Engine does not start… Normal cranking… 

The customer filled fuel just before parking the car in front of his house the previous night… 

No Malfunction Indication Lamps… 

Scan tool does not communicate with the car… 

Enough to send chills up an inexperienced electrician’s spine.

“Let us see why the scan tool is not communicating. Did you…” 

“Yes… I checked… it is communicating with other cars… Data Link Connector…? Yes, I checked… no problems… power is there… ground too…”

“What about Controller Area Network [CAN]? Did you check it?” 

“I checked both High Speed [HS] and Medium Speed [MS] CAN lines.” 

“Anything abnormal?” 

“No short to ground… no short to battery power…but…but…” 

“But…?” 

“HS CAN Hi and Lo lines have the same voltage. I checked other cars. The voltage is different… ” 

“Did you measure the resistance between the Hi and Lo lines?” 

“There is continuity…” 

“I am asking you – did you measure the resistance between Hi and Lo lines?” 

“No…no no Yes…” 

“How much?” 

“Zero… Zero ohms” 

“Is it OK?” 

“Nearly zero…” 

“You remember the body electrical training last week? The two 120 ohm…” 

“Oh no!… zero is not OK… it must be 60 ohms… two 120 ohms… one in the Power train Control Module and another in the Instrument Cluster… in parallel… There is a short in the HS CAN network…” 

“Take the workshop manual, find out the HS CAN diagram, identify the modules in the network… the short may be in one of the modules, disconnect modules one by one and you can isolate the faulty one easily. Sometimes you may not be that lucky… the short will remain even after disconnecting all the modules. In that case open the harnesses and follow the HS CAN lines…” 

And it turned out that the electrician was very unlucky… two hours later, after disconnecting all the modules, a few of them in really awkward locations, the short still remained. 

Now… looking for the short in the harnesses… where you start is very important… that makes the difference between a few minutes and a few hours… 

Luck eluded the electrician again… He decided to start from the DLC. 

DLC to IC to ABS module to TCM to PCM to…. 

It took nearly four hours to reach the short. 

But the short!

The electrician started form DLC, went to IC, ABS module… and finally came to the Power Steering Control module. This module is integral with the electric power steering pump located near the right side head lamp. The CAN lines to this module come from a joint connector located near the left side head lamp. As per the wiring diagram manual, there are no connectors or splices in the CAN lines between the connector and the module. The electrician confirmed the short in the lines between the connector and the module. He opened the harness and took out the CAN lines. Nearly two feet away from the connector there was a splice in the CAN lines. The lines branch off from there vanished into another harness. 

Wiring diagram manual does not show this splice. Where those lines are going?

 Finally the electrician reached the fuse and relay box in the engine compartment. The CAN Hi and Lo lines join the wiring harness going to the fuse and relay box and terminate behind a fuse slot. There was a 10A fuse sitting on the slot shorting the CAN Hi and Lo lines together!

 There are two fuses in the fuse box of this model that are covered. The cover has to be pulled out to access them. One of these fuses is related to ABS. We never noticed the other fuse slot. It is just mentioned ‘empty’ in the workshop manual. But there are many other fuse slots that are marked ‘empty’ in the manual. We checked other vehicles. That fuse slot was empty in all. We tried a fuse. Instant death for CAN!

“I came back from office, parked the car near my house, and the next morning find it like this…”

Some customers have very short memory…

He had a starting problem two days back… he took to a local garage… the apprentices there tried all they learned so far before identifying a dead battery… they changed the battery but forgot to remove the 10A fuse one of them had tried… and the customer brought the car to us directly from the garage… 

I am confused…Why the CAN lines are exposed in such a vulnerable point in such a robust machine?

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