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Starter Motor. Amps?

Thread starter #1
I was testing an old starter motor that I know used to work well.
I connected it to a car battery with about 20-30 amp cables and the insulation just melted. There was no mechanical load on the starter.

I also connected an analog amp meter to it.
Just the solenoid was using 15 amps.
But the whole starter motor was using somewhere near 100 amps.

I don't think it was spinning very fast.

Is this normal?

Dean Huster

Well-Known Member
No. Disclaimer: I haven't the foggiest idea what the actual currents are on these devices. However, a starter motor with no load should spin like a banshee (however they spin) and be a fairly low current, my guess would be in the area of 10 -20 amps at the most. Under load, the current will vary with the temperature (how loose the engine is), starter motor type and engine size. Expect the current on a warm engine to be in the area of maybe 100 amps, on a cold winter day closer to 150 - 200 amps. Fifteen amps for the solenoid (contactor) coil seems high to me.

Oh, and this is for a 12-volt system. On an older 6-volt system, expect double the current draws.

I think you may have a bad starter motor.



Super Moderator
Most Helpful Member
Screech said:
I connected it to a car battery with about 20-30 amp cables and the insulation just melted.
Because the cables were too thin for 100 amps.

Screech said:
Just the solenoid was using 15 amps.
But the whole starter motor was using somewhere near 100 amps.
My best guess is that would be about right.
Starter motors are usually series wound, take a lot of current and spin like banshees when there is no load.

Screech said:
I don't think it was spinning very fast.
Because a lot of volts were used to warm up your thin cables.

Screech said:
Is this normal?
Yes, I think it is.



Well-Known Member
Dean Huster is correct !
A starter motor in good condition with no load on it should run at about 20 to 30 Amps at a very high speed. The back EMF will be high hence a low current is drawn from the battery.

It looks that the starter motor under test by Screech could be poleing, has a worn bearing, burnt commutator or shorted field or armature winding and drawing excess current.
Screech should check if the pinion gear and shaft will turn around freely.

A starter motor from a normal 2 Litre petrol car draws between 60 and 200 Amps when turning over the engine when the oil is warm and thin.

Under winter conditions, this current can easily double when the oil is thick.

Diesel engines have a very high compression ratio e.g. 22:1 and require more powerfull starter motors. On average they draw between 300 and 500 Amps for average sized car engines 2 to 3 litre while on truck engines the current could easily reach 1000 Amps at initial turn over.

more to follow ( dinner ) :D


Well-Known Member

Check for suspect smell of burning or quick heating up which may be your problem.
Next step is taking it all apart, and take good note of it where it came from, can use a digital camera and take piccies as you go, be prepared for dirty hands from carbon dust, and old grease.

Normally a starter motor has a rating on it expressed in HP or kW. and direction of rotation, embossed in the stator housing.

Different types of starters are in use too.

Bendix: the pinion gear engages itself into the flywheel ring gear, by the slow inertia when the starter current flows. (used on older English cars, Lucas)

Forced engagement: the solenoid is mounted above the starter motor and assists in meshing the pinion prior to closing the contacts and allowing full current to flow. ( Petrol and diesel engines )

Shifting armature: the whole armature shifts and engages the pinion in one or two different stages. ( diesel engines, Bosch )

Reduction gear starter: Used over the last 20 years or so. A relative small high speed starter motor drives the pinion via an intermediate gearwheel, and has very good torque for starting and less demand on the battery. ( used on mostly Japanese cars and trucks )

Dyna starter: Comination of alternator/starter used on motor bikes.

Testing a starter motor is done on a testrig on which the starter is securely clamped, the correct meshing gear is engaged and a brake can be applied to increase the load on the starter to blocked armature condition.

Volt, Ampère and Tachometer are on the test rig to check current flowing, voltage across terminal and speed of the pinion.

The tests have to be done reasonably quick to avoid excessive heating of the windings affecting the test results.
Also good and clean connections are a must !
The resistance in these circuits are between 0.1 and 0.01 Ohms.

Ok that is my $0.05 worth.

