Consider a truck. The Electronic Brake Systems (EBS) ECU receives following message from some other ECU. Please explain this message. From where does some other module acquire the Suspect Parameters Number (SPNs) to send to EBS ECU. What does EBS do with the SPNs it receives?

 

My apologies for digging up and old thread... I was looking through this person's posts to get a better idea of them and came across something I had an answer to.

Consider a truck. The Electronic Brake Systems (EBS) ECU receives following message from some other ECU. Please explain this message. From where does some other module acquire the Suspect Parameters Number (SPNs) to send to EBS ECU. What does EBS do with the SPNs it receives?

The EBS would never receive this message from another controller. In a 1939 based system almost all messages are simply status messages broadcasted from an ECU to be read and interpreted by other ECUs unless it is a specific "request" message. There are probably some messages in the proprietary PGM range that do not follow this rule exactly, but they will have a specific ECU's destination address in the identifier.

All SPN data in a PGN is measured by the ECU broadcasting the message. If the ECU does not measure that particular data then all bits of that SPN will read '1' to denote an unused SPN. That is why the air compressor status is 2 bits. '00' would be off, '01' would be on, '10' would probably not be used, and '11' would denote the SPN is not used.

Although I only have experience with one particular make of truck I can say that all of the data in your posted message with the execption of the suspension pressure and air compressor status would all be measured by the EBS through sensors in the brake valve. A second set of pressure sensors were used for the dash gauges and, ABS although it was in the same unit as the EBS controller it treated the ABS as a separate ECU and it operated as a separate ECU, The suspension supply was not measured at all and there was no electrical connection to the air compressor.