gonefishn

Does anyone know much current the starter/solenoid draws when cranking?

Thanks,
John

Plasticman

Nope. But if you have an Ohm meter (VOM or DVM, etc.), it is an easy calculation to make on the solenoid.

Current (I) is equal to Voltage (V) divided by the Resistance (R).

Measure the Resistance of just the solenoid coil, and divide into 12 (for nominal voltage while pulling in the solenoid).

Example: If you see 6 ohms, divide 6 into 12 (for volts), equals 2 amps.

Note that the solenoid draw will be nowhere near the actual starter motor current draw. If I was to guess, I would say somewhere under 10 amps (judging from the wire gauge that supplies current to the solenoid).

As for how much the starter draws, I have "heard" up to 300 amps depending on a lot of factors (ambient temp, oil viscosity, size of engine, starter windings, etc.).

Plasticman

mechron

yep, 175 to 250, the lower the better. amp tested starters for years, about 200 is the norm...

EDIT: this is the amp draw through the bat cable with the starter motor cranking,just to be clear.

Matt Gruber

yep, my 61 is 200.
Quiz: who else can measure it with a $3-10 DVM ?

mechron

Quiz answer: Matt, i use snap on inductance ammeters. they have a U shaped channel on the back of the gauge, you just lay it over the outside of the cable and it reads the current running through the cable. it takes about 2 seconds...

gonefishn

Thanks everyone. The problem I'm trying to solve is a hot start problem with the starter and/or solenoid (when hot no cranking).

Have about 0.4 - 0.5 ohm resistance so at 200amps so I think my problem is too large of a voltage drop to engage the solenoid. (Very embarrassing when I can't restart the car until it cools down).

Plasticman

If the solenoid is not pulling in (when hot), you have a few options:

1st I would check the wiring to the solenoid for frayed strands (from the ignition switch), poor connections, etc.

2nd Would try a new solenoid, or the lighter solenoid spring that I have seen mentioned elsewhere.

3rd, I have had good success on another vehicle with just changing to heavier gauge wire to the solenoid (less voltage drop).

If the starter is turning over, but is just heat soaked (weak from heat), then a heat shield would be in order, or a stronger starter.

Plasticman

gonefishn

Thank you for the good ideas.

Was considering the stronger starter. My understanding is a OEM start only has about ~180ft-lbs of torque. When normal temperature it turns the engine over slowly and fires after about 2 - 3 turns. But when hot (immediately it restarts but after 10 min it won't) the starter won't even engage.

Will recheck the solenoid and connections to ensure they are good.

Also, does a heat shield really work well? Seems so hot down there it is hard to believe it reflects that much radiant heat. This would be the best approach and turn me around as a skeptic.

mechron

if i understand what you are saying correctly--NO cranking at all sometimes when hot. replace the solenoid, there is a break in the pull in windings. classic chevy starter problem. if there is a drop in voltage through the solenoid--REPLACE THE SOLINOID once again, the copper washer that connects bat voltage to the starter is pitted. in short, replace the solenoid, it's a simple job...

Plasticman

If you have headers, yes a heat shield works well. If no headers, should be no reason for one. Depends on how close the pipes are to the starter/solenoid. I do have headers and have not needed a shield, but have seen applications where that was the only way the starter would work when hot.

If your starter is turning over slowly when cold, might want to try a new starter. It only gets worse when hot due to heat losses in the starter and solenoid windings. Or at least take it down and inspect it for worn brushes, etc.

Another thing I have done is to increase the size of the positive cable going to the starter/solenoid (from the battery). It made my starter turn over much faster. Also check your ground cable and connections as well.

Plasticman

mikem350

OK Mat, how do you measure the starter amp draw w/ a cheapie DVM???

Plasticman

Would need to go through a shunt resistor (low ohm/high wattage resistor), and then measure the voltage drop across the resistor to calculate the current.

Plasticman

Matt Gruber

just measure the v drop on the heavy starter wire cranking, then the v drop headlights on only.
then headlight amps(up to 20)
v drop is directly proportional (starter wire is the shunt) or use battery to ground as shunt

mikem350

Thanks, Mat, I get it!! Will explain for posterity.

The bat cable acts as the shunt, and you will be reading small voltages (actually they better be else cables are bad ).

Then using the amp function directly measure the lights (pull the fuse and use 20 amp scale or higher) and compare. I would calculate the shunt resistance value R=V/I and repeat the V drop test for cranking

Matt Gruber

good.
Not to knit pick, but there is no need to calculate R.
use a solve for x equation
ie if
20 amp = 0.2v,
a 2.0v drop is how many amps?
answer 200
that example is easy enough to do in your head.

Note: cranking amps may be lots lower than starting amps. IIRC starting was ~300 on my 61. This is due to my aggressive spark advance.

If anyone wants to chime in with their results it would show they can really do it.
Hint:turn on enough stuff to get 12-16 amps; $3 DVM meter on 10a scale will read that ok.

The poster should check all connections for v drop. Ground to frame often overlooked and corrodes