View Full Version : Voltage drop test

08-01-2007, 01:45 AM
this boat is cranking slow the meter reads 2.1vdc measured across
A [battery +] and G [starter +]

what could my fault be ?

08-01-2007, 06:23 AM
The loss could be caused by loose or dirty connections anywhere along that circuit. The contacts inside the relay could also cause it. To narrow it down measure the loss at different points in the circuit. Measure between A & E , E & F , F & G

08-01-2007, 08:44 PM
Just battled through the same issue. My problem ended up being shot starter bushings creating armature drag and excessive amp draw within starter motor. This kills your voltage delivered to the starter. But before I assumed that was my problem I would test the output of the battery (at the post), the delivered voltage to the solenoid (cable resistance), adn cable resistance from solenoid to the starter. To confirm that I was battling a starter I used a jumper cable from battery post direct to starter motor, and when cranking my voltage dropped to less than three VDC. Rebuilt marine starter for 85 bucks and problem was solved. Good luck.

08-01-2007, 09:28 PM
Rektek- I assume you're using that relay diagram for convenience and there isn't a Bosch style relay trying to send current to a starter, right? A heavy duty relay, more like a solenoid switch should be used, not a Bosch style. If the voltage drops 2.1Vdc when you crank, look at it by connecting the meter to the + and - battery terminals. You may see a lot more that that as a drop, but due to the amount of current drawn by a starter, it's normal. You need to start at the battery + and keep going through the whole circuit, not just at one place, and then sum the various drops. A voltage drop across those points would not indicate a problem. If you start at the battery + and go to the ignition switch (B), then from B to C, and so on, it will be a valid test. Move from connection to sequential connection and sum them. Most will be very small. If you see anything large, other than the starter itself, find a way to correct it. Many times, a battery terminal will be the cause of trouble but it looks totally clean and normal. Same for a ground point on the block. Remember, the starter grounds through the mating surface and bolts, which all must be clean and unpainted, then the ground bolt on the block is the next point of contact, after which you go back to the battery negative. All voltages reference to the battery ground post, not the terminal on the cable and all voltage drop tests start at the battery + post, and not the terminal. It's easy to have a 2-3Vdc drop at one terminal alone. I have seen a 3+ V drop because of a corroded and dirty ground on the block. I have also seen a boat that ran well for the first 25 hours, then on the day before it was scheduled for its 25 hour checkup, it quit and wouldn't start again. The fuel pump relay ground was not connected well at the block ground and when they made a hard turn, high engine compartment temperatures caused the wire insulation to relax and the wire fell away from the ground bolt.

08-07-2007, 01:24 PM
Check for 12V from battery to the selenoid to the ignition switch. 12V all the way? Good. Then check voltage at starter when it engages, if it drops below 12V your starter is bad. I have a long thread on this issue if you search my name.

08-07-2007, 01:35 PM
I didn't get really spiffy starts until I replaced my "S" terminal wiring...both to and from the key switch - my boat was sluggish and eating solenoids due to lack of current...I went with 10 ga. as a fix - it FIXED it - 'nuff said...

Maybe it's overkill but for the cost of some 10/2 and a couple hour's time pulling it in - hey...

08-08-2007, 08:14 AM
I would check from A to E, A to F, E to F and so on. you need to pinpoints the part of the circuit with the problem, not the whole circuit.

08-08-2007, 08:15 AM
I would check from A to E, A to F, E to F and so on. you need to pinpoint the part of the circuit with the problem, not the whole circuit.

08-08-2007, 08:16 AM
sorry bout the double post

08-14-2007, 08:28 PM
Poor or corroded connection may cause unwanted resistance.
this can affect accuracy of sensor inputs and outputs.

Excessive unwanted circuit resistance may also cause circuit components to operate poorly or not at all if circuit voltage drops below normal.

Measuring voltage drop is a better way of checking circuit resistance than just performing a ohm check across a dead circuit.

The circuit must be energized for a voltage drop to be performed.

Suggested maximum voltage drops:
High current positive 200-500mv
high current negative 100mv

electronic pcm circuits 50mv,
light gauge low current circuits 90-100mv

Solenoid switch contacts 300mv

DVOM meters are cheap and should be carried on board in your tool kit.
get familiar with its function, make some checks when your boat is working fine so you're not caught off guard.

hope this helps