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Post by crunch81 on Aug 21, 2020 14:38:31 GMT
Good day People,
I do not have a osciloscope.
But I want to measure resistance between CAN High and CAN Low with a meter.
Can somebody tell me if I need to see round 60 Ohms? Are there two 120 ohms resisters in the CAN network?
Greetings Crunch
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Post by dv52 (Australia) on Aug 21, 2020 22:33:42 GMT
^^^^ why do you want to measure the resistance of the CAN wires and which of the CAN buses are you referring to? Rather than just giving you an simple answer to your question, I prefer to explain why! So, here's a diagrammatic representation of the set-up for a CAN twisted wire pair;
The "nodes" are the various modules. Notice that there are 2 x termination resistors which are placed in circuit (at both ends of the twisted pair wires) to absorb reflections of CAN messages. The values of these resistors are determined by the frequency of the message pulses and the maximum length of the twisted pair wires (and the maximum length of the tee-off wires to the nodes). But mostly, the value of the 120 ohm resistors are stipulated in the industry Standard for OBD - so it's a universal value for all cars. On VAG cars, I understand that the termination resistors are inside the modules, or they are integrated into the twisted pair - so you won't actually see these as discrete resistors.
Notice also how the 2 x 120 ohm resistors at either end of the twisted pair wires are effectively connected in parallel in the circuit. When 2 x 120 ohm resistors are connected in parallel - the resulting circuit resistance (i.e. the resistance measured across CAN_H and CAN_L)= (120 x 120) divided by (120+120) =60 ohms.
So if you place the leads of a good quality multi-meter across the CAN twisted wire pair and there is no CAN communications happening (i.e. the battery lead is disconnected, or ALL the connected modules are in their sleep state - including the Gateway module), you should measure 60 ohms.
Don
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Post by crunch81 on Aug 21, 2020 23:41:01 GMT
Hi Don,
This is exactly what I need!
I want to measure resistance to tell me if I have a open circuit in Can high and Can low. I know if I do measure 60 ohms it is not certain I do not have an open circuit somewhere but this measurement in combination with the volt measurements on the can wires with a multimeter can give me some information If I have any problems on the Can Network. I read somewhere that these ristance values may vary depending on vehicle so this is exactly what I need.
I have a DTC in the brake booster and I have sometimes a fault in the engine module about the brake booster. So I want to change the brake booster but also measure the 26 pin, 4 pin and the 2 pin connector on the brake booster.
Thanks Don.
Crunch
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Post by dv52 (Australia) on Aug 22, 2020 1:06:17 GMT
crunch:hmm....... the Brake module (J104) is connected to the Running gear CAN bus and the engine module (J623) is connected to the Powertrain CAN bus.
Don
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Post by crunch81 on Aug 22, 2020 6:10:49 GMT
Don,
I have a problem with the NX6 brake Booster Module.
Crunch
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