abecedarian

I don't mean to step your toes 4Crawler... I'm just confused.
but how can you calculate resistance on the wire from the voltage drop if you don't know the current (amps) being pulled? Given the standard equation of I=V/R (where I is Amperes), you can derive V=IR. However, knowing only the delta of V doesn't let you solve for I or R without assuming a value for the other. Are you assuming a standard amperage draw for the solenoid and starter under load to calculate the required drop in resistance in the wire?

4Crawler

iTrader: (3)

Just just the Web Catalog button and type in "Hot Shot Starter Relay" and scroll down to find:

Part# 30202 Hot Shot
and

Part# 30201 Hot Shot Plus w/engine bump switch


Well, if voltage at battery is good and voltage after igniition switch is bad, that is a good place to check. Take the "divide and conquer" approach, if good voltage at point A and bad voltage at point B, find some point C in between A and B and check there. If C is good, then you can eliminate C-B, but if C is bad, then divide A-C in two and test someplace in the middle. Repeat until you find the problem.

abecedarian

are we sure were not dealing with just a bad wire here? or possibly just a depleated battery that can manage to build up a decent plate surface charge on its own, but doesn't have the saturation in the plates necessary to supply the amperage necessary to maintain adaquate voltage for cranking?

Tomh4x4

lost me at resistance. I just dont understand why my truck crank when its cold and wont crank when its hot.

abecedarian

the net sumptom here is:
if it sets (cools off)- it cranks [possibly due to the battery recovering?]
after cranking then being shut off- it won't crank over again

maybe the alternator isn't sufficiently recharging the battery...?

maybe we need to get the engine running and check the battery voltage WHILE the engine is running?

4Crawler

iTrader: (3)

I have measured the current that the starter solenoid pulls and it is approx. 20 amps and I stated that at the beginning of that post if you will re-read it. And how did I measure that current? Simple. I hooked up a spare starter in series with an ammeter and a battery and read the current. It maxed out my 20 amp meter (19.99 amps) so it is likely something more than 20 amps.

Now yes, my calculations ignored the drop in current due to the added resistance in the circuit, I tried to make it simple. You could iterate to get the actual current given the solenoid and circuit resistance. But the point I was trying to make is that it take very little resistance to make a big voltage drop at 20 amps of current. Well under 1/2 ohm will do in the solenoid and keep it from working.

abecedarian

my apologies for missing that.
but wouldn't the starter draw even more current were it also expending power to turn the engine over (as in installed on the vehicle, turning 6 cylinders)? (and yes, I know the starters are [mostly] gear reduction type which has the benefit of requiring less current to operate the motor)
given that, wouldn't it take more current (than you're assuming) to crank the engine over and consequently the resistance in the wires could be calculated as less?

Tomh4x4

I have had the alternator checked out a while back and it was fine. I have a battery charger on it now. When I hold the key in start position it seems the battery dies fast. When held down the battery gauge will drop immediately and when let off it seems it to some battery power away. The weird thing is when the truck sits for about two hours to cool down completely it will crank right up. I can even turn it on then off and on and off. But if I let it stay on until it is warm nothing happens.

4Crawler

iTrader: (3)

Basic Ohm's law:
volts = resistance (ohms) times current (amps).

One possibility is that the resistance in that circuit changes between hot and cold. So to find out, continue as you are doing. For example you measured 9 volts cold and 1.4 volts hot at the solenoid. That is good in that you found a difference. Now work your way back to the battery (assuming the battery voltage is the same in both cases) and find out, for example, does the voltage at say the starter relay or at the ignition switch changes in a similar fashion or not. If not, then there is a problem in the wiring between the point where voltage is good and where it is bad.

abecedarian

Tomh4x4... the fact you say you've had the alternator checked and it's fine, and that trying to start the truck causes the battery to die quickly suggests to me, and anyone is free to comment on this...
... that either the battery, whether new or not, is having trouble maintaining a charge, or the starter is drawing far too much amperage.
The fact that the battery recovers on it's own from being depleted tends to suggest to me that there is a problem in the starter, not the battery, but even a battery with a dead cell can usually recover enough to start a vehicle once within two hours.

