possible damage from unhooking battery while running

Alrighty, I made a terrible terrible error tonight. I had my 142 w/ D-jet running and was going to check the idle RPMs and had my multimeter hooked up to the coil and the battery ground. Little did I know it loose and while the engine was running the negative battery cable came off. Many many expletives were uttered and I jumped to turn off the engine as it was still running fairly well. I hooked the battery back up and it wouldn't start....

It seems to be acting like it did when i had the dizzy unplugged and the injectors weren't firing. I took out the plugs and they barely a faint smell of fuel so i figure only the cold start injector is firing. I hear the fuel pump kick on for a few seconds when I move to pos 2, and also I can hear the clicks when I move the throttle.

SO what did I possibly screw up when i took the cable of, a sensor, the ECU, my mind, all of the above?? Any help is greatly appreciated.

it still ran with the battery terminal off? Cool...

I've killed a voltage regulator that way before, on a 240.

spe1983 said:

Alrighty, I made a terrible terrible error tonight. I had my 142 w/ D-jet running and was going to check the idle RPMs and had my multimeter hooked up to the coil and the battery ground. Little did I know it loose and while the engine was running the negative battery cable came off. Many many expletives were uttered and I jumped to turn off the engine as it was still running fairly well. I hooked the battery back up and it wouldn't start....

It seems to be acting like it did when i had the dizzy unplugged and the injectors weren't firing. I took out the plugs and they barely a faint smell of fuel so i figure only the cold start injector is firing. I hear the fuel pump kick on for a few seconds when I move to pos 2, and also I can hear the clicks when I move the throttle.

SO what did I possibly screw up when i took the cable of, a sensor, the ECU, my mind, all of the above?? Any help is greatly appreciated.

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Oh oh! 71 Green Book
Page 2:72
SPECIAL INSTRUCTIONS FOR WORKING ON VEHICLES WITH ELECTRONIC FUEL INJECTION.
#1 NEVER let the engine run with the battery disconnected.

It doesn't say what will happen,but it goes on to talk about the ECU

I have done it before - when you have a car with a bad alternator and a dead battery you need to start the other car, pull the battery and put it in the bad car and drive the bad car to where you need to go (within 10 -15 min drive).

It is probably not a good idea, but so far I have been lucky.

Redwood Chair said:

#1 NEVER let the engine run with the battery disconnected.

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Yeah thats exactly why I shouted as many expletives as possible while scurrying to the ign. switch cause all i knew was get the car off. Although the battery wasn't completely disconnected but it certainly wasn't in the loop. I know the battery kinda acts as a filter to keep the alternator(DC surging??) from damaging some electric components. Now i know the clicks and whistles of my briefcase ECU might be more open to this kind of damage that say a 240.

My best friend just laughed me when told him a while ago, and he said I didn't know you could kill something with out a soul, In order to need a soul you require self awareness(closed loop), but this open loop D-jet lacks a soul. I thought it was funny.

Anyone have anymore ideas or any sensors to check, I made sure all the connections were solid, and nothing has changed since it ran well a few days ago. Does anyone else have any ideas?? Thanks

Have you checked for spark at the plugs?

In an alternator equipped car, the battery is actually part of the load that is regulating the voltage. A battery in many senses works like a large capacitor and can absorb transients very well.

When a battery is removed from a car running an alternator, the voltage with briefly spike at several hundred volts before it got re-regulated. Depending on how the electricals on the car were designed, they may or may not have survived this.

Interestingly enough, I know all this because the heavy machinery industry (real life Tonka Toys!) used to only use batteries for starting the machines. After that, they just relied on the alternators to provide electricity. This was about 30 years ago. At the time, my father was working for a small company that designed an electric timer for lubricating them. In their first field trial, all of the machine failed after a couple of days. The reason: the voltage spikes. My father added some circuitry to protect the timers, and they never had another failure.

See, thats what I'm talking about. The thing about this car is its so old w/ the old electronics I'm hoping I didn't fry anything in the computer or the sensors.

