Voltage Drop when Cranking
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Laughing Gas
Joined: Aug 2010
Posts: 1,049
Likes: 230
From: Amarillo, TX
Voltage Drop when Cranking
As title says, my battery/system voltage is dropping to ~8V when cranking. Measured with a multimeter at the battery, the main fuse, the hot and switched terminals on the ignition switch, coil fuse, etc. The main reason for digging into this was a no spark/no start condition. Obviously, the coil isn't getting sufficient voltage when cranking to fire. I can run a wire directly from the + bat terminal to the Bat+ terminal on the distributor (I have an HEI), and it will start right up & run fine. I load tested the battery & it did show "weak", but even with the booster on it, the coil won't fire. I know the quick & dirty fix would just be to run the coil off a relay, but I want to know why this started all of a sudden. I have a universal Painless Wiring harness; any suggestions on where to start tracking this down?
Moderator
Joined: Jan 2001
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From: Virginia
If you have 8 v on the battery while cranking, and 8v at the distributor when cranking, then why does putting a jumper to the battery make it fire? Are you sure you have 8v on the battery terminal of the distributor when cranking?
Cargo Master
Joined: Mar 2008
Posts: 2,577
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From: Washington
Be sure to scrape a clean spot on the center of the battery post when you check voltage, compared to measuring on the battery cable.
The painless harness comes with a ballast resistor, did you use it. The instructions say to, what did you do.
If the starter truely does drag the battery voltage down to 8 volts, it's either a weak battery or the starter is drawing way to much current.
Good luck Jim
The painless harness comes with a ballast resistor, did you use it. The instructions say to, what did you do.
If the starter truely does drag the battery voltage down to 8 volts, it's either a weak battery or the starter is drawing way to much current.
Good luck Jim
Moderator
Joined: Jan 2001
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From: Virginia
Which truck is this on? The 1960 F250? Does it have a aftermarket tach? I would unplug the tach, and then plug the tach wire in at the distributor. If it still doesn't work, sounds like a wiring problem. If it does work, sounds like a tach problem.
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Laughing Gas
Joined: Aug 2010
Posts: 1,049
Likes: 230
From: Amarillo, TX
It does have an aftermarket tach; the entire instrument panel was unhooked during testing. I ohm'd out the tach lead to the battery negative & got a reading of 7.8. I didn't think this wire should be grounded, so I pulled back the jacketing to where I soldered the tach lead to the distributor pigtail. Checking the wire again with this connection exposed resulted in an infinity reading, which I believe is correct. Don't see how it could be grounding through all the tape & split loom, but re-did the connection anyway with a heat-shrink butt connector. Truck starts & runs now, but the tach isn't working
Hopefully, it's just a loose pin in my instrument cluster plug.
Hopefully, it's just a loose pin in my instrument cluster plug.
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Laughing Gas
Joined: Aug 2010
Posts: 1,049
Likes: 230
From: Amarillo, TX
New development - I decided to check all my gauge wiring with a multimeter. Just had my headlight switch crap out and have been having electrical issues since gauge install. Anyway, I discovered a short to ground in the power supply wire. I have the 12V switched power supply jumped from one gauge to the next, & when I measured between power & ground, it showed closed. I pulled off all the wires, & am still getting between 160 & 180 ohms between the power & ground terminals of the gauges themselves. The housings are all plastic, so no external grounding. Is that reading too low to be a problem, or do I have issues? Would jumpering positive & negative wires from one gauge to the next cause a problem?
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Moderator
Joined: Jan 2001
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From: Virginia
You certainly will get a reading between power and ground on any device powered by the battery. That is the "circuit" and you are reading through the components of the gauge or any other device. Like the radio. I am certain if you took the memory power wire from the radio and disconnected it, you would get a reading to ground from the power wire of the radio.
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