Regards, Raymond
Thread starter #6
Screech should check if the pinion gear and shaft will turn around freely.
It takes an effort to turn the shaft by hand.

I'll do some more testing when I find some thicker cables.

Thanks guys.


New Member
Screech said:
It takes an effort to turn the shaft by hand.

I'll do some more testing when I find some thicker cables.

Thanks guys.
But first find out the reason why it doesn't turn freely! One reason could be a worn out bearing and the rotor has mechanical contact with the stator. The gap between both is normally some 1/100 of a mm.
were the cables " jumper " or "booster" cables? the kind with the clamps on the end. if so i have seen some cheap ones that have insulation about a half inch in diameter to make it look like a heavy gauge wire but in reallity there is only like a twelve gauge wire inside.
Just the solenoid drawing 15 amps? Do you know what the voltage of the starter is? Maybe it's an old 6 volt and you're putting 12volts on it..
I have tested Chevrolet starters (unloaded) using 16 gauge wire without a problem, granted I don't run it for more than 20 seconds or so, but no melted wire..


Well-Known Member
I tried to measure the solenoid current on a small car starter motor.

The car is a Fiat Cinquecento, 900 cc, 4 cylinder. (about 55 cubic inches)

The starter motor is not geared, but it is a pre-engaged motor, where the solenoid engages the motor with the flywheel before the full current hits the motor.

I put an 0.47 :eek:hm: resistor in series with the solenoid and measured the voltage drop. There was a drop of about 6V and it didn't operate.

That makes it about 30 Amps to the solenoid when there is no resistor in the way. I think that the solenoid current reduces when the starter is engaged.

However, I am fairly sure that the 30 amps is very small compared to the full starter motor current. I think that 300 amps is a better estimate for that car.

I would agree with Rodalco's estimates of current.


Well-Known Member

The starter solenoid draws a reasonable amount of current. 10 to 15 Ampère sounds normal to me.

The idea of this to get the armature rotating, which makes engaging with the flywheel ring gear easier. When the pinion gear is meshed properly the main contacts close and full current is applied to the starter and full torque is available to crack the engine.

Regards, Raymond
Thread starter #12
Screech said:
It takes an effort to turn the shaft by hand.

I'll do some more testing when I find some thicker cables.
I got some jumper cables, and, what a difference that made.
I also opened up the starter motor. It all looks good and spins free.

Thanks for you help guys.
if it turns dificult by hand if it is asambeled and not if it disasambeld then it is something in the housing

maybe the shaft is not alinged in the housing or in thr stator winding house

a lot of starter motors are build up like 2 ends with the bearings and a stator winding housing in between but somtimes they don't have grooved conections to fit in eachother a drop on the floor can by enough to get you in trouble
also check your bearings

Hi, I'm sure you got your answer by now, but just wanted to post a little note.

Motors that start up from a stopped position will always draw much more current then when running, especially when a load is attached but even without. This is why AC units have a LRA Locked Rotor Amperage which is usually the start-up current. With no load attached like in your case, I doubt it would be as high, but still higher then the normal running amperage with no load.

Also, when you used undersized wiring you basically created a large resistor before the starter so out of your 12 volts, depending on the length and resistance of the wire your starter could have only seen a few volts and the majority was lost in heat generated.

The drop in voltage would make the starter spin slow, but depending on the winding resistance it could still have drawn a lot of amperage at a lower voltage. Before you rule your starter defective, try it on a fully charged battery with the proper cabling. Just food for thought, good luck. -Fred
I've heard 70A @ no load and 400A locked rotor @ 4v across the motor. Maybe 250A amps while cranking?
Be sure to restrain the motor when voltage is applied or you may get a quick lesson in conservation of momentum.

You can measure milliohms with a conventional DVM and a current source, like a 12v battery and several headlights in parallel, in series with the battery, lights and device under test.
With connection testing of house wiring use a hair dryer load and DVM.
The DVM leads should not touch the current inducing leads, it's a Kelvin setup.

I take it there are no orthodontists in Melbourne? :D
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