Still Having Power Issues

[fry61]

What/where is this "volt drop test" taken?

[Guest mcramsay]

Batt positive terminal to BATT + post on alternator, and battery negative terminal to alternator casing.

[fry61]

So you're loosing approx 6 watts moving the power up & down the alternator circuit, does not seem like that can be your problem.

[fry61]

? start pulling fuses - and see if the prob goes away - might flag the problem circuit ?

OR -- disconnect the +battery terminal & connect a low wattage 12 volt lamp across battery post & battery terminal -- is there any glow in the test lamp with all circuits switched off -- then start pulling fuses -- or convert to hand start diesel.

[Guest mcramsay]

I'm coming to the conclusion that either the battery is nackered or I've blown a diode in the alternator, which would make it loose 33% of its power output.

[Longboarder]

The bulb glow at 850 rpm suggests low volts from the alternator. PD across the bulb lights it. Bulb is 12v/2.2w. How many volts does it need to glow slightly? Probably 3 or more. So alternator is making 9v or less at 850rpm. Forget the amps. I don't think it's getting its excitatory volts in the right terminal. Can you check which terminal you are using to excite the alternator? What's it's terminal number or letter? (or it's just a duff alternator!)

 

Nigel

[Guest mcramsay]

Battery voltage was 14.2 volts with the bulb glowing... indicated on the dash volt meter and multimeter on the battery. I'm wondering if the diode in the circuit is messing with it

 

Tomorrow's work is to fully charge the battery and load test it. Run new wiring to the alternator to bypass the BMW loom and see how that works

 

If that fails the marlin is going on eBay.

[Guest mcramsay]

If the bulb still lights up I will measure voltage coming directly out of the excite terminal on the alternator, if it's less than the battery voltage then the alternator is duff. ( I am definatley using the correct terminal by the way, checked and double checked that one)

[knights_templar]

High resistance short circuit -- a high resistance in any of the normal circuits would lower the amps.

Yep Ohms law, but is a car a normal circuit.

 

how many times do we get an engine turning over slowly, and find a poor earth, resistance may be the wrong term I agree.

 

Poor connections tend to heat up, heat uses power.

[knights_templar]

Battery voltage was 14.2 volts with the bulb glowing... indicated on the dash volt meter and multimeter on the battery. I'm wondering if the diode in the circuit is messing with it

 

Tomorrow's work is to fully charge the battery and load test it. Run new wiring to the alternator to bypass the BMW loom and see how that works

 

If that fails the marlin is going on eBay.

Diodes are one way valves, meaning the current will flow one way and not the other.

 

Why you have one in circuit would be down to the alternator.

 

ignition goes to warning light and through the field winding of the alternator, light on, as the alternator charges the earth goes and there is on circuit, light off, the diode would stop a back feed through the warning light. should still work.

[Longboarder]

 

If the bulb still lights up I will measure voltage coming directly out of the excite terminal on the alternator, if it's less than the battery voltage then the alternator is duff. ( I am definatley using the correct terminal by the way, checked and double checked that one)

That's rather my point. Even at 850 rpm I'm thinking the alternator should manage 12v. Piddly amps but 12v. Lots of tintops tickover at 850 or less with no ignition light glow to show for it. The bulb will glow when alternator voltage output is a few volts different from the battery voltage. 'Flow' can be either way. Battery must be about 12 and a bit. So either alternator output voltage is piddly small (like 8 or less or you have a duff regulator in the alternator and it's a bit big (like 16.5 or more). At no time have you said/measured 16+ volts. So it suggests to me that the alternator output is low due to either no excite voltage to get it started or something duff in the alternator.

I believe you are overthinking the problem worrying about amps when you have an ignition light telling you the voltage output of the alternator is significantly low. I don't think you have problems with high consumption, just pisspoor production.

 

Nigel

[Guest mcramsay]

Thank you for the reply Nigel, I understand what you are saying but I am getting 14.2 volts at the battery with the car running. But I might not be getting the correct voltage from the field winding post on the alternator. Battery is on charge at the moment so I will check later on.

[Longboarder]

OK

 

Nigel

[Guest mcramsay]

So fully charged battery. Engine at idle 14.2 volts on the battery.

 

Warning lamp has 14.2 on the ignition side and 12.1 on the output side from the alternator.

 

Run a new length of wire to bypass the diode with the same results... so thoughts please? I would assume that the output on the field wire on the alternator should be 14.2 volts not 12?

[Guest mcramsay]

So called the battery/alternator repair place. Output on the charge light should match the alternator output voltage. Indicates a failed diode in the circuit which would reduce alternator output.

 

Time for a new alternator.