Hi

I recently noticed that during night time driving, the voltmeter drops steadily almost to the red bar when the auxiliary fan clicks "ON". The reading goes back to the normal (3/4) position when the fan goes off. Alternator upgrade (to 80 amps) has been done a year before. Daytime driving is all normal.

What might have caused this? Some say the culprit are the alternator carbon brass.

Measure the voltage at the battery with a real meter. The dash meter is not accurate enough to know what is really happening. The fan could be bad or the alternator or a bad connection on the line from battery the power distribution block. Measureing actual volts at the battery will be start to proper diagnosis.
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David Hunter

We have done what you just said in a reputable electrical shop. The shop even used a heavy duty ground cable and positive cable leads to check if the voltage drop will improve or not.

As I have stated before, it all leads down to the installation of a auto idle-up system that is vacuum activated whenever the AC turns on.

Thank you all for the invaluable inputs.

If the alternator checks out, then I'd suspect the fan motor.

Do some poor man's load testing. Un-fuse the aux fan, then start the car, and start turning on appliances one at a time in order of anticipated draw, e.g., radio, lights, blower fan, lighter. If the needle drops each time something is turned on, then recovers in a reasonable time, and the only appliance that causes the needle to drop and stay there is the aux fan, that's a clue. Low level short, resistance buildup in the windings, failing capacitor?

Thank you for the inputs. We already did that procedure, it seems that the aux fan draws the most voltage drain. I would like to know specifically the problem with the auxiliary fan.

Hi
I had the alternator load tested, checked on all the grounds and main lines and the battery. Everything was found to be ok. The conclusion: I need to install an auto idle-up to compensate for the electrical load drain during night time driving.

The conclusion: I need to install an auto idle-up to compensate for the electrical load drain during night time driving.

Howdy Dindo,

Unless your 240 is a door-to-door delivery van, this is not your problem. Even if your idle RPM is too low normal drive cycles will spin the alternator fast enough for long enough to provide full charge.

If the battery voltage continually falls during normal operation, one of two things is happening. Either the load is excessive or the charge rate is inadequate. It is that simple. I, like Ken C, tend to suspect the alternator side of this equation.

The only way to know for sure which is the case is to measure the battery input and output currents. Post the results the shop recorded.

Off topic -- What did you do about your carburetor problems?

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Mr. Shannon DeWolfe -- I've taken to using mister because my name misleads folks on the WWW. I am a 53 year old fat man. ;-)

You upgraded your alternator?

Frankly, among the other good suggestions, I'd suspect that your "new" alternator isn't all that it was claimed to be. Did you get it new, or rebuilt? And if the latter, was it manufacturer-rebuilt, or from a local "motor" shop? Or, did you just take it off another car (at a pick-and-pull)? It could have several possible ills (as already noted), such as worn brushes and bad diodes.

The carbon brushes are possible culprits. The brushes are easy enough to inspect. If you suspect them, check them. But...

The problem might be an open diode in the alternator. Or a bad ground. Or resistive power cable to the battery. Or weak cell in the battery. Or...

You see what I mean? You need to check some things. Start with the battery. Load test the battery. If it falls too rapidly and recovers too slowly, that is a problem. Google "load testing battery".

If you see this happen only when the fans come on, there just might be a problem with a fan motor.

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Mr. Shannon DeWolfe -- I've taken to using mister because my name misleads folks on the WWW. I am a 53 year old fat man. ;-)

Thank you for the inputs, I will do it step by step and keep you posted.

Hello,

I would be very concered that the fan kicking on did this. Suggest you disable the fan until you can sort it out. Does you fan have an inline fuse first of all? If not may I suggest you add one, that will prevent damage to other componets if the motor does start shorting out.

Easy things first, check the connectors to the battery, starter, and alternator for corrosion of the contacts and the wires up in the crimps for tarnish and green copper corrosion. Suggest replacing any that you can not shine up.

Check the ground wire from the block to the alternator, you have to get a good ground path on the alterator housing. You should have less than .2 ohms using a decent meter from the housing to the engine block. If you do replace it, size matters here. Use at least a 14 guage wire from the ground connection.

Go to an auto parts store that has a newer diagnostic testers. Advanced and Auto Zone both have systems that are pretty good. Someone who understands how the unit works is a big help, I have seen a couple of kids out in the parking lots that have trouble finding the battery. The test can vary the load on the system to check the voltage output, monitor for excess ripple (points to a bad diode)and measure the current output of the charging system. Now this is not the end all for system trouble shooting. But it will give you a place to start.

Regards,

Paul

Hi Paul

Thank you for the step by step diagnostics. And yes, the fan has its own fuse. I'd get right on checking all the connections. I will keep you updated on this.