Ignition circuit

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myredscimitar
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Ignition circuit

Post by myredscimitar » Tue Jul 02, 2019 1:13 pm

Electrical numpty here, finally found a bit of time to look at the ignition on my 1400.

Symptoms: No ignition light.

Split the small wire from the loom to the alternator and checked:

Ignition off: 12V on Alternator side, 0V on loom side. Ignition light off, that's OK because with ignition off there is no circuit to light the ignition bulb. My limited understanding tell me the 12V at the alternator is wrong?? Should be 0V

Ignition on: 12V on Alternator side, 12V on loom side. Ignition light off, that's OK because now got 12V from both directions so no current flow.

Engine at 2000ish rpm, battery terminals show 14 ish V so alternator is charging

Then I had a numpty thought. Engine off, ignition on, still 12V at loom side of wire split. If I earth the loom side, completes the circuit, ignition light should come on, checks ignition bulb is OK.

So I earthed it. No ignition light, and lost the 12V at the alternator loom. Guess that means I've blown the bulb?? If I'd thought a bit more before acting the bulb must have been OK before I messed it up, otherwise there wouldn't have been 12V at the alternator loom.

Just when I'd convinced myself the fault was alternator end it looks like I've introduced a new fault, and one that means dash out to replace a bulb .... plenty of scope for screwing something else up at the same time. Often happens when I play with the electrics.

At least non of Mr Lucas best quality smoke escaped!!

Going to leave it a few days for more thinking,

Keith


1600 CVH SS1 1988
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The Scimitar SS1 is not so much a car as a lifestyle statement; it basically says "I'm mad, and I don't care".

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Ignition circuit

Post by philhoward » Tue Jul 02, 2019 2:51 pm

You can't blow the bulb by earthing one side of it.

Assuming we're on about the brown/yellow wire here, so (assuming plug is unplugged from the alternator - and apologies for repeating your tests,but want to make sure I've got it right):

Ignition off - wire should have 0V (which sounds correct).
Ignition on - wire should have 12V, but lamp off (as it has no return path). If so, the bulb (and wiring to/from it) is able to pass "some" current. This is microamps though as your meter requires virtually no current to read a voltage - hence why it might be a red herring.
Ignition still on, temporarily connect that wire to ground, lamp should come on (as you've made its return path, normally made through the alternator). Remove your temporary earth and the light should go out and you get your 12V back.

What it sounds like is you have a high resistance connection (which might have now gone totally open circuit). It could be a bad connection on the bulb holder, or the flexible circuit board that makes up the back of the instrument pack.


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Ignition circuit

Post by AJL Electronics » Tue Jul 02, 2019 3:12 pm

Keith if you get stuck, pop it down here and I will test it for you.


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Ignition circuit

Post by myredscimitar » Tue Jul 02, 2019 7:05 pm

Hi All
Thanks for the replies Phil and Andy.

If I can't get to the bottom of this myself might have to take up your offer Andy.

Just having another look. The bit that has me totally confused:

Ignition off, brown / yellow wire from loom to the small spade connector at the back of the alternator split, but connector still on alternator. I'm getting 12V on the alternator end of the split wire ( why???? ), 0V on the loom side ( as expected ).

My understanding of the system was:

Ignition turned on

current flow through ignition light ( lighting it up ), through the field coils in alternator ( setting up magnetic field ) and to earth somewhere in the alternator.

Start engine, alternator spins and produces 12V. This 12V supply connected to field coils so alternator now produces its own excitation current. Now both sides of the ignition light are at 12V so no current flows, no light on.

So, why do I have 12V at the alternator end when the ignition is off? Should I? Obviously with 12V already there when I turn the ignition on no current flows in the ignition light circuit because its at 12V each end.

Anyone spot the fatal flaw in my understanding??

Rechecking what I did this afternoon I found that the flying lead I used to earth the ignition circuit was faulty ( read one end not in electrical contact with the other!! ). Checked again with a better lead, now with ignition on and the loom side of the split yellow/ brown wire earthed the ignition light comes on just as I expected.

Beginning to firm up on the idea that my fault is with the alternator somewhere??

Keith


1600 CVH SS1 1988
1400 CVH SS1 1987
Previous GTE 6b 1983
RSSOC 5432
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Ignition circuit

Post by philhoward » Tue Jul 02, 2019 7:27 pm

I’m slightly confused by the “split” on the brown/yellow wire?

I thought it was a single wire from loom into the small spade?


