I am trying to determine how many amps my starter draws when starting my car. I have a clamp on ammeter gauge and want to know how to test is. What procedure do I use? What should the starter draw during startup? My car doesn't start until I take my foot off of the starter button.
starter amps
- Thread starter chkolar
- Start date
Patrick L.
Well-known member
Just disable the primary circuit, such as, placing something plastic/cardboard etc between the points contact.
Place your ammeter across/around the starter cable.
Hit the starter and read the ammeter.
On 6v the draw should be under 450 amps. On 12v 250 amps.
It kinda sounds as if there is something drawing on the system which is not allowing enough electrons to the points. In other words the starter may be taking everything and when the starter is stopped spinning there is enough power to the points to allow the engine to start.
As mentioned, if 6v, make sure the battery leads are heavy enough to handle the load. The should be at least #1, however even bigger is 'more better'.
Place your ammeter across/around the starter cable.
Hit the starter and read the ammeter.
On 6v the draw should be under 450 amps. On 12v 250 amps.
It kinda sounds as if there is something drawing on the system which is not allowing enough electrons to the points. In other words the starter may be taking everything and when the starter is stopped spinning there is enough power to the points to allow the engine to start.
As mentioned, if 6v, make sure the battery leads are heavy enough to handle the load. The should be at least #1, however even bigger is 'more better'.
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Synchro909
Well-known member
I suspect you should have been referring to 12 v in one of those. Perhaps you could tell us which. Maybe in the first one you were referring to 12 v with the higher current.
BTW, the current for a 12 volt system will be double that of a 6 volt system if the starter hasn't been modified. That is, it will have 4 times the power. That's why they don't like 12 volts for long. Even the 5/8" shaft will eventually bend if 12 v is put through a 6 v starter. I've seen it too many times.
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w.michael
Senior Member
My own experience:
The starter would turn over normally, but the engine would not fire until I took my foot off the starter. I did not suspect any problem with the battery. I cleaned the cable connections and all electrical connections with no improvement. I finally replaced the battery and that solved the problem.
I don’t have the technical explanation for this. I just know what worked for me.
W. Michael
The starter would turn over normally, but the engine would not fire until I took my foot off the starter. I did not suspect any problem with the battery. I cleaned the cable connections and all electrical connections with no improvement. I finally replaced the battery and that solved the problem.
I don’t have the technical explanation for this. I just know what worked for me.
W. Michael
GRutter
Senior Member
you cannot measure current with a Clamp-On Ammeter on a DC circuit. They are designed for AC circuits. In a DC Circuit, the ammeter would need to be in series with the load.
DO NOT ATTEMPT to hook an ammeter in series with the starter. Due to the high current, the ammeter will not survive and could be dangerous if not properly fused.
DO NOT ATTEMPT to hook an ammeter in series with the starter. Due to the high current, the ammeter will not survive and could be dangerous if not properly fused.
Kurt in NJ
Well-known member
running free--no load 2700 rpm 50 amps @ 6V
locked --14 ft lbs 0 rpm 3.75 volts 560 amps
cranking --engine 100 rpm . 225 amps ,4.75 volt
brush spring -35-40 oz
locked --14 ft lbs 0 rpm 3.75 volts 560 amps
cranking --engine 100 rpm . 225 amps ,4.75 volt
brush spring -35-40 oz
Patrick L.
Well-known member
Oops. gotta start proofreading. Glad you saw that. I meant, 250 amp for 12v system.
Patrick L.
Well-known member
It depends on the type he is trying to use. I have one that works on DC. I bought it specifically for testing starters on a vehicle.
rotorwrench
Well-known member
Some amp meters use the Hall effect to measure current flow. They will work on AC or DC current. There will be circular magnetic fields around any conductor that has current flowing through it.
