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Pre engaged starter solenoid power consumption

PostPosted: Tue Jun 30, 2020 8:11 pm
by RichardHawkins
I am fitting a pre engaged starter to my 1968 S4 Elan, and belatedly realise that the ignition switch is probably not up to the job of conducting the extra load.

Does anyone know how much current is drawn by acre engaged solenoid, the only thing I can find on the web is from an MGB forum quoting 40 Amps.

Richard Hawkins

Re: Pre engaged starter solenoid power consumption

PostPosted: Tue Jun 30, 2020 9:34 pm
by mbell
I would have thought a amp or two. Any decent relay should be enough to handle it.

If the starter is not on the car it should be easy to test with a multi meter. Just power the solenoid and measure the current flow.

Personally I prefer to keep the standard setup with starter solnoid and bridge the starter's solenoid straight to the starter feed from the standard solenoid. Then you don't have a live large un fused 12v lead hanging around the engine bay.

Re: Pre engaged starter solenoid power consumption

PostPosted: Tue Jun 30, 2020 9:47 pm
by HCA
mbell wrote:I would have thought a amp or two. Any decent relay should be enough to handle it.

If the starter is not on the car it should be easy to test with a multi meter. Just power the solenoid and measure the current flow.

Personally I prefer to keep the standard setup with starter solnoid and bridge the starter's solenoid straight to the starter feed from the standard solenoid. Then you don't have a live large un fused 12v lead hanging around the engine bay.


???
An amp or two? A;ittle bit more I think!
Testing a motor on the bench will only give you a no load amperage that is unimportant. You need full cranking load to test a motor.
Not sure where your unfused 12V wire comes in I am afraid.. :?

Richard, The full load - if it is 40A - does not go through the ignition switch. I question the 40A though! Personally, I would think more like 100+A

The starter motor has two electrical connections - one with a large ring connector that goes directly to the battery, unswitched, using a 120 - 170A cable. The other smaller connector is a spade terminal that goes to postion 3 of the ignition switch via a thin cable in the loom.. This connection draws maybe 8A that activates a solenoid that in turn connects the battery to the motor for as long as the key is held in position 3.

In answer, your ignition switch will be fine!

Who is installing your starter? Please ensure s/he understands how to remove or re-use the existing solenoid and extend the wiring to the new motor. There are a lot of important connections here that could cause trouble in the wrong hands :shock:

Re: Pre engaged starter solenoid power consumption

PostPosted: Tue Jun 30, 2020 10:27 pm
by ericbushby
My Powerlite starter solenoid draws 6 Amps, but as I have left the original bulkhead solenoid in circuit, it is irrelevant as the ignition switch is still handling the same current as It did with the lucas motor
Eric in Burnley
S3SE DHC

Re: Pre engaged starter solenoid power consumption

PostPosted: Wed Jul 01, 2020 1:46 am
by mbell
HCA wrote:Testing a motor on the bench will only give you a no load amperage that is unimportant. You need full cranking load to test a motor.


We testing the solenoid not the motor. It's fair point that the current draw when fitted might be different for the solenoid but a bench test will give you a ball park figure.

HCA wrote:Not sure where your unfused 12V wire comes in I am afraid.. :?


And

HCA wrote:The starter motor has two electrical connections - one with a large ring connector that goes directly to the battery, unswitched, using a 120 - 170A cable.


That one there you're describing....

Re: Pre engaged starter solenoid power consumption

PostPosted: Wed Jul 01, 2020 5:14 am
by Andy8421
mbell wrote:Personally I prefer to keep the standard setup with starter solnoid and bridge the starter's solenoid straight to the starter feed from the standard solenoid. Then you don't have a live large un fused 12v lead hanging around the engine bay.


This is by far the easiest solution. It keeps all of the original wiring intact, requires no additional relay, just a small bridging wire on the back of the new starter solenoid.

Not sure why anyone would go down a different route.

Re: Pre engaged starter solenoid power consumption

PostPosted: Wed Jul 01, 2020 8:55 am
by MarkDa
+1 to using existing solenoid to power both elements of new starter.
As others have said this manages ignition switch and contains permanent live cable.
Final bonus is that no new heavy cable needed.

Re: Pre engaged starter solenoid power consumption

PostPosted: Wed Jul 01, 2020 10:40 am
by HCA
You are right of course!

It is funny how people can have different ideas on 'easy' - maybe because I like autoelectrics, ditching a surplus item and extending cabling to achieve an uncluttered end result is my 'easy'! :D So, yes, if anyone wants a cra re-wired for free, bring it down here.. :lol: :lol:

Re: Pre engaged starter solenoid power consumption

PostPosted: Wed Jul 01, 2020 7:19 pm
by MarkDa
It's a result of the lockdown - we've all got to much time on our hands to find elegant solutions to problems we don't have :) :)

Re: Pre engaged starter solenoid power consumption

PostPosted: Thu Jul 02, 2020 3:07 pm
by collins_dan
Richard, Keep the power to the starter going through the solenoid. Search the posts as I posted about my experience with insufficient voltage when going direct from ignition switch. Worked fine for many years, but starting experiencing a problem with insufficient voltage to engage the pinion. It needs a good 12 .5 volts. Checked everything to figure out why the voltage was dropping from the ignition switch to the starter and couldn't find anything. The solenoid was already gone from the previous owner creating this set-up, so I added a relay as I would have needed to run a new cable from solenoid to battery as PO had cut off excess when running it directly to starter. Works great now. I don't recall the rating for the relay, but it is likely in the old posting as I just followed someone else's recommendation. Good Luck. Dan

Re: Pre engaged starter solenoid power consumption

PostPosted: Thu Jul 02, 2020 7:24 pm
by RichardHawkins
Thanks everyone.

I will be cautious and assume the current drawn is more than expected. I will use a relay as my solenoid on the bulkhead/firewall had already been removed.

Richard Hawkins