Voltage drop

[seniorchristo]

Battery voltage registers 12.6 Volts at both the battery and at the solenoid with ignition OFF. With ignition ON it drops to 12.1 V. Voltage at the horn is only 11.8 Volts which is not enough. With the ignition on, the only draw seems to be the electric fuel pump. Are these drops normal?
Thanks
Chris

[Elan45]

Chris,

I'm gonna guess your battery has some time on it. I'll bet you've got a cell that is weak.

Take the battery out of your daily driver and replace the Elan battery with the one from your DD. Take the same measurements and I'll bet it will stay up.

Roger

[seniorchristo]

Roger
I think you might be right. I will give it a try tomorrow. Thanks
Chris

[Andy8421]

Battery voltage registers 12.6 Volts at both the battery and at the solenoid with ignition OFF. With ignition ON it drops to 12.1 V. Voltage at the horn is only 11.8 Volts which is not enough. With the ignition on, the only draw seems to be the electric fuel pump. Are these drops normal?
Thanks
Chris

If the points happen to be closed, you will also be powering the coil when you switch the ignition on. That's about a 4 amp load for a standard Lucas coil.

Still shouldn't give the voltage drop you mention if you are measuring across the battery terminals in the boot. As Roger says, that is more likely to be a clapped out battery.

If you have the airhorns / compressor setup, that never works very well (at least mine doesn't) if the engine isn't running and the dynamo / alternator isn't dragging the battery voltage up a a bit.

[nmauduit]

a common issue is surface oxidation at the junction of two connectors (esp. ground to chassis in the trunk) : this creates a contact resistance which does let a trickle current through, enough to show decent voltage while almost nothing is pulled from the battery, but does amper proper voltage to be maintained when more current is drawn. Maybe worth checking if thh battery is known good (similar behaviour can happen within an old battery internals over time)...

[alan.barker]

Bypass Ignition Switch and see what you get
Alan

[oldelanman]

Bypass Ignition Switch and see what you get
Alan

+1
Mine was causing voltage drop issues ....

[alan.barker]

Thanks Oldelanman,
it's nice that someone has the same idea.
I had this problem on my Classic TVRs.
Alan

[oldelanman]

Thanks Oldelanman,
it's nice that someone has the same idea.
I had this problem on my Classic TVRs.
Alan

My problem was that the indicators would slow or stop altogether when the brake lights came on (not ideal at road junctions), spent ages going through connections and earths before I eventually narrowed it down to the ignition switch.

[alan.barker]

Funny you say that on my Sprint the Indicators have the same problem.
I think i will change the Ignition Switch
Alan

[seniorchristo]

Connecting my truck battery gives similar results. When the car is running and charging between 13.2 and 14 volts the air horn struggles but works ok. The voltage drop could be the ignition switch so I'll give that a try. I dread any work under the dashboard . Thanks for everyone's help.
Chris

[alan.barker]

Don't forget it's better with a Relay for the Air Horns.
Alan

[RichardHawkins]

I am no electrical engineer, however I have noticed that many of our problems turn out to be a poor earth somewhere. Perhaps Bill Will could help.

Richard Hawkins

[h20hamelan]

how difficult is a voltage drop test

https://www.calculator.net/voltage-drop ... &x=64&y=21

can you measure against grounds on the frame too, as opposed to battery terminal

or a draw test

https://sciencing.com/calculate-amperag ... 28646.html

i suspect when you find the coil lit up, and some losses elsewhere. You will choose a big old harness refresh. The length of the battery wires (are they original-maybe time for new terminal post connections?), combined with other draws... I bet a can of WD40 and some dielectric could be of benefit. Have you added any grounds to the rear turrets, new fuse block, etc?

nmauduit summed it up, thought ied say it again. as I am about to go into dash and upgrade most heavy use wires and add relays with fuse blocks, extra grounds etc.

[2cams70]

I agree with the ignition switch theory. Look into that first. Pointless rewiring the whole car just for the sake of a couple of poor terminals or a dodgy ignition switch. Copper wires in themselves don't degrade.