A couple of weeks ago my battery light came on so I got my alternator and battery checked. Alternator came back bad so I replaced it and I charged the battery back up and it was deemed good. Car has been good except my aftermarket radio doesn't display anymore but I know it gets power because I can eject DVDs, until about a week ago when it wouldn't start one morning. I jump started it and it happened again in the morning about 2 days later. I checked my fuses for a draw and 3 fuses had current after the car was asleep, the radio, HVAC and the IPC. Before I give the measurements of the fuses I want to make sure I have my multimeter setup correctly. I also did an inline test of the negative battery cable and that number was 6.5 while my multimeter was set to 200mA DCA. Does that 6.5 number translate to 6.5 amps when the multimeter is set to 200mA? If so, I know 6.5 amps is too high for resting current. Below is a picture of how my multimeter is setup when I did the inline battery test. Is this the same setup I should use when testing fuses?

******Update******
After finding a conversion chart for milliVolts to milliAmps I believe I have the correct readings for the IPC, radio and HVAC fuses currents.
IPC fuse = 337 milliAmps
Radio fuse = 27 milliAmps
HVAC = 44 milliAmps
Are these readings too high and will cause my battery to drain overnight? Do these reading coincide with the inline battery reading of 6.5?

 

Also, should the 125 mega fuse have current while the car is started? Mine doesn't, can a blown mega fuse cause this drain?

 

it's the milliamp range, so 6.5mA


measure resistance


You don't want much more than 100mA when the vehicle is off. But you have to wait for everything to go to sleep first. Wait an hour after you've closed all the doors before measuring.

depends what the fuse does. You'll have to check a schematic. If it's feeding your cabin fuse block, then there should be some current at least, for your instrument cluster lighting.

no

 

I did let it sit for about a hour. I measured the fuses using the 200m DCV setting on the multimeter then converted the reading using a chart with the type of fuse. For instance, the radio fuse reading was .2 milliVolts and a 10a mini fuse, that converted to 27 milliAmps.

 

if this fuse is for the alternator, and it's blown then your battery won't charge.... which would also result in a dead battery.

 

Screw the chart that converts milliVolts to milliAmps! There is no such direct conversion! No two fuses will have the same resistance. They will be close to one another, but not identical.

The only way a conversion chart will work is if it is for a known resistance. Ohms law states Current (in amps) equals voltage (in volts) divided by resistance (in ohms) ( I = E/R).

To me it appears you are measuring the voltage drop across the fuse. Unless you have a high priced DMM that is calibrated and certified to be accurate at the sub-milliVolt magnitude, your readings are meaningless and you will be chasing your tail for a ling time.

Is this aftermarket radio one you recently installed? If so, and you did not purchase it and the installation harness from crutchfield.com, then that is problem.

 

I attached the chart i was referring to. It takes the millivolts measured by the multimeter of the fuse then you match that with the fuse type and amp then scroll down to get the corresponding milliAmps. It's an aftermarket radio but I've had this radio installed for 2 years now with no problem except for the display went out recently after my battery went dead and it wont come back on but the radio has power.

 

Again, that chart is really meaningless for automotive troubleshooting or any electrical troubleshooting for that matter in my opinion. It does not reference the fuse manufacturer or whomever authored it.

You are much better off just measuring volts and ohms with your DMM. There are too man variables in a circuit to accurately measure the voltage drop across a fuse. Plus, if there is any corrosion on the terminals in the fuse socket, that is going to change the resistance of the fuse and then those numbers become meaningless.

The only thing you really want to know about a fuse is if it is good (not blown) or bad (blown).

Also, I would not trust the low millivolt readings from an inexpensive DMM - especially one from Harbor Freight. I have several DMMs ranging from a couple of inexpensive ones that I consider to be expendable to a top of the line calibrated and certified Fluke 87 DMM. When I need to measure something in the under 100 mV (0.1 V) region, I would only trust the Fluke 87.

Inexpensive DMMs do have their place. They are adequate for measuring voltages in the range of a few volts to 500 volts (AC or DC). They are fine for seeing if the car battery has an acceptable voltage (say above 12 VDC) of if when I crank the engine if the battery voltage stays above 9.5 VDC. I certainly would not trust it for determining if the voltage is 12.00 VDC or 12.01 VDC

Also, I see you posted this same thing on the TrailVoy forum. I'll just re-post my comments there as well.

 

Thank you for the reply. I removed the radio and put the factory radio back in to see if it goes dead again. I'll update tomorrow.

 

Thank you for your help. I removed the aftermarket radio and put the factory radio back in and the battery doesn't drain anymore.