dramatically failed jump-starting

[scipio84]

Good morning guys.
Two days ago the battery of my car was almost died. So yesterday I tried a jump starting, very simple exercise.
So I connected the + of the charged one to the + of the worn battery and then the - of the charged one to the body of the "discharged car".
I turn on the dashboard of this last one (not making the engine run, only dashboard), and everything was ok, the lights inside the car were on.
So I turn the dashboard off and start the motor of the car with good battery, left it running for a while and finally I try to start the car with discharged battery.
Result: everything off. Not a sound, no smell, no smoke. Also the lights inside the can went off and no way to start anything.
The engine of other car continued to run without slowing down.

Two other notes.
The car was a BMW 320i, gasoline, 2 liters, year 1991 (pretty old), electronic injection and the battery was a 60Ah and I believe 480A.
The car with good battery is a 1.1 liters with a battery of about 50 Ah.

Now, I understand that such a smaller car can fail to start a 6 cylinders... but I can't understand the total black out and where can be the failure in the bmw, considering also that the small fiat didn't show any sign of fatigue.

Thanks a lot!

 

[Les Jones]

It sounds like the battery has failed totaly. Probably one cell open circuit.

Les.

 

[crutschow]

You may have blown the main (link) fuse from the battery that powers all the car's electrics except for the starter.
What is the bad battery voltage?

 

[scipio84]

Well, I don't know which is the main fuse. Has it any specific name? So that I can find it on any map or manual.
For the voltage, I measured it before trying the jump starting and it was 6,3V.
Didn't try after the tentative, I was too angry and sad.

 

[Les Jones]

Telling us what the battery voltage was at some previous time does not help solve the current problem.

 

[cowboybob]

Welcome to ETO, scipio84!

For the voltage, I measured it before trying the jump starting and it was 6,3V.

Presumably we're discussing a 12VDC system. 6.3VDC across the battery is indicative of, at the very least, a seriously compromised battery.

Remove any jumper cables. Recheck the BMW's battery voltage. Then, if you are comfortable doing this, disconnect the + battery cable and check the voltage level. If it's still low (i.e., less than ≈10VDC), the battery is toast.

 

[JimB]

Now, I understand that such a smaller car can fail to start a 6 cylinders

Only yesterday I jump started my 3.2litre Mitsubishi Shogun, using my Mazda MX5 (1.8litre), the operation went very well.
I hope the battery in the Shogun has held a charge overnight, I want to go out in it this afternoon.

I agree with crutters, either s fusible link is blown, or, there is a bad connection around the BMW battery or the battery earth strap.

JimB

 

[DerStrom8]

I had a 1984 BMW 325e a while back and one of the biggest problems I had was that the ground strap did not make good contact with the battery terminal. Make sure both the lugs and the terminals are clear of grime and corrosion and try again. If you still have no sign of life, it's likely a fuse.

Matt

 

[KeepItSimpleStupid]

So I connected the + of the charged one to the + of the worn battery and then the - of the charged one to the body of the "discharged car".

I have a "comment" about the use of the word body. The negative clamp should go to the engine, The purpose is two-fold. Remove the resistance of the negative battery cable on the jumped car and more importantly keep sparks away from the battery.

It doesn't take much to kill a battery. A simple battery hydrometer (under $5.00) will tell you the condition of the cells. Don't get the battery acid on your clothes. A hole will usually appear when you wash them.

I think it's simple battery dieing. The "main fuse" is generally not a fuse at all, but a wire of a lower gauge than everything else, but located in a spot where it would do little damage if it burnt.

I have had one issue where the car would not jump and I did something unconventional and NOT RECOMMENDED. The bad battery clamp was essentially removed and then jumped without a connection to the car. It was re-tightened after the engine started.

 

[shortbus=]

The "main fuse" is generally not a fuse at all, but a wire of a lower gauge than everything else, but located in a spot where it would do little damage if it burnt.

Having worked where they made that wire, called a "fusible link", the insulation on that type wire is made of a teflon (PTFE) type of plastic. This is to prevent it from burning or catching fire. In GM cars it is normally an orange color.

 

[scipio84]

So I connected the + of the charged one to the + of the worn battery and then the - of the charged one to the body of the "discharged car".

I have a "comment" about the use of the word body. The negative clamp should go to the engine, The purpose is two-fold. Remove the resistance of the negative battery cable on the jumped car and more importantly keep sparks away from the battery.

This is not my engine bay, but same position of everything. As you see the negative of the battery is connected to the suspension dome. I put the negative of the jumper lead on the border of the engine bay, where the red arrow is.

**broken link removed**

 

[crutschow]

You do not want to connect the negative terminal to the car body (who told you to do that?).
It should be to the engine block as KISS stated.
The ground connection between the body and the engine is not designed to carry the starter current and that connection may have opened when you tried to start the car.
Check the resistance between the car body and the negative terminal of your battery.

