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MG MGA - WOSP Starter trigger current

I fitted a WOSP starter a few years ago and used the original starter switch to just switch the solenoid current. The starter switch, an NOS Lucas one, is playing up even with the lower current it has to handle and I am thinking of adding a separate switch in parallel to the starter switch. However, I cannot find any specification for the current drain for the solenoid trigger only so I can size my extra switch. Does anyone have any definitive information about the "trigger" current. I think it is potentially up to 40A at first contact possibly dropping to 10A.
My other option might be to drill a hole in the existing starter switch and try putting some switch contact lubrication onto the switch contacts.
John
John Francis

Hi John
I have been looking at various different online discussions about automotive starter solenoids and most quoted between about 6 and 8 amps current draw on the trigger side of the solenoid.

I would have thought that it probably isn't as much as that, but if you are concerned, you just need to install an amp meter in the trigger circuit to measure the current draw.

I recently bought on of those clamp meter types that will measure AC/DC voltage and amps.
I used mine to measure the starter motors current draw.

You could test the trigger amperage with the motor off the car, just fix the motor into a vice, then rig up an earth to the starter, finally a switched connection to the starters trigger terminal, you could use a simple lighting switch for that. No need to connect the main started cables.
Then, put the amp meter into the circuit ( or clamp the amp meter onto the cable). Then flip the switch to operate the solenoid and watch the meter.

Heres a link to a similar meter to the one I bought.

https://www.amazon.co.uk/KAIWEETS-Auto-ranging-Multimeter-Temperature-KC604

I did try a WOSP starter motor to my car (it was the LMS 002 model), but I couldn't get it to work on my MGA. It would engage and easily start the engine when I pressed the start button, but when I took my finger off the button, it would not disengage and the engine would continue to spin the starter motor.
I think this was because I have a sort of hybrid mix of engine back plates, HighGear early MGB type bell-housing and a 5-bearing engine, which causes the pinion gear to be mis-aligned with the flywheel starter ring.

So, I still have a standard inertia starter motor although I do use a push button on the dash combined with a starter solenoid, rather than using the original starter switch.

If you don't want to buy a clamp on meter John, you are welcome to borrow mine for a few days, it shouldn't cost much to post it to you, just post it back when you have finished with it.

Cheers
Colyn

PS attached a pic of the starter switch on my dashboard.



Colyn Firth

Sorry, the Amazon link didn't work, here is a picture of it though. You can cut and paste the link I posted into the Amazon website search box and this will open it.

The offer of loaning you the amp meter still stands though.
Colyn


Colyn Firth

Hi Colyn

Thanks for the suggestion. The difficulty measuring the current is that the initial current, which might be 40A ,may only last a few ms so it will be difficult to pick up without a storage oscilloscope. The hold current could be measured quite easily with a clamp meter. However, with everything on the car I'd ideally just like WOSP to provide the details. What rating is your dashboard switch?

I'm not sure I really needed a high torque starter but at the time I was having real difficulty getting the engine to start until I had corrected my error with the ignition timing. I was hoping the reduced current through the starter switch might help to make it more reliable and last longer.
John Francis

Colyn
Nice carpet!
Graham V

Most decent meters would have min/max (peak) settings.
If it's going to draw 6/8 amps as suggested by Colyn, I doubt it'd flick much past 20 initially
Maybe a little 40a relay in the solenoid circuit --That would take any excess load off the switch.

willy
William Revit

Colin and Willy

I have actually received this reply from WOSP today below. I have bought a 25A momentary switch and am wondering where to mount it as a back up. As I have a Derrington cross-flow arrangement my rear carburettor blocks access to the switch (and jolly nearly hits it!). I think I will increase the size of my trigger wire from the switch to the motor. The instructions didn't say it needed to be up to 20A. It really needs to be 1mm2 minimum for a 0.5m run from the original switch to the motor.

----------------------------------------------------------

Hi John,

It all depends on the unit but as a general rule of thumb on our automotive range:

In rush / pull current is 35A (fraction of a second).

Hold current is 12-13A.

I’d suggest 20A fuse. A starter solenoid relay is never a bad thing.

Kind Regards
Luke

Bespoke starting & charging systems – OE Quality – No Compromise.
“Engineered solutions to real problems”
WOSPerformance Ltd is registered in England & Wales (No.07987687) at:
Unit 8 Anglo Business Park, Smeaton Close, Aylesbury, Buckinghamshire, HP19 8UP, UK
John Francis

Cheers John.
Draws a bit, but nothing over worrying.
Yes a decent wire to drive the solenoid would be a good idea--or-a relay down close to the starter using the original trigger wire as it'd be drawing not much at all.
The 'usual' sign of needing a relay is poor solenoid performance with a hot engine as in just a clicking noise or no action at all when trying to start.

willy

Being a bit of a Derrington nerd, can I ask what serial numbers are on your head---just out of interest.
There's a few come out of the woodwork here lately.
Just did a .040" up 1800 with an early small chamber head converted to B valves with twin webers for a friend, worked out quite well.
William Revit

This thread was discussed between 02/07/2024 and 03/07/2024

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