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Voltage drop after engine start

EFI Wiring Fundamentals

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Discussion and questions related to the course Motorsport Wiring Fundamentals

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Hi,

After relocating the battery to boot i started having voltage drop after engine starts.

In the log i see a cranking voltage at about 9 volt, then engine starts, voltage rises to 14 volts and then after 2 or 3 seconds the voltage drops again to 9 volts and engine stalls. This only happends once, usually when cold start, the next startups runs fine with no voltage drop!

Could it be the alternator not charging for the first seconds ?

Battery is grounded to chassis in boot only and engine block is grounded to chassis in engine bay. Starter and alternator power wiring is unmodified from stock and are connected to power wire from battery.

Any hints ?

Regards,

Paulo

The first thing i would check is alternator. Takes seconds to check....

It is not going to be the alternator, as the voltage is dropping well below even normal battery volts.

I would suspect a wiring issue, possibly a shart or the starter re-engaging for some reason.

If it were not for the voltage drop, I would be wondering if it were a mixture of the fuel pressure dropping on initial start and a high draw from the pumps as they re-pressurise the lines - on that, are you running the pumps directly off the battery through a relay, or from the ECU?

Gord idea about the fuel pump could be right.

By your username, it would appear it's a S-chassis, and the original wiring do share the fuel pump voltage with the Ignition ON signal to the ECU, which usually leads to weird voltage reading in logs of some aftermarket ECU. Of course it depends on your ECU and the way the wiring has been done...

First thing would be to use a multimeter to look where does the voltage drop occurs, if it drops directly at the battery, or only in a specific circuit, what is connected in that circuit, etc... or a quick test would be to get the engine running, remove the fuel pump fuse, let the engine die, put ignition on, then put back the fuel pump fuse and make the fuel pump run while looking at the log, if you see any drop, you have your answer.

The car is a S14a (euro LHD), running ECUMaster Classic (not the Black). Fuel pump is a Walbro 255 High pressure version, is wired to the battery (in trunk) via a relay and a fuse, ground on chassis in boot. Starter and alternator are both oem and none of them was modified or touched from the factory harness.

Fuel pressure is fine, pre and post crank, all channels in the log seem to be ok, except the bat. voltage.

The voltage i´m talking about is seen at the ECU Logs, see the attachment file from a cold start at about 10* Celsius.

As you can see the car cranks fine (voltage drops to about 9 volt while cranking but thats ok) , after start i get 14 volts for a few seconds, then the voltage drops and engine stalls. When i crank for the 2nd time, it works fine, no issues as you can see in the log

Notice that the Lambda is 0.73 after crank for a few seconds but if i lean it, the enginegets very difficult to start

Attached Files

Couple of Q's:

What size cable have you run through the vehicle from the battery up to the front Paulo?

How have you made the connection between the original positive battery connection in the engine bay and the new cable coming from the rear?

Definitely your your multimeter out and pop the terminals across the battery. See if the drop is occurring at the battery itself, this would be good info to have :-).