A 2014
Corvette with 1500 miles came in with a complaint of
intermittently the radio and/or Nav system would shut off at
times for 2 or 3 minutes and then come back on by itself and
operate normally. Scanning the infotainment system, U0028
was set as history in the radio. This signifies a MOST ring
break in the past.
Looking at MOST operation, the MOST network is a two wire
twisted pair infotainment data bus that runs 100 times
faster than high speed GMLAN. It is a unidirectional ring
network that has a master module that serves as a MOST to
LSGMLAN gateway. There are other satellite modules in the
MOST ring. Requests are usually supplied by other non MOST
modules connected by a LIN network, The request is sent by
LIN to the MOST module which sends it along the ring network
and watches for a return. Each MOST module watches the
module before it in the network for data. This occurs in an
downstream position only as the ring is unidirectional. The
modules are assigned places in the MOST ring by the master
module and assigned a numerical position ID number.
Still with me? If a module loses data from the module
upstream of it in the ring, this is known as a ring break.
At this point, the module first losing data will declare
itself on the ring as the Surrogate MOST Master Module. This
happens ONLY while the ring break is active. Surrogacy is
cancelled if the communication is restored. When the ring is
broken, only modules downstream of the break will send
information to the radio which usually is the MOST master
module. Ring architecture can be viewed on GDS2 as the Last
Working MOST ID of Node XX. XX will be the assigned ID
number of the module. The numbers will increase from node 1,
(usually the radio) through the other nodes in a downstream
direction.
On this vehicle the node IDs are as follows....
Node 1 - radio
Node 2 - Audio Amplifier
Node 3 - Human Machine Interface Control Module
Node 4 - Instrument Cluster
The Surrogate MOST Master Upstream Position value on the GDS
display was NONE as the ring break was not evident at the
time that I looked at the vehicle. How do we diagnose it? I
had two options. One would be to check the entire MOST
network ring for module/connection problems. The other would
be to test at the time of failure when the ring break
reoccurs which would supply me with information about the
declared Surrogate MOST Master module. I drove the vehicle
with a laptop, monitoring data. After 12 miles, the radio
shut down. Looking at the radio data stream I saw Surrogate
MOST Master Node Upstream Position value was 2. I drove back
to the shop and the radio started working again after a mile
or so.
Now I know the module that declared itself as the Surrogate.
The Surrogate MOST Master Node Upstream Position value again
reads NONE as the problem is not occurring again and
Surrogacy is no longer enabled. Although the MOST ring
information is transmitted downstream, we need to look
upstream for a diagnosis. The upstream position set was
position 2 so we need to count backwards on the ring from
the master module to find the position, not the assigned ID.
The last MOST module on the ring is the Instrument Cluster
ID 4. This will be in position 1 counting upstream. The next
module is the HMI module, ID 3. This will be upstream
position 2. Therefore the HMI module was the module that
established Surrogacy during the ring break. Still with me?
The HMI module is not a failure point and is operating
correctly. It just lost communication with the module
upstream of it in the MOST ring that was transmitting
downstream to the HMI module. So, the ring break must have
occurred BETWEEN the next upstream module which is the Audio
Amplifier, ID 2 and the HMI module, ID 3 in the MOST ring.
Checking the wiring pin fits at the HMI module inputs, I was
able to re-introduce the ring break by wiggling the wires
and found a loose pin fit at one of the connectors. A new
terminal fixed the problem.
------------------------------------------------------
intermittently the radio and/or Nav system would shut off at
times for 2 or 3 minutes and then come back on by itself and
operate normally. Scanning the infotainment system, U0028
was set as history in the radio. This signifies a MOST ring
break in the past.
Looking at MOST operation, the MOST network is a two wire
twisted pair infotainment data bus that runs 100 times
faster than high speed GMLAN. It is a unidirectional ring
network that has a master module that serves as a MOST to
LSGMLAN gateway. There are other satellite modules in the
MOST ring. Requests are usually supplied by other non MOST
modules connected by a LIN network, The request is sent by
LIN to the MOST module which sends it along the ring network
and watches for a return. Each MOST module watches the
module before it in the network for data. This occurs in an
downstream position only as the ring is unidirectional. The
modules are assigned places in the MOST ring by the master
module and assigned a numerical position ID number.
