failure is a specific unit and not the wiring, in which case all the Communication problems usually refer to that failed unit,  unless that unit is shorted the CAN and causing a total fault, but let’s wait to see the analysis of the system step by step to understand it clearly.

CAN check with multimeter.

Remember that in order to diagnose a CAN fault it is essential to have the vehicle’s electrical diagram in concrete.

Steps to test a CAN with two terminating resistors:

1. The first step to check the CAN with a multimeter is to disconnect the vehicle battery. In this case it is not a safety measure, but the resistance measurements will not be correct if it is not disconnected.

2. Using the electrical diagram, locate a CAN point that you can easily access, many of the control units and much of the wiring is behind the dashboard, it is very important to learn how to diagnose faults by disassembling the vehicle as little as possible, At least that’s what I always try.

3. Without disconnecting any connectors, measure the resistance between CAN-H and CAN-L, when everything is correct it should be about 60 Ω. Any value less than 57 or greater than 62 should make us suspect an incident. A false contact, for example, can cause a sporadic failure of the CAN due to vibrations, a failure that may not be present at the time of your detection, but even so, it will most likely find a resistance a bit higher than that specified.

4. Locate one of the CAN terminal resistances in the electrical diagram, disconnect the unit that contains it and check the unit by measuring the internal resistance between the CAN pins, as we have already mentioned, the resistance must be about 120 Ω.

5. Once a unit with terminal resistance has been disconnected and checked, measure again at your starting position and verify that the resistance between CAN HIGH and CAN LOW is 120 Ω, corresponding to the other terminal resistance that is still connected. Any value outside the range 117 – 123 Ω should make you suspect an issue.

6. Plug in the unit you removed and repeat the steps removing the other unit containing terminating resistor, measuring the resistance between pins of the unit and the wiring, you already know which is the correct value range, don’t you?

7. We must also discard a possible shorted to ground, to do this measure the resistance between each of the CAN lines and ground. With all the units connected, the resistance obtained will not be infinite, because there will always be a path to ground through the electronics of the units, but it must be at least several KΩ.

So far how and what measurements have to be cheked and all the data on correct range, but what to do if we find an anomaly?

8. Do the measurements from different points of the installation and follow the path of the current between the two probes of the multimeter in the correct measurements, you will be able to discard areas that do not present an incidence in the electrical diagram. You can take advantage when you disconnect the terminal resistances and also measure from the connector of that unit.

Resume:

1. Electrical diagram.
2. Easy access point.
3. All connected 60 Ω.
4. Internal 120 Ω.
5. Only one Terminal Resistance 120 Ω.
6. Another terminal resistance 120 Ω.
7. Shorted to Ground.
8. Sectors discarded.

Find the exact point of CAN failure.

What to do if you find a problem in the measures?

Let’s see a real case:

A vehicle has various CAN communication faults, it also displays a large number of fault lights on the cluster.

In the following diagram we can see that it is a CAN system without GATEWAY, in which the DLC diagnostic connector (OBD) is directly connected to the CAN,  in addition the vehicle has a second  diagnostic socket that we can also use to make measurements on the CAN, both of which are very easy to access, without the need to disassemble anything in the vehicle.

We disconnect the battery and measure resistance between CAN-H and CAN-L from the DLC diagnostic socket (OBD):

This measurement value was 80 Ω. We repeated the same measurement from the auxiliary diagnosis connector and obtained the same value. As you know, this clearly shows that there is a wiring problem, probably a poor contact that increases the resistance of one of the lines, but where is it?

The CAN BUS of vehicles can be extremely complex, with many meters of cable, intermediate units and connectors, so we must isolate areas that we know are correct from those that present the problem, for this we are going to disconnect one of terminal resistances and repeat the measurement.

In the case of this vehicle we see that one of the terminal resistances is inside a unit that contains many bus connections, which does not help us at all in a diagnosis of this type, for this reason it is decided to check disconnecting the motor unit, the second terminal resistance.

We confirm that the unit is not the cause of the failure by measuring its terminal resistance, which is approximately 120 Ω, as it should be in a correct system. The other unit with terminal resistance  is checked in the same way.

We repeat the resistance measurement from the DLC (OBD) and auxiliary diagnosis sockets with the motor unit disconnected, obtainingin both cases a correct value of approximately 120 Ω.

By obtaining a correct measurement of 120 Ω, we know that the path traveled by the current until it finds the only terminal resistance that the system now has, is correct. In addition, we can also discard  all the areas that are not involved in the measurement of the terminal resistances, since in the previous measurement, with everything connected, we found a clear problem when getting the value of 80 Ω, 20 Ω more than the correct value.

Having limited the cause of the problem to the small part of the schematic, it is very easy to find the exact point, in this case, the only remaining connector on the CAN was checked, finding a pin that was not correctly anchored on the connector.

Diagnosis in other types of CAN BUS.

CAN con GATEWAY.

If we have a failure in a CAN with Gateway, the only thing we would have to take into account is that we must carry out the measurements directly on the CAN line that has the fault, they cannot be carried out on the diagnostic connector if the vehicle has the Gateway between the DLC (OBD) and the CAN.

CAN without terminal resistances.

In the case of having to diagnose a fault in a CAN without two terminal resistances, you can check the wiring as you would with anyone who wasn’t a CAN. I like to short-circuit the two lines of the CAN and make measurements of resistance from the furthest possible point to the short circuit. In this way you can check it as explained above, just expecting to always find a short circuit instead of 60/120. Don’t forget to discard a possible shorted to ground and a possible short circuit between both CAN lines that is not the one caused to diagnose.

Oscilloscope.

With practice the above checks should take no more than 30-40 minutes, but what to do if you haven’t found any anomaly. If no abnormality is found during cheks described above and you continue believing that there is a problem in the CAN BUS, we may be facing a specific unit that causes the problem,  or in the event that the CAN is shorted to another wire, either positive or some signal.

In this case it is best to use the oscilloscope connected to CAN-H and CAN-L to see the Waveform and try to find out what happens. We can see below the correct waveform of a CAN:

You may have more or less signal traffic depending on the information that the control units are transmitting, for example, with the vehicle running we will always see more signal traffic than with the engine off.

In the case of finding a clear example of derivation of the CAN with another signal, such as the one that  you can see below, I am disconnecting units one by one or dividing the CAN BUS by zones, until I get the correct waveform and succeed in isolating the section of the Bus that is causing the fault.

The previous case was a failure caused by a water shorted in a connector that included the CAN lines. The fault occurred in a vehicle with several very complex CANs including Gateway and with many units involved. It was resolved in less than three hours by following the steps outlined in this article.

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