Unknown load on electrical system

Evan · Apr 30, 2008

I believe there is a high load on my electrical system when my 1991 Explorer is running. Voltage is around 12v (it was always 14v in the past) but sometimes dips even lower. It is not consistant. The alternator and battery are new. I just replaced the alternator, and I have a feeling that this mysterious load is what caused it to burn out. Battery holds its charge fine when vehicle is off.

Is there something that would commonly cause this? It is occurring with the engine started and NO accessories (lights, blower, etc.) on.

mjonesjr · Apr 30, 2008

Have the alternator retested. I believe it is a bad alternator or bad alternator wiring.

MAKG · Apr 30, 2008

An open in any part of the charging circuit can cause this.

Did you measure voltage at the battery or across the alternator? You should do both (BAT to case). If you get different answers, the problem is in wiring. Find the voltage drop and you got it.

If the voltage is the same with the engine running at 2000 RPM as it is not running, the problem is not load, but rather the charging system being disabled. If the running voltage is measurably higher but not high enough, it may still be the charging system (excessive resistance or a blown diode in the rectifier, etc.), or it may be load.

If you think it's load, put a clamp-on ammeter around the power feeds coming off the starter relay, and you'll get a very large current measurement.

Evan · Apr 30, 2008

Voltage across the alternator and battery is the same. I also took the alt in for testing and it was good.

Today after a 15 minute drive (in 65 F weather) I opened the hood and the alternator was hot to the touch. I could only touch it for about 5 seconds at a time. I don't know if this is normal or not.

I visually inspected all the main wiring (bat, alt, starter relay, power box) and it looked good.

I was thinking that if something is overdrawing enough to lower my voltage this much, shouldn't it blow a fuse? My wiring is clean and mostly stock, and any custom wiring is fused.

MAKG, I don't own a ammeter. What do they cost? Would it be a beneficial tool at this stage? I do own a nice miltimeter.

I still have voltage in the "Normal" range on my volt gauge. Today when I started the truck the voltage looked good. As I started driving it went lower and lower (but still in normal range). Eventually when I went to use the blinker, my temp and oil gauge were moving up and down in rhythm to the blinker?? This is a somewhat confusing and annoying problem. :annoyed:

MAKG · May 1, 2008

Ammeters run the gamut from very cheap to extremely expensive.

You want the clamp-on type; you don't have to remove parts to use it.

Here is a very cheap one: http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=95683

Conventional inline ammeters typically won't handle more than 10-20A without blowing a fuse, and your charging system can easily exceed that, particularly if the battery is low. Clamp-ons can handle quite a lot more (this one has a 1000A scale).

Evan · May 1, 2008

MAKG said:
Ammeters run the gamut from very cheap to extremely expensive.

You want the clamp-on type; you don't have to remove parts to use it.

Here is a very cheap one: http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=95683

Conventional inline ammeters typically won't handle more than 10-20A without blowing a fuse, and your charging system can easily exceed that, particularly if the battery is low. Clamp-ons can handle quite a lot more (this one has a 1000A scale).

So this measures the inductive field in order to determine the amperage? Which means no cutting or stripping wires. Very cool.

I will look for one today and do some testing tonight, then post the results.

mjonesjr · May 1, 2008

MAKG said:
Here is a very cheap one: http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=95683

That one MAKG doesn't seem to measure amps. Most of the clip on voltmeters only measure voltage.

Ammeters are rather expensive for a AC/DC Ammeter. I have never seen one that was cheaper than $100.

This one however measures AC amps, but not DC amps.
http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=95652

Here is a Craftsman that is a AC/DC Ammeter and Voltmeter:
http://www.sears.com/shc/s/p_10153_12605_03473756000P?keyword=ammeter&vName=Tools&cName=Electrical+Shop&sName=Multi-Meters,+Testers+&+Accessories

MAKG · May 1, 2008

It says it measures AC amps, but that's not good enough for the task at hand.

There is a cheaper Craftsman that claims to measure DC A:

http://www.sears.com/shc/s/p_10153_12605_03482369000P?keyword=ammeter&vName=Tools&cName=Electrical+Shop&sName=Multi-Meters%2C+Testers+%26+Accessories

That one is $60. Not exactly $10, but it's better than $100.

For the kind of drain being sought here, it doesn't have to be all that accurate (though it's probably fine -- I'd expect the AC isn't so great, though).

Evan · May 1, 2008

MAKG said:
It says it measures AC amps, but that's not good enough for the task at hand.

There is a cheaper Craftsman that claims to measure DC A:

http://www.sears.com/shc/s/p_10153_12605_03482369000P?keyword=ammeter&vName=Tools&cName=Electrical+Shop&sName=Multi-Meters%2C+Testers+%26+Accessories

That one is $60. Not exactly $10, but it's better than $100.

For the kind of drain being sought here, it doesn't have to be all that accurate (though it's probably fine -- I'd expect the AC isn't so great, though).

I'll head to Sears after work and look around. Given the number of times I'll use this tool, $100 is a little expensive, but $60 is pretty reasonable.

mjonesjr · May 1, 2008

I may actually go buy that Craftsman you posted MAKG. I have wanted an AC/DC ammeter for a while now.

MAKG · May 1, 2008

I've been surviving on a traditional ammeter. One you disconnect stuff for. Which works great as long as the currents aren't over 10A. I've popped fuses more than once.

I find that for automotive applications EXCEPT for the one being investigated here, voltmeters are substantially more useful.

What I'd like to find is a 2 or 4 channel DSO for $60. With an HV probe, of course.

Evan · May 1, 2008

Well, I went to Sears, Harbor Freight, Mills Fleet Farm, and a few auto parts stores. Nobody sells a DC ammeter around here, and several employees told me that they didn't exist. :temper:

I really don't want to wait a week to get one shipped while my truck sits in the garage "broken".

Back to one of my questions: if there is something drawing that much current, shouldn't it blow a fuse?

Roadkill · May 1, 2008

You know what comes to mind? I had this happen on my JCB.

The cables from the battery, starter, and alternator all had corroded inside not only the connectors but the cables as well. This caused a BIG current draw on the whole works. They looked fine from the outside but once I took them off one by one and stripped some insulation I found a butt load of corrosion all through the whole set. Didn't cost much to replace them and it solved a big headache.

MAKG · May 1, 2008

Roadkill said:
You know what comes to mind? I had this happen on my JCB.

The cables from the battery, starter, and alternator all had corroded inside not only the connectors but the cables as well. This caused a BIG current draw on the whole works. They looked fine from the outside but once I took them off one by one and stripped some insulation I found a butt load of corrosion all through the whole set. Didn't cost much to replace them and it solved a big headache.

Umm, no way.

Corrosion does not increase current. It decreases it or even cuts it off.

Evan would have found this with his voltage drop test were it what was going on.

Roadkill · May 1, 2008

Corrosion caused a rise in resistance. Thus causing a resistive load much higher than normal on those cables as well as low readings on the voltage gauge. This was evident once the cables were removed and measured for resistance.

Unknown load on electrical system

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