Still with me? If a module loses data from the module
upstream of it in the ring, this is known as a ring break.
At this point, the module first losing data will declare
itself on the ring as the Surrogate MOST Master Module. This
happens ONLY while the ring break is active. Surrogacy is
cancelled if the communication is restored. When the ring is
broken, only modules downstream of the break will send
information to the radio which usually is the MOST master
module. Ring architecture can be viewed on GDS2 as the Last
Working MOST ID of Node XX. XX will be the assigned ID
number of the module. The numbers will increase from node 1,
(usually the radio) through the other nodes in a downstream
direction.
On this vehicle the node IDs are as follows....
Node 1 - radio
Node 2 - Audio Amplifier
Node 3 - Human Machine Interface Control Module
Node 4 - Instrument Cluster
The Surrogate MOST Master Upstream Position value on the GDS
display was NONE as the ring break was not evident at the
time that I looked at the vehicle. How do we diagnose it? I
had two options. One would be to check the entire MOST
network ring for module/connection problems. The other would
be to test at the time of failure when the ring break
reoccurs which would supply me with information about the
declared Surrogate MOST Master module. I drove the vehicle
with a laptop, monitoring data. After 12 miles, the radio
shut down. Looking at the radio data stream I saw Surrogate
MOST Master Node Upstream Position value was 2. I drove back
to the shop and the radio started working again after a mile
or so.
Now I know the module that declared itself as the Surrogate.
The Surrogate MOST Master Node Upstream Position value again
reads NONE as the problem is not occurring again and
Surrogacy is no longer enabled. Although the MOST ring
information is transmitted downstream, we need to look
upstream for a diagnosis. The upstream position set was
position 2 so we need to count backwards on the ring from
the master module to find the position, not the assigned ID.
The last MOST module on the ring is the Instrument Cluster
ID 4. This will be in position 1 counting upstream. The next
module is the HMI module, ID 3. This will be upstream
position 2. Therefore the HMI module was the module that
established Surrogacy during the ring break. Still with me?
The HMI module is not a failure point and is operating
correctly. It just lost communication with the module
upstream of it in the MOST ring that was transmitting
downstream to the HMI module. So, the ring break must have
occurred BETWEEN the next upstream module which is the Audio
Amplifier, ID 2 and the HMI module, ID 3 in the MOST ring.
Checking the wiring pin fits at the HMI module inputs, I was
able to re-introduce the ring break by wiggling the wires
and found a loose pin fit at one of the connectors. A new
terminal fixed the problem.
------------------------------------------------------
Maybe you knew, maybe you didn't but most of us have been very active at doing this for the last fifteen to eighteen years. BTW James has another one out that he wrote after this one and both of them demonstrate that the cars can only test themselves so much, and the tools we have otherwise cannot diagnose the cars. It takes techs, dedicated, skilled, and committed way beyond what the compensation for the effort returns. The ugly truth about flat rate is for now that Corvette paid James straight time. If I was I the stall next to him while he was fixing this and doing customer pay front end and suspension work, I would have generated three to four times the income that he got paid.
Now how does that make any sense?
We always hear the advice of work smarter not harder. That simple line essentially suggests there is no reason to attempt to learn how to solve that Corvette's problem and is the heart of why it is difficult to find qualified technicians today. The need for speed and the one sided evaluations of the techs from the flat rate perspective has worked to discourage us from making the kind of investment in ourselves that we could grow into being the kind of technician that James is. What's even worse is many of the comments and especially the political pressures in the bays serves to duly punish anyone who genuinely tries. I hope you really hate seeing it written that way, in so much as to not just try and shout it down but to look closer at what really has been going on in all of the shops, and start managing to correct it. Straight time for that diagnosis and repair, what a joke. Soon the Corvette will go off of the straight time and that would probably be assigned the .3 like all of the other GM diagnostic routines that actually still pay anything. Of course the tech could beg for additional time which may or may not be granted.