I know it gets spark as there isn't anything to destroy on the spark side of a d-jet car. Plus it seems like the cold start injector is making it kick over just enough another indication that the spark is fine. Humm I'll try it again when i get home but I don't think its looking good. Thanks

you probably blew the whole damn thing up. in a MS equipped car, things go smoke when that happens, and I spect might do as well on an lh car.

stands to reason djet would be along the same lines.

The ****er runs. I got in it today after work, and the damn thing started w/ my foot at WOT, and the restarted when it was hot no problem. Thats a relief. Thanks guys for helping me think through this, all I needed was some patience.

My somewhat limited experience with LH cars suggests that they're fairly robust with regard to unhooking the batt while the car's running. Before I knew any better, I did it with my car probably about a dozen times, which did eventually kill the voltage regulator, but all other electronics are fine. I know two other people who've done it several times with their LH2.4/3.1 cars without any effects (I've since explained to them that it's a bad idea).

When a battery is removed from a car running an alternator, the voltage with briefly spike at several hundred volts before it got re-regulated. Depending on how the electricals on the car were designed, they may or may not have survived this.

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Not unless that engine is turning more than 15K rpm with bullet-proof rectifiers. No way. The self-excited alternator would fry the diodes shortly after the ac output exceeded their peak reverse voltage limitation. Which I can guarantee you is not several hundred volts. They're spec'd for amperage first, voltage second.

Well Spe, you taught me the lesson not to do this with my car lol.

Interesting - one of he basic tests to isolate whether you have a battery or alternator/generator problem is to start the vehicle, then disconnect the battery. If it runs, then the battery is a problem because the alternator/generator is running the car. If it dies, then the problem is NOT the battery.

A battery is in the car simply to provide a starting capability. The alternator/generator is there it primarily to provide all the electrical power for the vehicle and to incidentally keep the battery charged so you can start the car again.

All the regulation happens in front of the battery - not after it. Look at what the battery is primarily tied to - a big red wire to the starter for all the battery current to run thru and spin the engine. The wiring for charging is not heavy gauge, light gauge such as you find in a trickle charger.

The only warnings ihave seen is with newer cars when you disconnect the battery – not older ones. Besides, a little 9v battery tied into the cigarette lighter socket keeps the electronics happy most of the time…..

No that's not correct

The above test is only applicable for Generator equipped vehicles. Never disconnect the battery with an alternator charging system. I've seen people get away with it and I've also seen people ruin their alternators. But it's not a viable test for alternator equipped vehicles.

Also if you study the charging system layout on a Volvo. The heavy red wire from the alternator runs from the alternator to the starter. It's connected together with the large positive battery cable onto the starter. The battery gets charged through it's positive cable. Everything else in the car runs off the positive battery cable after the battery is a load on the alternator. Not before.

Volvo started using alternators in the 60s. Like many car makes.

I think all of the above posts were close.

The alternator is a three phase generator with each phase rectified by a pair of diodes and the output is summed to a single DC node or point.

The voltage is maintained constant by a regulator the samples the output voltage and controls the current flow to the armature slip rings and finally the rotating magnetic field.

200 peak reverse voltage is possible for the high current diodes used, it is not that high for a diode. I doubt you would get anything close to that though unless the regulator is very unstable in design.

The output of a three phase alternator is quite current stable that is why they do not require current regulators as did the generators.

In most cases the regulator does not require a filtered supply so it would work fine without a battery but it would have some three phase ripple and the electronics might not be damaged but would be somewhat in question as far as it performing well.

Doubtful the voltage would vary too much also but it really depends on the design of the voltage regulator. They vary quite a bit in quality even from the same maker.

Well it's been almost 5 months since this happened and I didn't have to replace anything. She still fires up almost instantly and hasn't had a charging issue. I was thinking that they might have over-engineered (surprise surprise) the voltage regulator for extra protection to the ECU in case this happened. But I am glad that nothing terrible happened, just scary when messing around with 35 year old electronics.