Lotus Elan

Headlight Failure

PostPost by: simonriley11 » Fri Mar 24, 2006 8:35 am

On my 1973 Plus 2 130/5 after about 1 1/2 hours of running with the headlights on they and the sidelights cut out completely (and the headlights which are the failsafe type also retracted). Everything else was working normally. I checked the fuses (on the dash) and nothing was wrong. Then after about 2 minutes there was a just audible 'click' from behind the dash and the lights worked again. We ran for a further 5 minutes or so and the lights cut out again. We then waited and again the 'click' after a few minutes and we were back in business.
I've read somewhere about a thermal cut out or something, is that my problem and what can I do to fix it?
Thanks in advance for your help
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PostPost by: rgh0 » Fri Mar 24, 2006 9:14 am

Sounds like the thermal cut out to me. If you have high power headlight globes fitted this can cause the problem. It can also be just due to age of the thermal cutout.

How you want to fix it depends on your preferences about how to redesign the Lotus electrics. But the right selection of relays and fuses eliminates the problem

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PostPost by: simonriley11 » Fri Mar 24, 2006 11:11 am

Just spoken to Keith at the ever helpful Paul Matty's. They suggested that it could be dirty connections in the loom for the headlights/sidelights causing resistance and therefore extra current consumption and so the cut-out has actually done it's job. Does this ring any bells with anyone before I spend hours messing about with the wiring loom, or is it more likely to be that the cutout (almost certainly the original and so 33 years old) is tired and just cut in too soon.
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PostPost by: rgh0 » Fri Mar 24, 2006 11:56 am

High resistance due to dirty connections causes lower current flow and dimmer headlights. This would make the thermal cut out less likely to operate not more likely. If you bulbs are standard wattage and the circuits have not been played with to add extra load to the head light circuit in someway then it sounds like a tired thermal cutout.

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PostPost by: poiuyt » Fri Mar 24, 2006 5:06 pm

If there are resistance connections in the viciity of the thermal cutout it may generate heat and activate the cutout

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PostPost by: john.p.clegg » Fri Mar 24, 2006 5:35 pm

Steve
Grasping at straws with that one aren't we?

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PostPost by: denicholls2 » Fri Mar 24, 2006 9:04 pm

I don't think Steve's grasping at all. While Rohan is right that the headlamp brightness is as good a relative indicator of current flow as an ammeter, the low current flow can still generate large amounts of heat.

Arc welding is based on the heat generated by a barely open circuit, for example.

Most electrical fires are caused by the heat associated with bad connections and the need to jump an arc rather than conducting full current which would generate little or no heat. Any heat thus created is applied to the wiring and conducted through it. The closer this is to the thermal switch, the greater the effect. And hopefully the switch opens before the insulation melts.

When my Europa radiator fan connections were bad, the problem manifested itself as an override switch too hot to touch. Likewise, when the bullet connectors feeding the headlamps grew tired and the lamps dim, I knew I had found the problem area when I reached the place where the wire became too hot to hold.

A suggestion I've found helpful is to use dielectric grease (Vaseline) on all bullet connections. You'd be surprised how many problems can be made to disappear just by basic cleanup and this attention.

Another is to NEVER uprate your headlamps or any other current draw in a Lotus without first uprating the wires that feed them. The original application was marginal when new and is now 30-some years old. Adding demand to a weak electrical system is a great way to turn your Lotus into a molten pool of resin.

A common form of unwitting temptation of fate is uprating your charging circuit with a larger than original alternator. When your new 75-amp alternator puts the ammeter on the pin if the battery is a bit flat, what do you think is going on with the wires designed to carry no more than 35 amps and any bad connections along the way?
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PostPost by: rgh0 » Sat Mar 25, 2006 12:43 am

Steve

I agree that if the high resistance connection was where the wires joined on to the thermal cutout then the heat generated in these joints could heat up the cutout itself and cause it to trip early. A high resistance point anywhere else in the circuit would not affect the cutout.

Another potential cause would be if the high resistance point was the switch contacts inside the cutout itself getting dirty. The cutout has a bimetallic strip that heats up with the current flow and trips at a certain temperature which correllates normally with a certain current flow. The click is the the bimetallic strip flipping the contacts from closed to open or visa versa. Dirty contacts would cause extra resistant and also feed extra heat into the bimetalic strip cuasing it to work at a lower current flow. Spraying some electrical contact cleaner inside the cutout may fix the problem with any dismantling.


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PostPost by: john.p.clegg » Sat Mar 25, 2006 6:50 am

I still think it's a long shot.....but maybe worth a try,,,,

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PostPost by: M100 » Sat Mar 25, 2006 9:38 am

The only wiring diagram I have of the +2's shows just fuses which if sized correctly to the cable will protect the circuit just fine. Why Lotus saw the need to add a thermal trip surprises me.

How is the cutout wired? Is in in the common from the dip switch before the circuit splits for the individual lamp fuses?

There are plenty of thermal cutouts available (about 6 quid + vat) that would probably fit. Does anyone know what the original trip setting is?
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PostPost by: JJDraper » Sat Mar 25, 2006 6:25 pm

Sorry to be a bit dim (!), thermal cut out? Where? I don't remember seeing one anywhere on my +2, but it is a bit of an oddball when it comes to wiring and may not have one. My only experience of a thermal cutout is the headgasket......

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PostPost by: ianf » Fri Apr 07, 2006 8:55 pm

Simon,

Sorry if this one is dead but sounds like another earthing possibility.

Ian
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