Run without a battery?

Discussion in 'Mechanic and Repair' started by dahammer, Jan 17, 2014.

  1. dahammer

    dahammer LawnSite Member
    Messages: 227

    I have a Scag Tiger Cat that is a couple years old now. This past summer the voltage regulator lost good ground and it drained the battery. I fixed the bad ground at the voltage regulator, let the mower charge the battery, and didn’t have any more trouble.

    It had been a couple months since I’d ran the mower, so today I decided to start it up and allow it to run for a little while. Well the battery was dead (around 10.5 vdc), so I had to jump it off. Once I had it started, I removed the jumper cables thinking I’d see if the battery would take a charge. As soon as I removed the cable, the engine died.

    I figured I had a charging issue again, so I removed the voltage regulator, cleaned all of the contact points and put it back together. Still no joy. Soon as the jumper cables are removed, the engine dies. Certain that the engine should run once started, regardless of the condition battery (or even without a battery), I began trying try to diagnose what I thought was a charging system problem. Well, it appears to me that the charging system is functioning correctly. With engine off and ignition on, I have battery voltage on the purple wire (center) at the voltage regulator. With the engine idling, I have 13.09 vdc on the purple wire at the voltage regulator and 15.0 ac on the stator wires. At WOT, I have 14.43 vdc on the purple wire and 37.8 ac on the stator wires with the voltage regulator connected. With the voltage regulator disconnected, I have 45.8 ac on the stator wires.

    But still it dies as soon as I disconnect the jumper cables. I could have sworn that earlier in the summer when the battery was drained (at an even weaker state than it is now) that it ran fine with the battery disconnected once I fixed the bad ground at the voltage regulator. Was I mistaken? Is there some circuitry on these machines that requires a battery in order to keep the engine running? Just to check, I disconnected the battery completely, started the engine with jumper cables and soon as I disconnect one of the jumper cable leads, it dies.
  2. whiffyspark

    whiffyspark LawnSite Fanatic
    Messages: 6,385

    Not necessarily. Bad batteries can do that. Happens on full size vechiles all the time.

    Charge the battery and try it again. Or even replace it they're not that expensive.
    Posted via Mobile Device
  3. unkownfl

    unkownfl LawnSite Gold Member
    Messages: 3,837

    Bad charging system. How many amps is it putting out to the battery from the purple VDC wire? If you're not running anything the charging system has to be able to sustain the engine running or you would have a run down condition even with a battery in place. Don't disconnect the battery just the jumper cables.
    Last edited: Jan 17, 2014
  4. BigFish

    BigFish LawnSite Platinum Member
    Messages: 4,115

    Look, the charging system seems to be OK. Depending on how the machine/engine are wired, and what engine ya got : it may or may not stay running.
    Some/most motors have a solenoid at the carb and some require 12VDC at the coil module.
    Ya need to go over yer wiring, including the ground/s, and get a battery afore ya really screw something up.
  5. ducnut

    ducnut LawnSite Bronze Member
    Messages: 1,566

    Get yourself a Battery Tender Jr for each mower you have and plug it in, anytime the mower is in the shop. That'll help extend your battery life and eliminate one variable from your electrical issues. If the LED light on the Tender is always showing a charge condition, it's a given the battery is no good. I'm averaging 9-10 years of battery life, on my mowers and motorcycles, using Tenders.

    Anytime a battery is in a state of discharge, it is sulfating. When the sulfation on the battery's plates grows to the point of contacting the next plate, it grounds the cell. I'll just about bet money your battery has done that. It's taking more juice than your engine can produce and is killing the engine when you pull the jumpers.

    Always completely charge any battery, before use, via an automatic charger. The slower, the better, which is why I like Tenders. Your engine's charging system will never completely top-off a battery. Lastly, relying on your engine's charging system to overcome a battery's discharged state is very hard on the charging system. It's meant to maintain charge; not completely recover a dead battery. Don't do it.
  6. dahammer

    dahammer LawnSite Member
    Messages: 227

    The battery is done. It won't take a charge. But I had the battery disconnected completely from the mower and the mower will not stay running without a good battery connected to it. That seems very odd to me. I've got other small engines, one of which is a Kohler Command CH730, that run fine with no battery connected.

    I had assumed that the charging system would provide 12vdc to the fuel cut off solenoid once the engine is running, but perhaps it doesn't. I'll check that. It's an instant kill when I remove the jumper cables, just like I had turned the key off, which could be the solenoid.

    It will run fine once the battery is replaced but I still wander if the charging system is working correctly if it will not support the engine without a battery.

    I'm not sure about the amperage output, as my cheap DMM will not measure anything beyond 500mA. I'll have to pick up a better one and check that. However, just for curiosity sakes, I borrowed the 25amp voltage regulator off of my CN730 and put it on the scag (which has a 15 amp regulator) and got the same result. As soon as the jumper cable is disconnected, it dies immediately.

    I'm wandering if there is not something in the interlock module that requires 12vdc directly from the battery and if not present shuts the engine down. But like I said earlier, I could have sworn that it ran fine earlier in the year without a battery.
  7. ducnut

    ducnut LawnSite Bronze Member
    Messages: 1,566

    My background is in the powersport industry. There are many machines that will not run without a battery, even with jumpers hooked to it. Also, many will not even fire a plug or injector, without a minimum voltage value seen by the ECM.

    Without seeing specs in the service manual, I'm guessing. But, your charging numbers seem low. I think you should be seeing voltage numbers over 14, even at idle. That's what I'm used to seeing, but, not 100% sure with mower engines. I should've caught this earlier.

    You should post up all pertinent engine data, so hopefully one of the site's techs will come along and be able to offer correct voltage numbers.
  8. dahammer

    dahammer LawnSite Member
    Messages: 227

    It's a simple system, no ECM involved. It's just a basic Kohler CV740. There is a cut off solenoid on the carburetor. It could be that the interlock module shuts off the voltage to the cut off solenoid if it doesn't sense 12vdc from the battery, I'm not sure.

    The voltage was 13.09 vdc out of the voltage regulator at idle with the dead battery connected, as well as the jumper cables to a good battery. I assume the voltage was low because the dead battery was pulling it down. It shot up to 14.43vdc at WOT.

    Anyway, I'll investigate it further when I get a chance. I just figured someone here could tell me if these mowers were set up to where they would not run without a battery connected.
  9. whiffyspark

    whiffyspark LawnSite Fanatic
    Messages: 6,385

    Just put a new battery in it and go from there.
    Posted via Mobile Device
  10. Valk

    Valk LawnSite Silver Member
    Messages: 2,720

    ^^^ +1 ^^^

    I use 1 battery tender and move it/leave it on each battery for about 1 day/battery every couple of weeks during the Winter.

Share This Page