Electrical issue?

[dave]

Evening all,

So I have stripped back my loom as far as I dare (pics to follow) but have a couple of small issues. When I first start her up I get a small spark between the inlet manifold and brake master cylinder as at the moment they are very close. Also where the engine earth connects to the frame it gets very hot, so hot that a tiny bright pin sized light glows and the plastic clamp that is securing it is melting. Any ideas chaps?
Thanks Dave

[kiwicanfly]

For the hot spot issue have you got a good metal on metal contact? No paint or anything between?

[dave]

No paint, still on a bare chassis it's only clamped with a plastic spring clamp.

[daydreamer]

I'm not an electrical, MX5 or Excocet expert but from 1st principles I suspect that the "effective" engine earth provision is inadequate.



If the earth cables are there and sized appropriately then check that each connection point is metal to metal and firm with no rust film, no rust inhibitor, no paint, no powder coat or other insulating material in the connection. The bolt or fixing device itself should not be the main connector, that can work but is less ideal.



If you are seeing a pin light glow at one of these connectors then the connection is not good, check it is bare shiny metal to bare shiny metal with a good amount of flat, firm contact, at least 1 square cm of bare metal to bare metal contact.



I think most modern setups would have several earth routes from the engine, normally one very significant earth strap capable of carrying the starting current for the starter motor. In my experience, others may have a much more extensive professional experience, that most significant earth strap is one piece that connects the starter motor case, engine-g-box, chassis and negative terminal on the battery. If this main earth provision was inadequate I would expect you to have difficulty turning over the engine or for the engine to "turn over" or "start to turn over" slower than expected.



If the engine starts, and I understand your does, then I expect the next major current will be the charging current from the alternator. This will often expect to get it's earth through the alternator bracket to the block and then through earth straps connected to the block, head , gearbox. If you have a powder coated, painted alternator bracket that paint/powder coat can interrupt the intended earth route and the charging current may route through a "too small" earth route. If the alternator has no earth route then it just won't charge and you'll get a red "no" charge light showing on the instrumentation " assuming your instrumentation " has that feature and you've not stripped that out.



Is there just one earth strap or is your glowing earth strap connector joint the one from the Alternator ?

[dave]

Hi,
No charge light coming on and checked to see if alternator is charging battery which it is. I think I'll give it all a good rub down and clean up and ensure it is secure. Would this explain a spark between the inlet and master cylinder as it is trying to earth there?

[daydreamer]

I speculate and assume as follows;

I am assuming that:
earth connection from engine starter motor case to battery negative terminal is NOT one piece. 

If you have the engine earthed to chassis and then battery -ve connected to chassis elsewhere then this explanation may make some sense

I speculate that:

RE: spark on start.
Start current is huge when the starter motor is activated and engaged. If the engine to chassis earth is constrained by a poor connection,  current demanded by the starter motor will try other routes , jumping through the air "Spark" takes a quite a demand , never seen it before in the scenario you describe but I imagine it is possible if the gap is small enough and the earth is sufficient to engage the starter motor get the cycle of current demand going through the starter motor.  A poor connection works ok at low loads but increases in resistance as it gets loaded ,   at some point the resistance through the air might be less than through the faulty earth route.

Re glowing earth connector
While running and immediately after starting, the alternator will try to charge the battery and supply other electrical loads.  This can also be a significant current, more than any fuse can reasonably take which is why the starter and alternator are two unfused electrical devices in any setup I have seen.
The charge light will go out with just the slightest trickle of surplus current from the alternator, so long as there is a fraction more supply from the alternator than all the electrical loads combined then the charge light will go out.
just after starting, the battery will take quite a bit of charging and the alternator charge circuit will be using the engine earth.  This constant current , if in excess the earth connector capability will lead to the connector getting hot and in extreme examples glowing, melting and catching fire. 

Now I move to fantastic speculation, consider this along with sun spots and spooks as the cause of your problem.

I suspect in this scenario there is an ironic acceleration of the connector joint heating process as current from the alternator is wasted heating the joint.  To an "intelligent" alternator charging controller that heated joint looks like an electrical load such as lights or heater fan and the alternator will produce more and more current , up to its maximum as the "intelligent" charging controller believes there is a large load to support.

All that speculation is of course based on quite a few assumptions , not least of which is that I know what I am talking about.
now it is late , have to take my meds and go to bed.

[dave]

😀😀 Thanks for the advice, will be cleaning all earth points and making sure they have good connections tomorrow and will let you know how I get on

[gwnwar]

Do you have the OEM ground strap from the battery going to the frame and and to the rear of the PPF near the diff. make sure the PPF and threads clean to bear metal. also the Ground strap from under the rear of exhaust manifold to the frame.. Check all grounds with OHM meter looking for a 0.00 reading. Make sure surfaces between the engine and trans,starter, alt. PPF to trans..

[dave]

Thanks for the advice guys, after giving every ground point a good clean up all seems well.