Now, you are saying that you can start and stop, then start again, as long as it's done quickly (right?), but if you let the engine warm up the starter will no longer function?

4Crawler

iTrader: (3)

There are two parts to the starter, the starter SOLENOID and the starter MOTOR itself. The starter SOLENOID is basically a big relay and it switches the 100s of amps that the starter MOTOR pulls. The starter SOLENOID is otherwise independent of the starter MOTOR, and it'll pull whatever current it wants regardless of the starter MOTOR running or not. That 20 amps or so that the starter SOLENOID is pulling while the key is in the START position is why you see the lights dim even though the starter MOTOR is not running.

Now to be correct, when the starter MOTOR is running and pulling 100s of amps, the battery voltage will drop due to the load and thus the voltage at the starter SOLENOID will also drop and thus the current it is pulling will drop proportionately. This is why you need a good solid voltage at the SOLENOID in order to have it NOT drop out due to that inherent battery voltage drop when the MOTOR is cranking over.

And you are correct that the starter MOTOR will indeed pull loads of current when it is trying to start the engine.

abecedarian

Thanks!

I'm just trying to wrap my head around what you're thinking.

(but I'm going to add that when the voltage drops, the current increases. )

Tomh4x4

Correct if done quickly while cold I can start turn off start and turn off several times. If I start it leave it running for a while then turn it off I get nothing the start relay clicks and thats it. I have a battery charger hooked up to it now and is almost fully charged. I went to crank it and nothing happened the engine temp is not COMPLETELY down yet though. My battery is reading 12.3 and 12.3 at the starter cable. When I hold the meter on the battery the voltage is dropping. Not real fast but it will bounce between 12.3 and 12.29 and then drop to 12.29 and continuing what could that mean?

Tomh4x4

I just checked the voltage at the starter solenoid and it didnt read anything.

abecedarian

a good battery charger should bring the battery up to around 13-14 volts, then if it has a 'float' cycle, the battery should flucuate between 12 and 14 as the charger turns off an on. if it has an 'equalize' setting, the charger should bring the voltage up to 15-16 volts for a while, then drop down to the 'float' settings.

I don't know your situation, but if it were me, I would be taking the starter in and getting a warranty replacement. I'd cut my losses with the solenoid. Something is obviously not right. And (in my opinion) everything is pointing at a problem with the starter, whether it's the solenoid or not. Heat soak should not affect the starter that way, that quickly. So maybe there's a clearance or tolerance issue in one or more of the parts you've purchased.
I don't know what 4Crawler would think of this, but were it mine, I'd be getting another new starter assembly to elliminate some of the variables, particularly if I had a warranty on it.

abecedarian

are you referring to the small wire? it won't read anything if you are not turning the key to start it, unless I'm mistaken about the operation of the starter.

4Crawler

iTrader: (3)

Sounds low for a fully charged battery, should be up in the upper end of the 12 volt range, not the lower end.

And if the voltage at the starter solenoid is as low as you are measuring, then the starter (solenoid and motor) is likely NOT the problem. If you are not giving the solenoid enough voltage to function properly then of course it will not work, but the problem is that "you are not giving the solenoid enough voltage to function properly", so of course it won't work. Find out why the voltage is so low.

abecedarian

that would depend on which connection on the solenoid is being measured, no?

if it's the battery connection (the large stud) then one would expect battery voltage readings.
if it's the conection from the ignition switch, one would expect no voltage.

Tomh4x4

Ok I just cranked it up. Cranked it about 4 times and then it stopped again. I felt the wire at the starter relay the black wire across from the black and white wire. It was pretty hot. Any ideas?

4Crawler

iTrader: (3)

Only one external connection to the solenoid coil, that is the small clipped on wire. That is the one that turns the solenoid on and off as you turn the ignition switch to the START position and back to RUN.

The big wire that is held on with a nut is what supplies the solenoid CONTACTS with power to send to the starter motor. That will always has 12 volts to it as it is directly connected to the battery.