Cranking voltage drop

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Cranking voltage drop

Postby Vertigo » Sun Apr 15, 2018 6:25 pm

I was messing around in my AW11 with the COP and CAS stuff that I have, and noticed that my Megasquirt was shutting down while cranking. Ive noticed this in the past with the distributor too. Did some digging, and found that the voltage is dropping to 7.5v. This is probably far too low for the MS to function, and in any case, it should be more like 10v. Its even worse when my intercooler pumps are running, and the fuel pump is wired on constantly too at this stage. Disconnecting both of those results in the voltage of around 12v at the ECU, but dropping to like nothing when cranking, which is odd.

Ill be completely rewiring the engine bay very soon, but Id like advice as to how to avoid drops this low. If nothing else, I could use a resistor in line with the start power feed, but there must be other options. The starter motor is a later model Corolla type (I think its a gear reduction type, at least sounds better than the older type), and its sitting on the intake side of the gearbox.

Also, what voltage drop is normal for the gear reduction type 4AGE starter motors? 2v?

Edit: "the motor torque falls with the square of the voltage and the voltage across the motor" - so a resistor inline wont be a good idea.
Last edited by Vertigo on Sun Apr 15, 2018 8:04 pm, edited 2 times in total.
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Re: Cranking voltage drop

Postby whynot » Sun Apr 15, 2018 10:51 pm

I would be having a look to see if you can pin point the voltage drop before you rewire the car otherwise you could end up having to re work your wiring job. It's usually to do with earthing and I believe the aw11 has a tendency to develop bad earth connections. Grab a multi meter and check the battery voltage when cranking to start with. Then keep the meter on voltage and measure from the negative post to the block and chassis while cranking. Any voltage you read while doing this is voltage drop and you can then look at improving that part of the wiring. Once you know it's not from the battery to the chassis or the block then start testing closer to the ecu but it's more likely the main wires.

I kept the factory power wiring when I re did my aw11 and the ecu read about 10.8v when starting and that was after putting the battery in the front.
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Re: Cranking voltage drop

Postby matt dunn » Sun Apr 15, 2018 11:20 pm

Yes first thing to check is the actual battery voltage while cranking.

It happens even on standard Toyota's at work sometimes,
the starter motors are so good on them that they will crank fine with a buggered battery,
but not enough voltage to run the ECU.
Usually you can tell as the car will start when you release the key,

It also used to be a problem with LINK ECU's as they dont like low voltage.
When I wire cars with aftermarket ECU I usually run an IGN relay to power the ECU and bugger all else,
and that does help. The relay needs a decent feed from the battery too.
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Re: Cranking voltage drop

Postby Vertigo » Mon Apr 16, 2018 1:21 pm

Thanks guys, Ill do some checks.
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Re: Cranking voltage drop

Postby jondee86 » Tue Apr 17, 2018 8:24 pm

Because I happened to make a log while starting the car on Sunday :)
16V 4AGE with s/c and AE92 reduction starter. Voltage at the ECU. EWT 8 deg.

11.8 ignition ON before cranking
8.2 when starter first kicked in
9.7 while cranking immediately before engine fired

I ran a fresh wire from the battery positive to a relay that supplies the ECU and
a few other items. However, the only item that is active on that circuit during
cranking besides the ECU is the LC-1. Running a fresh wire with a decent amount
of copper inside eliminates any bad joins or corrosion problems with ancient
factory wiring.

Make sure your ECU has a good ground to the engine and the ground path
engine>chassis>battery is solid with no voltage drop. Cranking is always going
to pull the system voltage down, but having a good battery with a full charge
will help minimise the drop and hopefully prevent your ECU re-setting.

Cheers... jondee86
1984 AE86 Corolla GT Liftback, NZ new... now with GZE
spec small port, twinscrew s/c and water/methanol injection :)

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Re: Cranking voltage drop

Postby Vertigo » Sun Apr 22, 2018 8:40 pm

Had time to look today, and found a 1v drop from the battery to the body. Unacceptable! After redoing the mounting point for the ground cable, its now dropping much less.
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