Battery

[seight]

I've been working on a few cold start and warm up things on my megasquirt and I've been having real problems with subsequent hot starts. It's got me thinking my battery just isn't up to all the work. I'm currently running with a PC 680, I've been looking at switching to a Redtop 30, anyone got any experience of the latter?

A side issue may be that my fan is just wired to a radiator switch so it's on as soon as I turn the ignition on and during cranking. So I may look at hooking it up to my ecu somehow.

Thanks Mike

[richardpope50]

A side issue may be that my fan is just wired to a radiator switch so it's on as soon as I turn the ignition on and during cranking. So I may look at hooking it up to my ecu somehow.

Mike, my fan draws 24amps on start up and 18 thereafter so guess that's where the problem is. In fact for the very reason you say, I have just inserted a simple switch on the dash cutting off the relay circuit so I can start when engine is hot such as turning off engine in standing traffic. Easy to switch back as soon as engine starts.

[DaveEFI]

If the fan really is causing a problem if running when cranking, you could wire it to the aux side of the ignition switch, which is killed when cranking. If using standard Lucas wiring colours, this is normally brown/green.

Or as you say, use a spare MS port to drive the relay, so you can programme it as required. But note you may have to add a relay driver circuit (depending on MS version etc) as the processor ports can't drive a standard relay directly.

[seight]

Thanks for the quick responses.

My fan is on a 15amp fuse so must be a bit smaller than yours Richard.

I'll see if I can find a feed on the correct side of the ignition to turn the fan on. Is this the kind of thing I could do with a relay to avoid breaking into my loom too much? i.e. use an aux feed to turn a relay on/off and that relay is in the constant live to the current fan fuse?

Mike

[DaveEFI]

Yes - always use a relay. 15 amps is a large extra load to put where it's not allowed for.

But if you start a log with TunerStudio after switching on then starting the car, you can see just how far the battery volts are dropping when cranking. MS is usually fine to well below what you'd normally get down to when cranking - about 10v or so.

The log shows the actual volts MS is seeing.

[ChrisJC]

What is the symptom?, slow cranking?

Chris.

[richardpope50]

As above, always use a relay.

I have an MS2 Extra and my FIDLE port is set to switch on the fan via a relay when the engine gets hot.

Pins / circuit are simple:....

From ECU to Relay pin 85 (that will then earth) 12v from an ignition on fuse to Relay pin 86 to switch on main fan feed.

Main fan 12v 15 amp supply to Relay pin 30 and Relay Pin 87 direct to your fan's 12v supply terminal. (Fan's earth stays as is.)

If you want a dashboard switch to cut fan when starting, place on/off switch between ECU and Relay pin 85.

Hope all will agree (as this is my circuit!).

[seight]

Hi Chris, yes a little slow cranking when it's hot but I'm not sure what normal is, I think I was seeing max 170.

Plus after several failed attempts all the gauges in Tuner Studio started going a bit crazy. I put my charger on the battery and after a few minutes it eventually started and TS went back to normal. I didn't have time to take a good look at voltage level logging in TS as it started to pee down. I should be able to do it all again tomorrow as I'm 'working from home' and I'll take a bit more notice of the numbers.

I went for the PC 680 battery partly due to size and I had heard other V8s using it. But maybe it's just not got enough 'spare' in it for my motor.

I'll see if I have something spare on my MS for switching the fan.

Mike

[ChrisJC]

It sounds to me like your earthing might be a bit suspect. Even if the voltage is a bit low on the battery, I don't see why readings will be erratic, unless the earth is jumping all over the place.

Is there a good earth path between starter motor and battery negative?

Chris.