 

[scipio84]

Ok crutschow, I'll do it now. Do I have to disconnect the battery and check the resistance between negative a car body? Correct?

 

[KeepItSimpleStupid]

In this case, probably not. You can always get in the habit of measuring voltage first before trying to measure resistance.

Eliminate the battery first. Hydrometer or battery swap.

The engine is "almost" insulated from the chassis. There are motor mounts at the front which are rubber and the exhaust my hang from insulating straps. The radiator hoses are insulating.

Water is slightly conductive. The drive train may offer some decent path to ground though a whole bunch of rotating stuff.

You have linkages, but with drive by wire, those connections may not have been there.

The sensors such as MAP, TPS, temperature, ignition, oxygen end up connected to ground somewhere.

With the two vehicles I have now, I'm not familiar with the ground situation. I do know that there was a frame bond between the front and rear seats in a wiring harness I was troubleshooting. I seem to remember a frame bond in the engine compartment in another vehicle I owned.

The current to run the starter is meant to flow from the (battery to the engine block) to the starter though the starting solenoid (usually part of the starter) and back to the positive terminal of the battery.

(battery to the engine block): When you use a jumper cable, that section of wire is not used and replaced by the jumper cable negative.

One of my vehicles had a wingnut that could disconnect everything but the starter cable.

So, yes the frame of the car is grounded. the question is where.

 

[debe]

Fords had a problem here in Australia with one model Falcon, there was a wire to bond the body shell to the Engine. This wire had a poor earth conection to the body, the end result of this was blown tracks in the engine computer. The wire from the body to engine is mutch smaller than starter cables, its possible you have damaged this wire by putting the jumper neg to the body. And possibly other components. The Ford problem I mention was a factory fault & was covered by warranty.

 

[crutschow]

Ok crutschow, I'll do it now. Do I have to disconnect the battery and check the resistance between negative a car body? Correct?

Yes, the safest way to measure resistance is to disconnect the battery. I think your battery is probably shot anyway.
Then measure the resistance between the body and the negative battery terminal. It should be no more than a few ohms.

 

[scipio84]

Yes crutschow, the battery was completely died. At the end I tried a new one...
But can you please explain better? I don't understand (and excuse me for my ignorance with electrics) what is the sense to measure the resistance between negative pole and heart; isn't that the internal resistance of the multimeter? I mean, battery is connected on the positive to car harness, negative is disconnected from the body with the heart lead, but connected to the body via multimeter. I'd understand if had to measure the resistance of the negative lead...
thanks!

 

[cowboybob]

... negative is disconnected from the body with the heart lead, but connected to the body via multimeter. I'd understand if had to measure the resistance of the negative lead..

Not sure what you mean by the term "heart lead".

Anyway, what crutschow is suggesting is for you to measure the resistance, from the end of the disconnected negative battery cable, to the body (better to use the engine block). This resistance, ideally, would be 0Ω but certainly (as suggested) no more than a few ohms. It's just a check to confirm a good connection between the battery's negative terminal and the rest of the electrical system's negative connections.

 

[scipio84]

Not sure what you mean by the term "heart lead".

Sorry, wrong H, I meant earth lead, so lead from negative to body.

... to measure the resistance, from the end of the disconnected negative battery cable, to the body (better to use the engine block). This resistance, ideally, would be 0Ω but certainly (as suggested) no more than a few ohms. It's just a check to confirm a good connection between the battery's negative terminal and the rest of the electrical system's negative connections.

I'll try for sure, but this is what I don't understand: the negative cable is disconnected; I measure the resistance between the negative and the body or the block; the resistance between the negative and the body or block is only the resistance of the multimeter, that will compensate itself, so it will (should) be surely about 0.
If there is a problem with the negative cable, I believe I should measure the resistance of this cable, so putting the multimeter in parallel to the cable, pointing the negative and the body; in this case, if any problem with this connection happens I'd see a high value of resistance.
Feel free to correct me where I'm wrong, I don't want to exit burned from my garage

 

[Les Jones]

You measure the resitance with one meter lead connected to the end of the negative lead that you have just removed fom the battery (NOT the negative battery terminal.) and the other meter lead to some CLEAN metal on the body. Then move that meter lead to some clean metal on the engine block. I can't understand why you will not measure the battery voltage as it is now. I am still convinced the battery is faulty. I once had a similar problem with my wife's car. I started the car (The starter turned the engine as fast as normal.) I stalled the engine and when I turned the key to start it again there was not even a click from the starter solenoid. I drilled holes in the plastic on the top of the battery so I could get the meter probes to the link bars between the cells and found one cell compleatly open circuit. If you doe eventuly get round to measuring the battery voltage first measure it with no load on it then measure it again with some load. For example a headlamp bulb or a 50 watt 12 volt halogen MR16 spotlight bulb.

Les.