PeachParts Mercedes-Benz Forum - View Single Post

jay_bob · #27 02-11-2013, 01:38 PM

I agree with Whunter, the battery is breaking down under load or you have a bad connection.

Zach, are you measuring the output voltage at the GP relay with the harness plug in place or removed?

I assume you are measuring the voltage with the plug out, therefore you are getting the system voltage with little to no load. Although I would like to see something north of 12 V in this case. 11.99 to me looks like the beginnings of weak battery syndrome.

When you apply the huge load to the battery when hitting the GPs, you will pull down the system voltage. This is because you have internal resistance in the battery. You are setting up a voltage divider between the voltage drop across the internal resistance of the battery and the resistance of the GP system.

A little definition - a "cell" is a single element that produces voltage by chemistry. A "Battery" is a group of cells. An AA is a cell. A 9-V or the big black thing under your hood is a battery. The cell voltage is dependent on the chemistry, I forget what's in an AA but an everyday alkaline one makes 1.5 V, while a NiCd rechargeable makes 1.2 V. The lead-acid cell makes 2 V.

I'm sure you would accept that you cannot take 8 AA's (even though that makes 12 V) and try to start the car. This is because the internal resistance of a AA is much higher than a lead acid cell. All the voltage gets dropped across the internal resistance of the cells and that leaves none to start the car. (Don't actually try this, the AA's will see the load as a dead short and this will probably cause them to explode...)

A car battery is not a single cell, it is six 2-V cells connected in series. A common failure mode for a car battery is that one of the cells fails but the others are still healthy. If one of those cells has failed it brings down the entire battery, since the sum of the cell internal resistance is the total internal resistance of the battery. It is conceivable that you could still have dome lights, or the radio works, or the clock keeps time, because the current draw is low and not enough to where the high internal resistance becomes a factor. But when you hit the battery with a high load you cannot overcome that high internal resistance and the voltage collapses.

The first thing I would try would be to pull the battery and take it to a chain parts store, and have them put a load tester on it.

If that is not feasible for you to get to the store, or your battery checks out okay, try the following, you will need a helper. Be sure both the glow harnesses are plugged in at the relay.

Take your voltmeter and set it to dc volts. (I don't know your level of familiarity with a meter, so forgive me if I over explain things.)
Put the probes on the battery terminals, not the clamps, but the terminals themselves. This should be at least 12 V. Have the helper turn the key to preglow, quickly record the voltage then turn it off.
Have them turn on the headlights and the high beams, check the voltage, turn the lights back off.
If the voltage drops to 10 V at this point you probably have a weak battery and can rule out the rest of the system.

If you get closer to 12 V at the battery posts then let's go hunting for the weak connection. If your meter is inexpensive and has discrete ranges for the dc volts, put the meter on the 2 V scale first. If you get a reading near zero then step down to the 200 mV scale. If it is auto ranging (only one choice for DC volts) then be sure to pay attention to the display as the numbers may be Volts or MilliVolts (1 mV = 0.001 V). I am assuming you are using a digital meter that is auto polarity. If you have an old school analog meter you may have to swap the locations of the probes if the meter swings backwards.

Put one probe on the + terminal and the other on the + terminal clamp. Have the helper hit the glows quickly again, you should have nearly zero volts (looking for bad connection at + post of battery).
Do the same with the - terminal and - clamp (looking for bad connection at -).
Put one probe on the - terminal and the other on a clean spot on the block. Again hit the glows quickly, you should have near zero volts (looking for bad ground straps).
Put one probe on each screw of the glow fuse, hit the glows, should get near zero volts. This will probably be several tens to hundreds of mV, owing to the resistance of the fuse.
Put one probe on the + post of the battery, the other on one of the glow relay fuse screws, hit the glows, should be near zero (looking for bad main wire between battery-starter-glow relay).
Put one probe on the glow fuse screw, the other on the glow plug terminal nut, one at a time, hit the glows, should be near zero (looking for bad individual wire in GP harness from relay to plugs).

Good luck and stay warm, I won't tell you what the weather in SC is today...