Brake sensor wires

[zeke]

I am still troubleshooting my poor charging problem in the 300TD

Alternators go in putting out proper voltage (~14.1-13.7V), but after a couple of weeks, performance declines and they stop working.

It has happened 2 or 3 times (once there was a bad battery in the car as well, so I am not sure which came first.)

While I am waiting on a replacement alternator I am digging into the wiring.

The wires coming to the 3 port junction just behind the expansion coolant tank all look good - I had previously cut out the burned ends and spliced new terminals on.

But, the brake sensor wire on the PS is pretty rough. The outer insulation had cracked, so I wrapped it with electrical tape. Now, the electrical tape is showing signs of heat distortion, and the modular plug on the cabin side had melted and warped and was burned right through right above where one of the wires connects with a socket. The connections themselves have corrosion in them. So clearly this wire must be replaced, and I need a new modular socket on the cabin side as well.

My question is if you think this is enough of a problem to cause my alternator failures.

On a recent trip, about 4 hours of continuous highway drving with only the headlights on (no radio, AC, interior lights, or anything in the cig lighter) led to a 60% loss of charge.

[scottmcphee]

Put an amp meter between the battery and +ve lead and see how much she is drawing while running.

[zeke]

thanks, will do!

[scottmcphee]

If you don't have a clamp type DC amp meter, the best way to avoid blowing meter fuse ...

1. car off
2. remove +ve lead from battery
3. place a jumper cable from +ve on battery to the car lead
4. start car, leave running
5. place meter probe tip on +ve battery, other probe tip on lead to car. Make sure these probes are firmly on their game. Meter set to highest DC Amp setting, wires plugged into correct holes on meter
6. now remove jumper lead, now the circuit if flowing through the meter

If you just hook up the meter to take load, and then start the car, you will blow the meter fuse, every time.

[Yak]

Originally Posted by zeke

I am still troubleshooting my poor charging problem in the 300TD

Alternators go in putting out proper voltage (~14.1-13.7V), but after a couple of weeks, performance declines and they stop working.

It has happened 2 or 3 times (once there was a bad battery in the car as well, so I am not sure which came first.)

While I am waiting on a replacement alternator I am digging into the wiring.

The wires coming to the 3 port junction just behind the expansion coolant tank all look good - I had previously cut out the burned ends and spliced new terminals on.

But, the brake sensor wire on the PS is pretty rough. The outer insulation had cracked, so I wrapped it with electrical tape. Now, the electrical tape is showing signs of heat distortion, and the modular plug on the cabin side had melted and warped and was burned right through right above where one of the wires connects with a socket. The connections themselves have corrosion in them. So clearly this wire must be replaced, and I need a new modular socket on the cabin side as well.

My question is if you think this is enough of a problem to cause my alternator failures.

On a recent trip, about 4 hours of continuous highway drving with only the headlights on (no radio, AC, interior lights, or anything in the cig lighter) led to a 60% loss of charge.

If the connectors are melting or burning through, you're getting a significant current draw. Since the brake pad wear sensors are nowhere near a critical item, I'd disconnect them completely until you can clean up the connections and replace/rebuild the harnesses.

I used a length of vacuum tubing and re-shrouded the wires on mine. The insulation on the individual wires was okay, but the outer tube/shroud was dry and crumbly. You may have an electrical hobby-shop project on your hands.

[zeke]

Originally Posted by Yak

You may have an electrical hobby-shop project on your hands.

I am so afraid of that!

OK, so I didn't have to start the car at all - I realized that the brake sensor is activated in the last accessory position of the ign switch.

In the 240D, the yellow light comes on before ignition.

In this car the Alt light used to come on prefire, then stay on after ignition until I hit the throttle the first time. Can't remember the last time I saw the yellow brake light.

Now neither light comes on.

So I tested the voltage across the sensor wire leads and with ign off, it is 0, with ign in last position it is ~12 (GP relay was active in that position too, so it droppped duing that).

