Starter Motor Circuit Disaster
Discussion
Hi Guys,
Thought I was just putting the finishing touches to my rebuild and I went for the first start and I've had a bit of a disaster.
Everything electric was working (fuel pump priming fine etc). When I cranked the engine over it sounded very lazy like it had a flat(ish) battery. It would only crank for about 5 seconds before stopping. I thought it might still catch so I carried on (for rather too long it turns out). Smoke started coming from the passenger footwell and I've melted some wires.
The melted wires come from the negative battery terminal and are spliced into two black wires of the loom immediately after the ECU. The loom looks like it might be ok but I guess the spliced in section was an inferior grade of wire. I assume this will be wiring for the alarm since it looks like an after-thought. I'll post some pictures later as I'm at work right now.
I've now checked the battery and it was reasonably fully charged.
I'm not sure what the cause is but I would guess it's an earth that isn't working properly (maybe the new chassis powdercoat is insulating it).
If the engine was too stiff for the starter to turn over; could that cause this effect or would the starter just not turn it over? Not sure why this would be but this was my first full engine rebuild.
Anyone any thoughts of the likely cause? What should be the normal path for electricity to return from the starter to the negative battery terminal?
Cheers,
Simon (feeling somewhat deflated)
Thought I was just putting the finishing touches to my rebuild and I went for the first start and I've had a bit of a disaster.
Everything electric was working (fuel pump priming fine etc). When I cranked the engine over it sounded very lazy like it had a flat(ish) battery. It would only crank for about 5 seconds before stopping. I thought it might still catch so I carried on (for rather too long it turns out). Smoke started coming from the passenger footwell and I've melted some wires.
The melted wires come from the negative battery terminal and are spliced into two black wires of the loom immediately after the ECU. The loom looks like it might be ok but I guess the spliced in section was an inferior grade of wire. I assume this will be wiring for the alarm since it looks like an after-thought. I'll post some pictures later as I'm at work right now.
I've now checked the battery and it was reasonably fully charged.
I'm not sure what the cause is but I would guess it's an earth that isn't working properly (maybe the new chassis powdercoat is insulating it).
If the engine was too stiff for the starter to turn over; could that cause this effect or would the starter just not turn it over? Not sure why this would be but this was my first full engine rebuild.
Anyone any thoughts of the likely cause? What should be the normal path for electricity to return from the starter to the negative battery terminal?
Cheers,
Simon (feeling somewhat deflated)
Disconnect the battery negative terminal and check resistance between it and the chassis. Then between it & the block..
Should be minimal, be warned that is not an acid test as the continuity can break when put under load.
You really need to fuse any extra grounds taken off the battery just for this reason BTW.
Should be minimal, be warned that is not an acid test as the continuity can break when put under load.
You really need to fuse any extra grounds taken off the battery just for this reason BTW.
Hedgehopper said:
Have you got a battery master switch? If you have and its turned off then your starter will try to use the two black wires you mention as an earth path....and will get very hot.
I don't think I have a battery master switch - at least I don't have one of the big obvious aftermarket ones sat on the terminal or in the footwell anywhere. I presume there isn't a factory switch hidden somewhere...Now I've more calmly assessed the damage things don't look as terrible as I thought.
This is the starter motor wiring which I'm assuming is correct. The thick red cable (hidden by the chassis) is attached to the same terminal as the many brown wires.
This is the start of the disaster zone where the two black wires are attached to the negative battery terminal.
Thankfully all the melted wire is in the footwell and doesn't continue far into the main body of the loom. In this shot the ECU is just out of frame to the left. Two thin black (undamaged) wires come out of the ECU and have the two melted wires from the negative terminal spliced into them. Then there's a short melted section before the two wires join into one thicker (undamaged) cable that disappears off into the loom.
Does this shed any more light on the cause? I don't know what this wiring is for but it looks like an amateur addition.
Am I still looking at the engine earth strap as the cause? I can't really see anything to suggest it wouldn't be fine.
spend - what fuse should go into these cables?
This is the starter motor wiring which I'm assuming is correct. The thick red cable (hidden by the chassis) is attached to the same terminal as the many brown wires.
This is the start of the disaster zone where the two black wires are attached to the negative battery terminal.
Thankfully all the melted wire is in the footwell and doesn't continue far into the main body of the loom. In this shot the ECU is just out of frame to the left. Two thin black (undamaged) wires come out of the ECU and have the two melted wires from the negative terminal spliced into them. Then there's a short melted section before the two wires join into one thicker (undamaged) cable that disappears off into the loom.
Does this shed any more light on the cause? I don't know what this wiring is for but it looks like an amateur addition.
Am I still looking at the engine earth strap as the cause? I can't really see anything to suggest it wouldn't be fine.
spend - what fuse should go into these cables?
The fact that the problem occurred while you were cranking seems to point to the current that is flowing to the starter while it is cranking.
Definitely check the resistance of the earth cable to the case of the starter motor and make sure it's not too high. i'd do that as an end to end check to make sure all the points in between have good continuity along the whole path.
One observation on the picture of the earth terminal is that it looks like it could do with a bit of a clean up and maybe a coat of vaseline. I think the earth from the battery goes to the earth point underneath the console, check that is fully tightened up if it has been undone.
As I don't know ht extent of your rebuild I'd also recommend checking the earth strap between the engine and the chassis.
