Chain reaction Mercedes
This diagnostic article takes you through the process our technical support team uses with problematic vehicles. We look at the issues involved and share how we resolved the problem. This an inside look, from the profound to everyday issues automotive workshops encounter. Problem presented to the technical support team…
We have a client’s SUV towed into our shop with a broken ignition lock that would not turn to the starter activation position. We have now replaced the mechanical lock assembly. Unfortunately, the vehicle now does not start although the key turns all the way. We have no fault codes relating to this issue. The technician who attended the break down is concerned over two issues. Firstly, the battery was significantly discharged. Secondly, he removed the relay from the AAM (all activity module which is like a BCM) and activated the starter motor. He is concerned that he probed the wrong terminals and damaged the engine ECU at one point or spiked it in doing so causing this problem.
With no relevant fault codes, a fully charged battery, confirmation that the starter does at least work from our technicians bridging exercise we need to investigate our immobiliser and key activity next. The spare key which was working was tried to no avail. Next, we recommended going into live data. This is the quickest way into the systems with the least amount of work. When we spot something, we can go deeper. On this vehicle we know the AAM module receives a constantly changing code from the key inductively which is sent on a communication line via an amplifier to the AAM, if correct the AAM lets the engine ECU know all is good and operate the starter via the start relay. The engine ecu controls the start process.
Scan the car
The Launch Auscan 3 scan tool is full of useful functions and made this next step very easy. We looked at the engine ECU for a, start enable signal.
There are over a dozen possible live data groups to select. The reason for this grouping is to make life easy but more importantly from a technical perspective it speeds up data transmission rates for the user. We cover this in detail in our scan tool training courses (Scan Tool – Level 1 and Level 2).
We can clearly see our engine ECU is not getting authorisation to start the engine, but the immobiliser looks to be working. Sort of definitely… maybe?
Measure
Next step effectively is which end of a piece of string do we start with. It is easier to start at the ignition switch with maybe a dozen wires which anyone with a few years under their belt could rattle off. The AAM module is connected to a mountain of wiring buried in an E box so that will be last. Firstly, let’s see if our key excitor ring works? It only has two wires and either will do. The workshop owns a ATS 6004 XM WIFI scope so we can measure anything with ease. The technician measured on one of the two inductive rings wires.
Flat out zero
It has no signal when the key was inserted! So, what about the amplifier? It has four wires. Two will be power and earth, other two signal out and in. Again, keeping things as simple as possible, the technician reports no power or signals but a good earth.
Let’s see if our ignition switch outputs are all working? They are close by and easily identified. All are good, except orange wire terminal 15C. Let’s power it up manually and recheck our key excitor coil.
We fitted a temporary power supply wire (15C) to confirm operation of system.
See below an ATS scope recording of the excitor ring with key inside coil working with terminal 15C. Perfect signals show the immobiliser is now online. Also, importantly the vehicle now starts. Job done.
Conclusion
Always try and find an easy way in. Use your scan equipment to quickly find what looks to be not as you would predict it should be. In this case it was relatively easy to see that the engine ECU was not getting authorisation to start.
Now is not the time to guess what the fault might be! Measure in a methodical way. By starting on the easy end of it we did not need a wiring diagram as we started with a well-known part of the vehicle. The ignition switch is the most likely to fail as it is a mechanical item which we interact with as the driver and it gets moved way more than for example an ECU.
Even though this vehicle is over 20 years old, issues like this are not simple to pinpoint without the correct equipment and “Swapnostics” is an expensive and risky game to play.
Tech support
Our AECS technical support removes the day-to-day stress present in your operation and thinks strategically. AECS technical support helps makes things easy and saves money for its subscribers, many times over than its cost!
Final thoughts
Incidentally, the technician asked the owner how often and with how much force the key was being turned during the non-starting of vehicle. More force was applied in an attempt to get it going was the reply. So, the switch ultimately failed, and the owner broke the lock. That was the start of our chain reaction.