voltage drop.electrical experts helpppp?

[jlcelaya]

well ive got a 72 chevy c10. and i seem to have a voltage drop. my truck wont turn on when the ac is on or lights. but once i turn it off the voltage goes over 12 it starts. so far ive tried a another good battery, changed relays. its just a small amout of voltage drop that wont make it turn on.

[69nova]

I want to make sure that I understand your issue first. Does your ac stay on while you are trying to start your truck (the fan itself)? It should be wired to the fuse box to a terminal that turns off when the ignition switch is in the start position. Also check to make sure that if you have more than one ground wire coming off the battery, that both are grounded good. I had a starting problem on one of my other vehicles (a ford) that all the current was trying to go thru the smaller grounding cable because the large cable had corrosion where it connected to the block. Is this a recent problem?

[Longhorn Man]

i'm not understanding the way you have this worded...
If you leave your headlights on, the engine won't start?
This other battery ... was it out of another running vehicle, or was it something sitting in the shed for 6 months... ?
A voltage drop describes something else all together that you haven't tested for.

[cparman]

Ok, here is my guess. I am assuming that the truck will not crank over with the a/c switched on or the headlights on? When you turn off those electrical loads it will?
I am wondering if you have a bad connection at the battery, or where the ground connects to the engine? My $.02 would suggest cleaning the ground connections at both ends of the cable, and doing the same thing on the positive side. Voltage drops can be bad connections.

[unclewatts]

First of all, where are you measuring the voltage at the battery? Or are you measuring it at the positive terminal of the starter? you could just simply have a bad battery cable.

Originally Posted by cparman

Ok, here is my guess. I am assuming that the truck will not crank over with the a/c switched on or the headlights on? When you turn off those electrical loads it will?
I am wondering if you have a bad connection at the battery, or where the ground connects to the engine? My $.02 would suggest cleaning the ground connections at both ends of the cable, and doing the same thing on the positive side. Voltage drops can be bad connections.

I agree with Cparman. This whole thing sounds very much like a poor connection.

[jlcelaya]

both read 12.47volts

[mclairmo]

First, you should not have the lights or a/c on when trying to crank the engine. These accessories draw a lot of amps and will compete with the starter for battery power. Also, make sure you have a ground strap from the engine to the body and from the body to the frame. Also, confirm that your batteries are really good. They should hold 12.6 volts with engine off and you should have well over 13 volts with the engine running. Make sure battery cables are correct gauge and are not corroded or loosely connected. Also, make sure the starter is in good shape. A weak one will draw more amps and be more sensitive to voltage drops.

[rwolf9653]

Crappy starter is what I was going to suggest. My jeep YJ is that way, if the battery is not at full strength it will not kick over the starter. Check and clean the grounds first though because that is free.

[jlcelaya]

new issue now. even if my volts are reading over 12 volts is still wont crank.seems like the volts dont get to the starter solenoid it is brand new(solenoid).also solenoid had new wires from solenoid to firewall. with all accesories off (normal opreation) and reading over 12 volts still wont crank the starter.

[Longhorn Man]

you tried giving it a jump?

[jlcelaya]

tried that. and sometimes it cranks and sometimes it doesant with a jump.

[mclairmo]

How old is the battery? Exactley what is the voltage reading? 12.6 volts is good, 12.1 volts may not be enough to operate an old/weak starter motor. If the battery will not hold at least a 12.6 volt charge consistently for many days, then the battery is dead. Make sure that full battery voltage is getting to the starter. If it is, then the starter motor must be bad, assuming that the new solenoid is working properly.

[mclairmo]

12.47 seems a tiny bit low to me. I like to see at least 12.6 or very close to it. Based upon my research, 12.4 volts is already considered partially discharged and down near 11 volts is considered completely discharged. I would put the trickle charger on them and get them up to 12.6 or more, if possible.

[fixit-p]

Check your battery cables as already mentioned and if you have a remote starter switch you can connect it to the "S" terminal on the starter and the hot post if it turns over then the cables and starter are ok and that leaves the purple wire (start circuit). IIRC the solenoid requires ~40 amps peak to engage and the purple wire goes from the "S" terminal thru the bulkead connector thru the neutral safety sw, thru the ignition switch, back thru the bulkhead connector, thru the alt splice, to the fusible link and to the positive batt terminal. A high resistance connection at any of those points would cause a voltage drop at the starter "S" terminal.

[mclairmo]

I always thought that the small gauge purple wire going to the small terminal on the solenoid was a relatively low amp 12 volt power source just used to activate the solenoid so that the real high amp power could then flow directly from the battery to the starter motor. I think if 40 amps went through that wire, it would fry it. Also, it would also seem to me that the solenoid should not require 40 amps to activate. That sounds more like the amp draw of a big starter motor not the small solenoid. However, I am no electrical expert. Someone correct me if I am wrong.

[fixit-p]

I've never put an ammeter on it but 40 amps peak surge is in the ball park and thats also the leading cause of the dreaded "hot start" problems we hear about. When the purple wire gets hot it's resitance goes up which prevents the solenoid from getting enough current to engage, bad connections or switches would have the same affect (high resistance). The starters amp draw once engaged could be around 250 and up.

[mclairmo]

Thanks. Makes sense since a battery can have anwhere from 500 to 900 cold cranking amps. I guess that starter motor really pulls a lot of amps.

[MrBlue]

Just my 2-cents.
Try taking a pair of jumper cables and connect the black from the neg terminal on the battery and connect to the engine, making a very large ground. If the engine cranks/starts, you need to check all the ground wires. Clean and replace as needed.
MrBlue

[rwolf9653]

Originally Posted by MrBlue

Just my 2-cents.
Try taking a pair of jumper cables and connect the black from the neg terminal on the battery and connect to the engine, making a very large ground. If the engine cranks/starts, you need to check all the ground wires. Clean and replace as needed.
MrBlue

Good idea! I will have to keep that stored in my memory bank for later.

[jlcelaya]

used another battery from another car reading 12.59 volts and acts the same. check cables groud battery cable to engine reads .05 ohms almost no resistance.so grounded good. battery (+) reading same voltage at battery post and solenoid.(12.59) could my starter just be failing me even with required voltage?

[fixit-p]

Whats the voltage at the purple wire "S" terminal when you try to crank it.

If you can connect a jumper wire to the "S" terminal and touch it to the batt positive post (if engine cranks or not) would help you narrow it down.

[mclairmo]

Yes, sounds like starter is bad. Does the solenoid click? Or do you hear nothing? I would remove the starter and bench test it with a good battery. Test the by putting power to the solenoid large and small posts to see of the pinion pops out and spins. If nothing, then put 12 volts directly to the starter motor post and see if the pinion pops out and spins. Don't forget to hook up the ground post of the battery to the starter body.

[mclairmo]

Solenoid is bad or not getting any power when you turn the ignition switch to start. Are you sure you have power to the solenoid?

[jlcelaya]

ran a jumper wire from the purple wire to the batt +. started the first 3 times but then started to crank but didnt turn on after the 3rd try.

[jlcelaya]

took starter to autozone they tested it and it was good. bought a new solenoid and installed it.welll see what happens now. will keep posted.Thanks for all the help guys. this is my first c10. moved over from a 5.0 mustang to a c10 and loving it. will post picture.