LS3 ECU Wiring Harness

[sideshow]

yer i get afew bits off them

[Jethrow]

Righto, next question ....

So I think the majority of the wiring is done now, and I have it to the stage the motor is cranking over. I think If I put fuel in, it might even start!

Now on the GMPP fuse box is a MIL light, which comes on when the key is turned on. Should go off when engine starts. The bulkhead harness also has an output for this same light signal, which I wired thru a small dash indicator light and then to earth. But it does not light up.

Any thoughts on this? Is it wired properly?

Similarly I have signals coming from the bulkhead connector that go to my speedhut tacho and speedo. Any notes on correct way of hooking these up? I have just run them as signals in to these gauges, not sure if a ground for the signal is required? The gauges do run independent power.

Thanks

[sideshow]

mil wire is a ground trigger
other side of globe is ign
it should go out after u start car
but if it does come back on u need to get ecu reprogrammed to turn whatever codes come on if the codes are not really important
speedo wire and tacho wire just go directly to each gauge
if speedo dont work u need to program it first
if still dont work u need a pull up resistor
if tacho doesnt work u need a pull up resistor

if after all this it still doesnt work u need to look at tune but they should have turned tacho and speedo signal on if ecu came with the gmmp harness

[Jethrow]

OK, so I need to add power to the MIL globe. Thanks again for your help sideshow.

[sideshow]

yes
if it stays on once u start it just dont connect it
wont affect how car runs

[Jethrow]

Wow - this electrics is tricky.

So my latest issue is that now my engine (LS3) suddenly cranks very slowly.

I am not quite ready to start proper but I have been cranking the engine in preparation and to date the engine has been cranking over well. But now all of a sudden the engine turns over very slowly when cranking.

It seems a very deliberate slow crank like the ECU is controlling it like that, but maybe I am off here. The battery is not flat (12.8V when tested) and to check I also jumped it from my other car but to no avail.

The only changes I have actually made to the car since the last successful cranking was to connect up the O2 sensors, and to remove the spark plugs for hopefully even faster cranking (I was hoping to see some oil press on the gauge before a real start).

I have been disconnecting the battery earth lead when not playing with electrics so I trust this is not bad for the ECU...

Anyway, I cannot see what I have really changed to cause this.

Have I burnt out the starter motor? I would have thought I would flatten the battery first, but I have not cranked so much for that to happen.

Have I failed my battery? It does test at 12.8V, but maybe that does not mean it has enough CCA left to spin the motor?? As I said before I did also jump start to no avail. Maybe I need to remove the battery from the circuit and just use another?

Should I pull the starter motor and inspect?

I believe I have good battery connections to starter motor and ground, and the motor has spun over properly before.

This is confusing me ...

[Modena]

The ECU doesn't control the starter, slow cranking means not enough battery juice or bad connections (+/- or both). If a jump start didn't help then it points to the wiring connections, also check the connections to the solenoid.

[Jethrow]

I was hoping you wouldn't say that, but it was as I suspected. Now I just need to figure out what changed as it did crank perfectly before ....

[Zedn]

Check all the main earth straps. That would be my guess.

[kyleb]

A bad earth will do it every time. Does the earth lead get hot when you crank and do you have an earth from the engine to chassis?

[sideshow]

bad earths are common when u fit battery in boot and u use the seat mount bracket for an earth as this bolts thru body to chassis but with so much fiberglass inbetween ive had some cars get bad earths
or your starter is stuffed
or your main power cable to starter has bad connection

did u crimp properly with proper tool or did u use a vice and crimp lug on end
or did u solder which is the worst way to attach a lug to starter cable

also make sure u use 2b&s cable or lug size 35 cable

[Gaz64]

The lead to the starter will have a bad connection, (overheated) from too much use.

Disconnect the battery and wiggle the starter battery lead.

The solenoid internal contacts get hot and melt the insulator.

Common on LS engines.

[Jethrow]

Thx for all the comments guys, a few things to chase up here. Gary, should I pull the starter and inspect the solenoid? Is it (the inspection) a simplish thing to do?

[400TT]

Tim, sorry haven't read the whole thread, so you might have done a few of things already.

As suggested check the wiring connections to the starter first, particularly the ends/lugs to deteriation where they are crimped/soldered to the cable.

Then try another battery, even if the battery tests fully charged it still might be faulty, starting to fail.

If no luck there, you will need to get your starter motor tested by an auto electrician.

Just make sure something else hasn't occurred. You haven't accidentily popped it into gear. Check if the transmission output is turning.

I hope the problem isn't engine related. Fingers crossed.

[sideshow]

jump start your car with another car
but have neg jumper cable go from jumper car engine block to ls3 engine block

and the positive jumper cable go from jumper car battery positive terminal to cobra battery positive terminal

it makes it hard sometimes if battery is in boot
and if battery is in boot theres higher chance of bad connections
do u know what size cable u used and is battery in boot
also when jump starting don't use the 20 dollar supercheap jumper leads

[Jethrow]

OK, you guys will think I am crazy .....

So I tried most the stuff suggested above, I pulled all the cable to and from the battery, cleaned and scuffed the ends to make sure all is well, and nothing was helping. I was about to try jump starting the way sideshow said (I had tried normal jump starting before) but my leads were a bit short and hard to find good quality ones on a Sunday. (Where do I buy good ones anyways?)

So what the hell, the main change I had done before was to pull the plugs out, so lets put them back in.

And presto - good cranking speeds again!!! WTF??

I do not understand this at all, but for now it seems all is good again?? Till the next time

[sideshow]

I got 6m jumper leads and they cost me 400 dollars 8 years ago
I can jump start most cars from behind that's the greek coming out of me hehehe

the problem might still be hiding in the background it its not there now
how did u attach the cable lugs on the ends of the the cable
and what size cable did u use

so it was cranking slow with plugs out
now u fitted them back in and its cranking fast?

[Jethrow]

Originally Posted by sideshow

so it was cranking slow with plugs out
now u fitted them back in and its cranking fast?

Yes - exactly that. I have no idea what is happening here which is a worry as a problem not fully explained is bound to re-occur!

The leads are the ones that came with the kit and are 4 GA. I had them crimped at my local Autopro store - the guy who owns it also has a Cobra (RMC) and is happy to crimp any battery terminals you buy from him with the big proper crimping tool he has.

[sideshow]

ok cool
just wait till it happens again
if battery is in boot then its more likely bad earth than bad power lead

[sideshow]

hey goto this website
AWG to Metric Conversion Chart
look at table and tell me which one is your cable
your 4ga wire is it 4/0 awg or 4awg as they very different
I use 2 awg which is 33.5 mm2 and takes a 35 sized lug
anything smaller diameter cable can cause issues when battery is in the boot
I use this cable for everything relating to battery cables