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Ignition circuit

Post by scimjim » Tue Jul 02, 2019 7:28 pm

There’s a connector on the brown/yellow down by the exhaust.


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Ignition circuit

Post by scimjim » Tue Jul 02, 2019 7:29 pm

Sounds to me like a diode has gone in the alternator and you’re not getting an earth path through the small pin with ignition off?


Jim King

SECURE DRY STORAGE FOR YOUR SCIMITAR

Current: SE5 (8Ball), TI SS1 (snotty), 1600 SS1 (G97), 1600 SS1 (C686CCR), 2.5TD SE5a (diesel 5a), 6 x random other SS1s.
Previous: SE5, 3 x SE5a, 2 x SE6a, 3 x SE6b, GTC, 2.9i GTC, 3 x 1600 SS1, 1300 SS1, Mk1 Ti Sabre, Mk1.5 CVH Sabre
Chief mechanic for: 1400 K series SS1 (Megan3), 1400 CVH EFi SS1 (Grawpy), 1300 SS1 (Number One) & Sarah's coupe.
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Ignition circuit

Post by myredscimitar » Tue Jul 02, 2019 7:39 pm

philhoward wrote:
Tue Jul 02, 2019 7:27 pm
I’m slightly confused by the “split” on the brown/yellow wire?

I thought it was a single wire from loom into the small spade?
Sorry Phil, that's me not being clear.

As you say it was a short single wire from the loom to the spade, but I split it and crimped a spade connector in to make it easier to get at and test.

Keith


1600 CVH SS1 1988
1400 CVH SS1 1987
Previous GTE 6b 1983
RSSOC 5432
The Scimitar SS1 is not so much a car as a lifestyle statement; it basically says "I'm mad, and I don't care".

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Ignition circuit

Post by AJL Electronics » Tue Jul 02, 2019 7:43 pm

Diode pack in the alternator is faulty.


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Ignition circuit

Post by myredscimitar » Tue Jul 02, 2019 7:51 pm

That's two votes for dead diodes then. I had just about convinced myself the alternator was at fault, but then got sidetracked by my non functioning flying lead.

What alternator do I need then?

Don't even think about suggesting changing diode packs, tried that once a few years ago ( when I had the 6b I think ), bought a heavy duty soldering iron specially to do the job, eventually burnt out new soldering iron without melting much solder, gave up and bought a recon alternator.

Keith


1600 CVH SS1 1988
1400 CVH SS1 1987
Previous GTE 6b 1983
RSSOC 5432
The Scimitar SS1 is not so much a car as a lifestyle statement; it basically says "I'm mad, and I don't care".

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Ignition circuit

Post by AJL Electronics » Tue Jul 02, 2019 8:04 pm

Is it CVH or Ti? I can't remember.


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Post by AJL Electronics » Tue Jul 02, 2019 8:16 pm

There is an auto electrician on the Innsworth Technology Park who could refurb it for you. Can't recall their name at the moment. They are hidden down what looks like a back alley.


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Ignition circuit

Post by AJL Electronics » Tue Jul 02, 2019 8:20 pm



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Ignition circuit

Post by philhoward » Tue Jul 02, 2019 8:28 pm

myredscimitar wrote:
Tue Jul 02, 2019 7:39 pm
philhoward wrote:
Tue Jul 02, 2019 7:27 pm
I’m slightly confused by the “split” on the brown/yellow wire?

I thought it was a single wire from loom into the small spade?
Sorry Phil, that's me not being clear.

As you say it was a short single wire from the loom to the spade, but I split it and crimped a spade connector in to make it easier to get at and test.

Keith
Mystery solved :D

Another vote for a duff diode pack here as well.


Phil Howard
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SS1 1600 Rooster Turbo; Sabre Mk1.5, Sabre Mk2
Previous: SE5/5a/SS1 No.1/SS1 Rooster/SS1 1800Ti/SE5a 24 Valve
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Never try and argue with an idiot. They drag you down to their level, then beat you based on experience.

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Ignition circuit

Post by myredscimitar » Tue Jul 02, 2019 8:33 pm

This the one I want?



Looks about the same, listed for all sorts of Escorts and Fiestas of mid 80's vintage.

Keith


1600 CVH SS1 1988
1400 CVH SS1 1987
Previous GTE 6b 1983
RSSOC 5432
The Scimitar SS1 is not so much a car as a lifestyle statement; it basically says "I'm mad, and I don't care".

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