Definitely check the resistance of the earth cable to the case of the starter motor and make sure it's not too high. i'd do that as an end to end check to make sure all the points in between have good continuity along the whole path.
One observation on the picture of the earth terminal is that it looks like it could do with a bit of a clean up and maybe a coat of vaseline. I think the earth from the battery goes to the earth point underneath the console, check that is fully tightened up if it has been undone.
As I don't know ht extent of your rebuild I'd also recommend checking the earth strap between the engine and the chassis.
Yeah it is an earth problem. The starter is drawing many Amps and that has to get back to the battery and will return anyway it can. If your main earth is high (ish) resistance then it will take an alternate path back to the battery, in this case your hacked wires.
I suspect that the main problem is not the hacked wires but the main earth connection. Possibly though the problem with your main earth has shown up an error in the way the hack as been wired.
To check the resistance, remove the battery neg cables and separate them then test the resistance to the clamp from the engine block, I.e. remove those extra small wires from the battery clamp before measuring. Thing is that an ohm meter (DVM) can not tell if a connection can handle high current, if you measure as is it will probably appear to be OK. What you need to know though is what is the resistance on the main high current cable, and I suspect this is your original problem, as said probably due to paint on chassis. Once this is sorted out then worry about the bodge wires.
I suspect that the main problem is not the hacked wires but the main earth connection. Possibly though the problem with your main earth has shown up an error in the way the hack as been wired.
To check the resistance, remove the battery neg cables and separate them then test the resistance to the clamp from the engine block, I.e. remove those extra small wires from the battery clamp before measuring. Thing is that an ohm meter (DVM) can not tell if a connection can handle high current, if you measure as is it will probably appear to be OK. What you need to know though is what is the resistance on the main high current cable, and I suspect this is your original problem, as said probably due to paint on chassis. Once this is sorted out then worry about the bodge wires.
So sorry to here of your disaster Simon. As stated by others it is definitely an earth return issue. ALL chassis earths MUST be solidly fixed and be in good contact (free from paint & corrosion) with the chassis. Where there is a stud mount (i.e. return from starter onto chassis) run a die over the thread and clean up thorougly, where a bolt through hole or another mounting clean up the mating surfaces thoroughly including the crimped ring terminals on the earth cables. Best of luck Pete
Not sure but there was a previous post on this same issue, from deepest Essex. Come on in Mr Essex. Understand from sketchy memory now it was rectified by re insulating the wires. With or without checking the earths? I have checked the heat on my wires on start up. No worries. Pete how is the brake servo project coming on, just hoping?
EGB said:
Not sure but there was a previous post on this same issue, from deepest Essex. Come on in Mr Essex. Understand from sketchy memory now it was rectified by re insulating the wires. With or without checking the earths? I have checked the heat on my wires on start up. No worries. Pete how is the brake servo project coming on, just hoping?
Hi mate, brake servo project took a back seat over the winter unfortunately. Had Griff off the road carrying out a complete suspension and wishbone refurbish. Also got wrapped up in designing and having made bush heat shields and various spacers, plus looking into designing a more accurate engineered mechanism to adjust the camber (those crude sliding top mounts!!). Anyway, I uncovered the servo parts over the weekend and now the Griff is back on the road and home jobs have subsided, I intend to satrt looking at this again. All the best, PeteHi,
I had a similar problem many years ago when I built an MGB based kit car, on the first start it earthed back through the choke cable and melted the plastic cover. Turned out I had left off an earth strap from the engine block around the rubber engine mounting blocks (which were effectively insulating the block), and back to the chassis.
Regards
Nick
I had a similar problem many years ago when I built an MGB based kit car, on the first start it earthed back through the choke cable and melted the plastic cover. Turned out I had left off an earth strap from the engine block around the rubber engine mounting blocks (which were effectively insulating the block), and back to the chassis.
Regards
Nick
Just to provide a happy update; the car is now electrically fine and started first time (for the first time in nine months).
Massive thanks to Shane Cooper at Motosparkz for coming over to replace the wiring and run some diagnostics.
It seems it was the new powdercoat on the chassis which had coated the insides of the engine mounts as well as the outsides. On a pre-cat, the OS engine mount is where the main engine earth goes to. Having cleaned that up and replaced the melted wiring, all was well with the electrics and she's suffered no other side effects. Just a few more little bits to tidy up over the bank holiday weekend and we should be good for our MOT on Tuesday.
Thanks for all your replys as always, they were very helpful. I've now got two big earth straps going directly from block to chassis. It's overkill I know but as LongBaz said, the car has never cranked over as fast as it does now.
Cheers,
Simon
Massive thanks to Shane Cooper at Motosparkz for coming over to replace the wiring and run some diagnostics.
It seems it was the new powdercoat on the chassis which had coated the insides of the engine mounts as well as the outsides. On a pre-cat, the OS engine mount is where the main engine earth goes to. Having cleaned that up and replaced the melted wiring, all was well with the electrics and she's suffered no other side effects. Just a few more little bits to tidy up over the bank holiday weekend and we should be good for our MOT on Tuesday.
Thanks for all your replys as always, they were very helpful. I've now got two big earth straps going directly from block to chassis. It's overkill I know but as LongBaz said, the car has never cranked over as fast as it does now.
Cheers,
Simon
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