Lotus Elan and Lotus Elan Plus 2 Forums

Is anyone out there in a position to provide me with some comparison volts drop figures?
I'm measuring some figures which certainly seem to be excessive even after cleaning all connections. The worst so far 1.9 volts to the final bullet at the reversing lights. Given the amount of time those lights operate I'll not worry too much about that.
I'd like some comparison figures for the drop measured at the large positive terminal at the solenoid, the input at the ignition switch and the drop across the ignition switch. Thanks.

Happy to help, but when you say 'voltage drop', what exactly are you measuring, and relative to what?

1.9V certainly sounds like a lot, esp. on a low current type of use like a rear bulb : after making sure that the ground is not the major culprit, I would try to norrow down the location of the higher unwanted resistance with a series of measures from source to device, to see if a contact resistance along the way would have increased du to oxidation or carbon deposit (if not it may be that a copper wire is actually oxidized under the isolation due to water ingress, and/or damaged wires). A poor contact can develop into a hazard because of the heat it can generate.

If you are a Club Lotus member ask them for the ‘Finding Lotus Wiring Faults’ bulletin, it describes the most reliable way to trace poor or high resistance connections, I’ve attached an excerpt here. It’s hard to generalise but you would be looking for a 0.5v drop as a good result.

Iain

Excerpt here....

Gents, thanks for the response.
I've had the Club Lotus bulletin for a lifetime but this is the first time I've really used it in anger so to speak.
Using just the reversing lights as an example I was shocked to measure the drop at the last bullet 1.9 v.
Measuring through the circuit I'm losing almost half of that between the bulkhead and boot (proved by jury rig wire between bulkhead and last bullet). The drop measured a the ignition switch input is approx 0.3v and more than another 0.2v being dropped across the ignition switch. The bulletin suggests a maximum of 0.5/0.6v drop across an entire circuit. Adding extra load to the circuit causes the figures measured at the switch to increase with associated warming of the switch. The figures obtained at the rear lights and indicators are both higher than the bulletin suggests is acceptable but not outrageously so. Voltage drop across all grounds I've measured has been very satisfactory.
The request for comparison figures was to put my mind at rest. My fear is that this is indicative of issues throughout the entire loom. Rear loom can be purchased for approx £60 and to construct one would be easy enough and probably about half the cost. Dash loom on the other hand would be a bit more of a challenge.

Like you I was getting weird drops all over the place. All due to poor earthing, cables twiched together - even a 1.5mm@ house wire back to the brake lights twitched to the existing and oxidised wires all over the place! Lost temper and just ripped the whole lot out and rewired with thinwall with each item (other than starter) returned on a dedicated earth wire to a central point and a deicated feed to earth. Engine earth strap connection to chassis tinned as is battery negative.

An ideal 12V auto system should have no more than 3 or 4% drop at each appliance.

Measured this evening the sidelights, each wired on a dedicated 0.5mm2 thinwall, absolute minimal drop - unreadable on my meter anyway.