Vehicle details: Mercury Mariner
Engine code: V6 2 stroke
Symptom: Non-starter,
Non-starter with cranking
Author: Ben Martins

Mercury Mariner | Mercury Mariner crank no start

Previously the only outboards I’ve worked on have been a single cylinder pull start which we tested by running in a wheelie bin full of water so when the opportunity to take a look at a V6 2 stroke marine engine came up, I wasn’t going to miss it!

The customer had reported that the engine would intermittently crank and not start which has been progressively getting worse to the point where it no longer starts. Not particularly useful if you’re in the middle of a lake, or worse at sea. Customer started to take a look and found that there wasn’t any spark being generated when the engine was cranking. 

This was quickly verified by capturing the crank sensor and a HT pickup around one of the coil packs.

What we noticed was despite the lack of ignition during cranking, as soon as cranking stopped there was a secondary ignition event. How can that be?

Looking at a wiring diagram we could see that the power supply for the ignition modules were also shared with the fuel pump. For the next capture we connected to an ignition module and by adding a current clamp to the fuel pump, we would then be able to see if the issue was just with ignition or with a possible power supply/ground.

In the above capture we have battery voltage at the ECU on channel A, supply at an ignition module on channel B and fuel pump current on channel C. We can see in channel A that as cranking starts we get a voltage drop as expected. There is rather a large amount of noise present which could be some of the problem but what is clear is that the power supply for the ignition module does not come on till after cranking has stopped. We can also see that the fuel pump doesn’t run either. So, onto finding the control of the power supply to the ignition and fuel pump!

The power supply is provided by the main relay and given we had a spare channel, we decided to take a look at the control of the main relay.

We’ve kept the same setup with the power supply at the ECM, supply to ignition module, fuel pump current and then the ground control for the main relay. First test was to see what happened with just a key on cycle. Here we can see that as the power supply switches on the ECU, the relay control is grounded and so current can flow to which in turn switches the relay to supply the fuel pump and ignition module. As the fuel circuit is primed, the ECU switches off the ground and we see the pump stop and the energy in the ignition module dissipate. 

It appears that the main relay loses its power supply during cranking. Once cranking has stopped it comes back which is where we see the power supply to the fuel pump and the ignition module. Hindsight being a wonderful thing, I should have looked at the power supply for the main relay as well but given the ECU is dropping the ground side but we don’t have any current flow, we can only have lost the supply, given current starts to flow to the fuel pump once cranking has stopped. 

Unfortunately we didn’t have a wiring diagram so we resorted to tracing the wires back manually, where we found both the supply and the ground for the main control relay wiring went back to the main ECU. Looking at the amount of noise present in the main battery voltage during cranking we turned our attention to the battery. When looking closer at the voltage drop during initially cranking, it got down to 4V so before going any further, we need to look at the main battery connections.

Looking at the battery it is clear that the cables have seen better days. We also noted that the battery fitted is not suitable for marine applications. Unknown to me till working on this boat, was that marine batteries are very different to normal car batteries. Marine batteries are designed to withstand the harsher environment and larger vibrations felt in a boat due to it being on the water. They are also made to be more sturdy so internally the plates and the walls are stronger to prevent short circuits. More information can be found here

A new marine battery was acquired and fitted and during cranking we looked at the voltage drop between the battery and the ignition live at the ECM. To make it easier we used a maths channel to show the difference. We do still have a lot of interference but we are between 6V and 7V drop during cranking which is better than our 4V with the previous battery. Still though the engine didn’t start. Going back to basics, we supplied the main relay with an ignition live. Subsequently when we did this the engine started every time.

It’s at these times you need to make a decision. We can see that if supplying the main relay with an ignition live, the engine will always start. Without this ignition live the supply is cut off during cranking and so the engine does not start. After testing the powers and grounds to the ECU we came to the decision that the likely cause of the power supply failure was the ECU. However, rather than replacing the ECU the customer was happy to run a separate wire from a fused ignition switched live which would supply the main relay while the ECU still had control over the ground side. Following up with the customer, we can confirm that the engine has started ever since and no further non start issues.

This might seem like an incomplete fix and it is just covering up the real issue. Whilst I do agree and I would have liked to see an ECU fitted and if that fixed the problem, the customer wasn’t prepared to pay anywhere between £500 and £2000 when the issue could be solved with a piece of wire and some solder. Sometimes we just have to do what is right for the customer at that moment in time, and the fix here was to fit the piece of wire.

I hope this helps.

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Case study: Mercury Mariner crank no start