If the starter is using up all the battery current on the start phase then the battery's capacity is severely limited. A way to check for a battery problem is to turn on the headlamps before a start attempt and see what they do while the starter is engaged. If the lights dim almost to no light output at all then there is either a problem with the battery or the starter. A good battery has a lot of cold cranking amps available if it's in good condition. This is a basic poor man battery load check. If lights come back up bright after the attempt then I'd check the starter. If the headlamps stay dim then it's likely the battery.
If the starter is using up all the battery current on the start phase then the battery's capacity is severely limited. A way to check for a battery problem is to turn on the headlamps before a start attempt and see what they do while the starter is engaged. If the lights dim almost to no light output at all then there is either a problem with the battery or the starter. A good battery has a lot of cold cranking amps available if it's in good condition. This is a basic poor man battery load check. If lights come back up bright after the attempt then I'd check the starter. If the headlamps stay dim then it's likely the battery.
Crankster
Well-known member
"Less than 250" sounds a little too vague to me, but I don't know for sure. These sound like OHV V8 numbers. A Flathead or early Ford in stock form at least, is not a high compression engine, I would think it would (or should) be considerably less than this. Probably closer to 150 amps?
For all that, a voltage drop test of both sides of the starter circuit would be useful. One probe placed on battery + POS post, the other to starter terminal (assuming NEG ground) should not exceed 0.5 volts whilst cranking starter.
Same test performed from - NEG battery post to a clean portion of starter case should not exceed 0.2 volts. Note that these are positive voltages, any voltage displayed on the meter represents electrons finding the voltmeter itself an easier path around than any loose, corroded or high resistance connections. High current low voltage DC starting and charging systems find almost any resistance however small to be a major defect, and as a practical matter almost impossible to measure effectively, so a "voltage drop test" has long been used instead to infer excessive resistance.
For all that, a voltage drop test of both sides of the starter circuit would be useful. One probe placed on battery + POS post, the other to starter terminal (assuming NEG ground) should not exceed 0.5 volts whilst cranking starter.
Same test performed from - NEG battery post to a clean portion of starter case should not exceed 0.2 volts. Note that these are positive voltages, any voltage displayed on the meter represents electrons finding the voltmeter itself an easier path around than any loose, corroded or high resistance connections. High current low voltage DC starting and charging systems find almost any resistance however small to be a major defect, and as a practical matter almost impossible to measure effectively, so a "voltage drop test" has long been used instead to infer excessive resistance.
Car doesn't start until you take your foot off the starter button??? Your starter may be dragging the voltage down to the point where it is killing the spark. If possible, I would try a different starter but first make sure battery, starter and cable connections are clean and tight. Maybe try hand cranking.
katy
Well-known member
Attachments
Patrick L.
Well-known member
There ya go ! That one looks a whole lot newer than mine ! Everything ends up showing its age.
Synchro909
Well-known member
With respect, I think you have it backwards. The resistance of the starter doesn't change when you put 12 volts through it. With double the voltage, double the current will flow.
Resistance (R) is calculated by dividing the voltage (V) by the current (I). R=V/I.
Clearly if you double the voltage, the current doubles to keep the resistance (R) the same.
The power of an unmodified starter on 12 volts is 4 times what it is on 6 volts.
Power (P) is V x I. If bother have been doubled, the P is increased by a factor of 4.
That is why an unmodified starter running on 12 volts will turn over the motor so fast, the fan would nearly pull the car along!!
I expect the cranking current on 6 volts to be about 180-220 amps and double that for 12 volts.
Patrick L.
Well-known member
12 volts needs less amperage to do the same job that's why the cables are smaller.
12 volts 250 amps is equal to 6 volt 500 amps.
When you double the voltage you only need half the amperage.
Bill
Synchro909
Well-known member
You are correct up to a point. To do the same job, requires the same power so with double the voltage, you only need half the amps. Trouble is, a 12 v starter doesn't do the same job as a 6 v one. It spins the motor faster because it has more power, hence more than half the amps.
Everybody knows what happens to a 6v globe if you put 12v through it. so we use a different globe (higher resistance) to give the same power. With double the voltage and the same power, the current is halved. THAT is why we are able top use thinner cables with 12v.