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MG Midget and Sprite Technical - electrical puzzle
|I have found a wire in the harness of my '74 midget that shows definite signs of getting hot. |
I noticed it while working on the engine bay a couple days ago and took time today to unwrap the harness so I could follow the wire and check for any other damage. Fortunately it is the only effected wire.
The part I cannot figure out is why it is even there. It runs from the center alternator post and connects to the solenoid post. It seems to me that it is redundant because there is a heavier wire performing the same function. The other thing that seems strange is why would this smaller wire connect so securely to the solenoid post while the heavier wire is only a push on fitting.
Then the question as to why it got so hot in the first place. I think it is because the heavier wire lost its good connection over time so the smaller wire had to carry the entire load but that is only a guess.
I sure would appreciate your thoughts on this subject,
I think you're correct about the reason for the overheated wire, in that the thicker wire has had a bad connection at some point leading to the thinner wire carrying more than its fair share of current. I believe the reason for having the thinner wire is to sense the voltage at the solenoid. On some of the earlier alternators the voltage at the solenoid was feed back via this thin wire to the regulator within the alternator. This meant that the regulator got an accurate reading of the voltage as opposed to sensing it via the thick wire. The problem being that as the current increases through the thick wire there's a voltage drop across it, this means the regulator gets an inaccurate reading. I think on later alternators the two connections are linked inside the alternator making this connection redundant.
|redundant reply deleted (Bob beat me to it)|
|You could open that plug in the photo and see how good the wire-to-90 degree connector is on all the wires|
And check wires and connectors at soloniod end
You can buy a new plug kit, plug and the three connectors but the kit I got was very cheaply made plus crimping them is awkward as they're 90 degree
|FYI, here is a simple schematic of the alternator circuit.|
As Bob noted, there are two wires between the Alt "B" terminal, and the "hot" side of the starter solenoid (called, "starter switch" in this image).
|The light wire is the one with the loop connector on the stud. The heavier wire is the one on the right side of the stud. The one with the spade connector to the left of the stud goes to the dash and back. And the one attached at the side in the middle is from the ignition switch.|
What I don't understand is that the smaller wire connects to a common on the switch and on the alternator. On the Lucas the two large 90 degree spades on the alternator are both common hot. In Norm's diagram both are connected to the B side. And the "I" side is to excite the alternator (wire to the right in the photo above).
In other words it sure seems that the two wires are parallel and connect to common connections at both ends.
I installed a 72 amp Lucas alternator in my TR6 which has the same terminal and plug configuration as this one in the Midget. The TR6 only runs one wire back and because of the increased capacity of that alternator I was considering adding a second wire like what I found in the Midget.
You may wish to have a look at the below link, If you scroll to the bottom you'll see the connections for the alternator. Looking at this and reading the paragraph below it would suggest the thick and thin brown wires in your plug have been transposed. If this is the case and you also had an old alternator in your car at some time in the past that had the external sense connection it would also explain why the thin wire was over heated.
|Bob, I do believe you have found the problem. My armature is currently in the shop so I will check that when I get the unit back.|
Thanks you guys for all your help.
|I had some time this morning and took a closer look at this.|
I found the nut on the solenoid post that tightens against the spade terminal plate was not tight. This could cause a condition where the current would be conducted more through the smaller wire causing the wire to heat when the battery condition called for a higher charge rate.
So, when the guy doing the work on my alternator gets back from deer hunting and does my alternator I will check the connections according to the article Bob provided.
The other thing is that the wiring diagram I have (purchased on e-bay) does not have that wire in it and I still wonder if it is needed, even Bob's article seems to infer that it is not. In that case I would change the wiring to reflect what Norm shows in his drawing.
This could be important.
In an effort to cover all bases I just found some new information. This is from my B/L Sprite/Midget Workshop Manual.
The wiring diagram for late '73 through '74 cars shows the wiring to be like what I have with the positions in the alternator plug the same as mine.
BUT, the wiring diagram for US import shows the wires position in the plug reversed.
This would seem to indicate at least the possibility that different alternators were used on US imports.
|Tom why not double up on the two connections with the same sized wires|
More modern users of Lucas alternators seem to all use the same size wire on both plug connections
The thin wire was as stated, to use when most alternators were "machine sensed", but this technology seemed to vanish quite quickly and almost all alternator regulators now rely on "battery sensing"
I simply doubled up on the wiring when I converted my car from dynamo to alternator and have had no such problems.
Both thick brown wires terminate on the battery side of the starter solenoid so the battery stays in the charging circuit
The twin browns can be seen in this picture taken during my wiring tidying up session a month or so ago (OK, not awfully tidy yet but you should have seen it before and this was only partway through!)
It's much prettier now though and at last the fuses are sorted too :-)
|Bill, while I was over-thinking this you state the obvious. And of course you are right!|
This thread was discussed between 19/11/2010 and 21/11/2010
This thread is from the archive. The Live MG Midget and Sprite Technical BBS is active now.