VM-1000 Current Indication Troubleshooting
The VM1000 uses a hall effect current sensor. The sensor responds to amperage by generating an induced voltage of 0.036 volts per amp flowing through the alternator load or battery discharge cable (depending on how you have the sensor installed). Here's how I would recommend troubleshooting this problem:
First thing to check is the wiring between the sensor and the DPU. Older syle sensors (with tan colored PC board) will have a tendency to peel off or break off the wire contact points unless properly installed with strain relief for the signal cables. Newer style sensors have the green PC board and a more robust blade/receiver design but can crack at the base of the soldered blade assembly unless properly installed with strain relief for the signal cables. Also look for the shielding to be grounded only at the DPU end, otherwise you can generate a ground loop.
Next, trace the signal cable back to the DPU looking for excessive bends (internal wire breaks possible), chafing (shorting the signal wires), or obvious external breaks. On older screw type terminal DPUs, take a close look at the connections J3-13, 14, 15, and 16 terminals. Make sure the signal wires are secure in the terminal, tug slightly on each one and make sure the screws are snug. Don't overtighten the screw terminals or you'll break the wire or worse yet strip the small driver screw that opens and closes the lock jaw mechanism. On serial connector type DPUs, carefully remove the 37-pin P1 connector, remove the shield, and inspect the connections to pins 30-33 for security/continuity, then reconnect P1 with shield removed for subsequent tests.
If the wiring is fine, then move on to power on testing. First, for screw terminal type DPUs only, remove the J3-13, 14, 15, and 16 wires from the terminals, tape them up away from the DPU. Apply power to the unit and look for "00" amps indication, anything other than a zero indication and you have a DPU problem that will have to be fixed. There's no easy way to do this on the serial connector DPUs other than to use a pin extractor and pull pins 30-33 and reconnect the P1 connector. Reapply power and look for the 00 amps indication. With power still on the system, using a voltmeter, check the reading from J3-15 (red) to 16 (black) or pins 31 (red) to 30 (black), proper voltage is 10VDC steady. Anything other than this and you have a DPU problem. If the DPU passes these tests, reconnect all removed wires/pins (and reinstall P1 connector for serial DPUs).
Next, with the sensor reconnected to the DPU, apply power to the system and check for proper excitation, reference, and signal voltages. Using a voltmeter, check the reading from J3-15 (red) to 16 (black) or pins 31 (red) to 30 (black), proper voltage is 10VDC steady. Next check the reading from J3-13 (green) to 16 or pins 32(green) to 30, proper voltage is 10VDC steady. Finally, check the reading from J3-14 (white) to 16; here the zero current voltage should be 5.0VDC. Depending on how your amp sensor is installed, this voltage will either increase or decrease by 0.036V per amp. For example, on my aircraft with avionics bus turned off, the only overhead current is to run the VMS, a few indicator/warning lights, and the master relay, about 2 amps total battery draw, so I would see 5.0 - (0.036 x 2) = 4.928VDC (I have the amp sensor wired so that current from the alternator to the buss is positive indication and from the battery to the bus is negative indication). You can then apply a heavy draw, such as the landing light, to see a corresponding change in the VM1000 indicated amperage and corresponding voltmeter reading change as well. If you do not see the proper voltages here, then the sensor is bad and needs to be repaired or replaced.
If you need repairs or replacements for your system, I'll be happy to help out. Feel free to email me direct as I don't check the forums here every day. Good luck on the troubleshooting!
