Electrical gremlin - help needed please
 

Hello brains trust. Before I go outside for another spell of troubleshooting in nearly 40 degree heat I thought I'd throw this one out there.

Somewhere in my electrical system there's a leak or bad earth (or other) causing the starter to kick over when I switch the engine off.

It happens like this: when turning the ignition off, as the key passes from ACC to OFF a pulse goes down the IGN (start) wire from the ignition barrel. This is the "exciter" for the starter solenoid relay, so the starter kicks in briefly while the engine is spinning down. I can minimise damage to the starter / ring gear by pausing at ACC until the engine has stopped cranking, then OFF - obviously this is not ideal.

My initial thought was to check the ACC circuit, since it seems related. At the junction where I share the ACC feed from the ignition barrel, I run ACC power to the fuse box for all components that need ACC power. When I disconnect this wire the solenoid no longer fires when turning off - problem solved except I no longer have accessories. :D

This only used to happen when my heater fan was running as I switched off - managed by turning off the heater first. Since fitting a car stereo and amp (always powered by ACC) it happens all the time. My H4 headlights are also dim (hi beam is fine though), which may or may not be related - bad earth?

I doubt there's a silver bullet here but if anyone has any tips or tricks for troubleshooting this kind of thing, please share. I'm really just fishing around randomly at the moment with my multimeter - hoping there's a more methodical approach I can take.

Because of the prior issue, this may not be relevant, but the dim headlamps made me post anyway. I deal mainly with Fords and I have noticed in the past that if the battery voltage drops too much, the solenoid wants to stay connected. I realized that when the starter was engaged the contacts in the solenoid would get lightly arced together, I'd have to disconnect the battery to get it to stop. The issue resolved itself after the battery was brought back up...just a thought. This is on vehicles with a separate solenoid, but I guess it could apply to a starter with the solenoid mounted on it...but they have a heavier spring mechanism to return it.

I think it's the gmmp engine loom hahahaha

Do u have starter relay
I never run one if its manual unless car has immobiliser
Which I'm sure most cobras have
So could be something to do with immobiliser or how u wired the starter

If your lights are dim just run a temp earth direct to batt neg to see if it fixes it

I have found sometimes if battery is in boot there is no real good spots for an earth to chassis and I've once hooked an earth to the frame under roll cage where seat belts bolt to
And because it goes thru fibreglass I had a bad earth

Tested it by using two jumper leads as earth from batt neg to motor and fixed it so then I found fault and cleaned up bad earth

But to crank engine over it must be bad earth which makes power trigger your start relay

I wire my start relays different to some people so less chance of this happening

Paul. I was wondering whether it might be a faulty ignition barrel. Could you put a diode from the ACC lead from your switch to stop the "back feed" through the ACC circuit. I might be totally off track here but maybe your ACC circuit is getting a feed from somewhere else? Cheers Gregg

Sideshow, it's one of your looms. Well, the ECU/engine part is anyway. Not your fault though, I wired the rest of the system.

Yep starter relay is the one that's "tripping". I've disconnected the immobiliser for now.

The car starts perfectly. It's when turning off the ignition, going from ACC to OFF that it blips the starter (via the starter relay) for a fraction of a second. If I disconnect the ACC wire from ignition barrel to main fuse box, the problem goes away.

So how do I find bad earths using a multimeter? I tried measuring resistance but the readings were all over the place and inconsistent even when testing the same spot.

Thanks Woodz, I'll come back to this when I get around to fixing my headlights.

Hi Gregg. I fitted a brand new ignition barrel and switch because the original Ford Laser barrel was stuffed. I checked it earlier and all seems OK inside. I thought about fitting a diode or even a relay but I prefer to find the source of the problem rather than use a workaround. Thanks though - it might be my fallback position!

Paul. I was thinking maybe it was a back feed related to the Alt circuit. I was thinking that whilst the engine is still spinning down the alt circuit is still providing power etc to some circuit. Just a thought. Good luck. Cheers Gregg

It actually happens without the engine running - turn the key from OFF to ACC then OFF again.

I've had a breakthrough. I removed one fuse at a time and tried the key each time to see whether I could isolate the problem circuit. Turns out when I remove the fuse for the pos feed to the amp (open circuit) the starter problem goes away. The earth to the amp has 0 ohms resistance (good) so it must be the positive that has a bad contact. I've used a heavy gauge wire with really chunky strands so I'll try a thinner wire and see what happens.

Bad or worn switch
 

I have had a bad switch from a good supplier. The problem I found was similar in that the key switch would break ignition between the start and "on". The car would crank, start and when I released the "start", it would shut off if I was not quick to release the key. I had to let the spring flip the key by releasing it. I replaced the switch and it's fine. It sounds like you've just got a bad switch that has bent a prong inside the mechanism and now it bridges the "Start" when you switch from other positions.

It sounds like the problem happens when you use the switch. You can also try to temporarily wire 2 switches to substitute your ingition key switch. Ignition and start separately. This way you can see if the same action happens when you turn on the ignition and then "start". If it's not the switch then the same sort of problem should be duplicated in some fashion. Or, just change the switch with a new one.

Solved
 

Xack, thanks for the suggestion. I don't have switches etc at my disposal right now but I like the approach.

After running new wires to the amp with solid earth etc, nothing has changed. So I decided to install a relay for the ACC power to the fuse box, triggered by the ignition instead of a straight feed from the ignition switch. PROBLEM SOLVED. :cool:

In hindsight it wasn't a great idea to draw power for all accessories through the ignition switch and I still don't know where the leak is/was but after two days messing around I just don't care.

Thanks for all the suggestions - believe me it helps just knowing people are out there feeling the pain!

Time for a beer :D

Hey sambo
I've had this issue before on an old Datsun
I wired up a turbo timer and it happened to be in gear when I was testing it
Went from ign to off and car started and rolled back abit and I just got out of way

The problem was ignition key switch etc etc

The contacts in back of key was touching start as u turn key back

If your car is regoed then just fit a fancy start switch button in the dash

Thanks Jim. Sounds like you were lucky there! I'm all good now, no accessory load through the ignition switch seems to have fixed the problem.

Another thing might be some old switches have the resistance start wire
Which goes to coil + to bypass ballast resistor and that causes issues

But if u fixed it by running a relay sounds like feedback thru acc circuit
Only thing I run off acc circuit is stereo stuff
Everything else goes thru ign circuit
I run 50 amp wire for ign circuit and that normally does the job
Enough for wipers and heater and gauges and blinkers and rev
Everything else runs a relay

If u do get time to test it more
Pull out all acc fuses then add one at a time till u get motor cranking again when switching off
But this means putting it back to how it was when it played up

Good it's all sorted now