ELECTRICS QUESTION Lotus Elite S2

[Howard Cox]

Hi folks. Can someone help me with this please.

To me it defies logic. The voltage disappears when connected to a test bulb.

[EXCEL V8]

How accurately are you measuring voltage?  At 11.8v the battery is virtually flat; at 12.0v not quite flat.  With a battery in that state of charge the smallest interruption in the earth path might be enough to extinguish the light.  I'd charge the battery and test again.

Pete

[Howard Cox]

Hi Peter, thanks for your reply.

I’m measuring voltage with a multi meter.

I should say the battery in my diagram is the battery fitted to tge car, not a separate test battery and the output measured is 12.4 v direct off the terminals.

I find it hard to believe that just swapping supply source of 12.4v at the +ve at the battery to 11.8v supply from the harness.

Also, when connected from the harness supply and the bulb doesn't light up, the meter reading changes to zero.
All voltage is lost.

[EXCEL V8]

How many harness connections have you tried?  Does the problem occur at all harness connections or just those, say, furthest from the battery?  What happens at the engine bay end of the large positive feed cable?  Have you looked for contemporaneous parasitic drains on the battery?

Pete

[Howard Cox]

Just the one Peter.

The bulb in question is the reversing light bulb (21w) The harness connection to that particular bulb is 11.8v. If connected to a 21w test bulb and straight to Earth terminal, bulb won’t work. If I connect the bulb to the positive terminal bulb works.

All winkers work ok, All brake lights work ok, all side lights work ok. Both headlights and beam work ok. (Now the lift motors have stopped, working 2 days ago) ugh !!

Neither reverse lights, neither fogs work (though at various tests one of each reverse and fog have worked)

Driving me nuts.

 

[EXCEL V8]

Can you get to the electrical connectors on the reverse light switch?  Try connecting to the positive side of the switch and then to earth - that may tell you if the switch is dodgy, or the wiring from the switch is dodgy.

Pete

[Howard Cox]

Ok thanks Peter, will try tomorrow 👍

 

[Clive59]

you have a high resistance point somewhere. for a 21W bulb, that means I = 21W / 12 = 1.75A

That is an equivalent bulb resistance of R = 12/1.75 = 6.8R      so  if you have only a few ohms resistance somewhere, it will seriously drop voltage across the bulb.

Could be a crimp connector, oxide layer on a connector, etc...

A multimeter has an input resistance of a Megohm or more, so the current it draws is insignificant if you measure the volts at the bulb end, so no IR drop.

[Howard Cox]

Thinking on what you suggest,  the wiring from the reversing light switch goes to the connector for the rev light bulb, which is reading 11.8v. This voltage appears when I select reverse, (suggesting the switch works)  but disappears when I connect a test lamp to Earth. Can't seem to get my head round this, must be missing something. But will try from inlet of switch to Earth via test lamp…….if I can get under the car.

I know the Fuel Flow lock valve is working (it's new, I put it in) and it's branched off the reversing light switch inlet as per wiring diagram.

[Clive59]

The actual reverse switch could be the culprit, just gone high resistance, by a few ohms. The fact you have a solid 12V at the switch and doesn't fail when fuel lock is on suggests this.

 

[Howard Cox]

Thank you Clive for your input. Just realised you're not the previous contributor. Apologies. Will investigate tomorrow. 

[TAR]

Are you testing this with just a bulb or are you using the bulb holder as well? They are notorious for getting oxidised and causing connectivity issues. 

When you say the voltage disappears, do you just mean the bulb won't light or you measure zero volts on the multimeter?  

[Howard Cox]

Hi Tim,

using test bulb in a bulb holder. Lights fine when connected directly to battery. When fed from harness connection (11.8v) will not Light. Put a multimeter in line and reads 0v.

[EXCEL V8]

I had a similar situation some years ago with a night storage heater - had 240v at the socket but no heat!  Turned out to be a dodgy wire leading to the socket.  I'm sure that's what you'll find eventually.  Can't really be anything else.

Pete

[TAR]

10 hours ago, Howard Cox said:

Put a multimeter in line and reads 0v.

A multimeter set on a resistance scale must be connected in parallel not series. If you're connecting in series it definitely wont light the bulb 

do you have a voltage drop at the reverse switch? There should be a green wire feeding 12v into it and a green/brown wire feeding the reverse lamps from the switch.