Alternator Voltage Drop Testing

A loss of proper voltage or a weak signal from an alternator
can cause more than just lower than normal battery voltage, but
also can affect the rest of the electrical system as well. 
Determining where the problem is can be a challenge but a
little research and some diligent effort the job of finding a
voltage drop in a charging system can be narrowed down to
just a few common places.

Symptoms of a Voltage Drop from the Alternator

Voltage drop symptoms vary depending on the severity or cause
of the actual voltage drop. Some of the signs are:

•inoperative systems
•sluggish, lazy, or sporadic operation of some systems
•devices that work sluggishly or erratically during periods of high electrical loads
•no-starts or hard starts
•high sensor or computer voltages
•erratic engine or transmission performance
•false trouble codes in the memory of any on-board computer

A voltage drop can be defined as; “Voltage that has changed from the source to the load, without first going through the load.” The real challenge is finding it.

Defining a Voltage Drop

There are several factors to consider when checking for a voltage drop.  Things like the type of circuit and how it is setup in the vehicle.  Connections are the primary cause of voltage drops.  Weak, corroded, or overheated connections are going to be the prime places to look. This requires not only a look and test of the positive side but also the negative side of the charging system.
Some manufacturers use an in line main fuse to the alternator output lead, while others use a fuseable link.  On the modern car the voltage regulator control is part of the PCM while the battery indicator (charge light) in the dash has only a cursory job in the role of determining the charging system quality.  (Most older cars the dash warning light was directly connected to the voltage regulator and the regulator were connected directly to the alternator.)

Checking for voltage drop

Start with a good ground, which would be the negative battery post.  Everything electrical in the car relies on this very post to actually complete the electrical path to every type of circuit in the car.  Somehow, someway every circuit will use the same ground path.  The normal voltage drop on a wired circuit should not exceed .1V or 100mv.  (Connectors for wired circuits should not exceed 0.0V drop from the male to female connector.)

Common Connections to Check

Start to look for the source of the voltage drop at the source of the voltage, namely the battery connections.  I prefer to work from that direction most of the time while following the wiring schematic for the vehicle.  (Never fails the problem will be on the other end if I start at the battery… but of course… if I start at the other end it will be at the battery… go figure.)
Examine the connections carefully.  Especially with the screwed down fuseable links that are in popular use these days.  Some of these are covered with little plastic windows on the sides of the fuse box areas.  Take the time to actually look at these or using your multi meter read the voltage drop on either side of the connections.  (Keep in mind… connections should have 0.0V drop)
Do the same with the negative side as well as the positive side.  Both are just as important.  Another thing to keep in mind is that no matter how complicated or intricate a circuit can be they all have the same thing in common when it comes to voltage and grounds.  They all have to return to the battery at some point, and the rule for voltage drops still applies.

Of course with most computer circuitry voltage drops are even more critical than on other circuits.  Their starting voltage is already reduced to 5 volts or less and if there is a weak connection or a broken ground lead the voltage drop may seem small but to these circuits it’s a big deal.

Other Connections to think about

I’ve mentioned connections, but did I mention switches?  A switch is nothing more than a moveable connection.  Ignition switches, toggle switches, headlamp switches…etc… are all nothing more than a connection.  Switches are another common place to look for a voltage drop when all the other connections check out fine.  Checking the switches in both positions and record the differences.  Any differences between the two values would indicate a fault in the switch.

Power Voltage drops

Voltage drops don’t always have to be stationary, or constant.  A lot of times you’ll run across a voltage drop that seems to be only there when a load is present on the circuit.  A good example is a charging system at rest (engine not running) shows a perfect signal from load to source and back, however when it’s started the voltage drop is present.  Sometimes very severe, sometimes no charge output at all can be read at the battery while at the back of the alternator everything seems perfect.  This type of power voltage drop can be the hardest to locate.  One of the primary reasons can either be connections as with any other type of voltage drop to internal problems with the battery (plates gone bad) to the wire itself not allowing the current to flow. The last thing that would be a suspect in this problem would be the alternator, but don’t rule it out until you’ve checked the current flow as well as the voltage output. 

Voltage drops are common and one of the biggest culprits in false diagnostics or changing of parts that weren’t bad to start with.  It’s always a good practice to check for any voltage drop when testing an alternator circuit before rushing in there and changing parts.  Make voltage drop testing part of your normal procedure for alternator output diagnostics.

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