Then I tested current from the hot lead of the brake wire plug on the PS (melted one) to the neg. terminal of the battery.

No continuity down to 200k range, and there it read ~163 ohm

Moved to the DS for comparison: No continuity down to the 200 (smallest) scale, but there it pegged and the meter screamed at me. Back up, and it shows no continuity at larger scale???

Not sure how to interpret this.

[Yak]

Originally Posted by zeke

I am so afraid of that!

OK, so I didn't have to start the car at all - I realized that the brake sensor is activated in the last accessory position of the ign switch.

In the 240D, the yellow light comes on before ignition.

In this car the Alt light used to come on prefire, then stay on after ignition until I hit the throttle the first time. Can't remember the last time I saw the yellow brake light.

Now neither light comes on.

So I tested the voltage across the sensor wire leads and with ign off, it is 0, with ign in last position it is ~12 (GP relay was active in that position too, so it droppped duing that).

Then I tested current from the hot lead of the brake wire plug on the PS (melted one) to the neg. terminal of the battery.

No continuity down to 200k range, and there it read ~163 ohm

Moved to the DS for comparison: No continuity down to the 200 (smallest) scale, but there it pegged and the meter screamed at me. Back up, and it shows no continuity at larger scale???

Not sure how to interpret this.

I'm not sure what you're saying/measuring here. If you put an ammeter to measure current from the hot lead (+12v) to the neg side of the battery, it should have sparked, then blown the fuse since that would be a dead short from +12 to ground. Unless by "hot" you mean temperature 'hot' (melted) and not voltage 'hot' (+12v) and you're looking for resistance/impedance (ohms) and not current (amps).

Can you tell what color wire you're measuring: brown or brown/red? If brown, then you should have very low ohms from that pin to ground. If brown/red it could give some funky readings since the meter may get confused by the added current from the car in addition to what it's providing.

Does your parking brake light work? What about the low fuel light? These lights share a connection on the cluster wiring trace that goes back to the alternator. It's possible a short in the cluster or at the brakes may be contributing to killing the regulators in the alternators.

Can you set your meter to Amps (current) and then connect from the brown/red wire on the "car" side of the sensor to ground and measure what happens with the ignition on? You should see a low current draw that equates to the light bulb in the dash and see the bulb glow. You're basically replicating the pad wear sensor closing. Assuming something like a 1.2 Watt bulb, the current ought to be 0.1 Amp (P= IV, 1.2 W = 0.1 A * 12 V)

[compress ignite]

Zeke,

I don't believe (You) (or anyone else reading this) has understood the significance of your sentence:
"The wires coming to the 3 port junction just behind the expansion coolant tank all look good - I had previously cut out the burned ends and spliced new terminals on."

HOW DID THE ENDS GET BURNED?
(Just cutting off the SCORCHED ends of the cable does not "FIX" the CAUSE)

Did this chassis suffer a FIRE?
[It MAY in the Near Future,with the symptoms you describe]

Find the CAUSE of the Power Transmission Cables BURNING and then you can
confidently fix the problem.

Unsolicited Rant:
If I've got Alternators "Going South"
(I don't know what else to call it,'Cause you can't tell us How,or What FAILED in the Generation System.)
AND I'm seeing Burned Power Transmission Cables...
I'm not risking Myself or the Chassis by operating the Vehicle.
UNTIL the Anomaly is COMPLETELY diagnosed and rectified!

[carnut]

Alot of cars will not charge properly with an inoperative battery warning light to load the field windings in the alternator. I would fix the reason the light does not come on in the run postion. Good chance that will fix the low output. All the Mercedes cars I've owned over the years never required an alternator replacement, but they all needed voltage regulators eventually and were hard on the alternator belt, wearing it out or loosening sooner than my other cars.

[carnut]

My 86 BMW will not charge if the "BATTERY" light burns out in the speedo cluster as an example.

[zeke]

Will go play with ammeter tonight once the kids are asleep.

CompressIgnite - You are right - there is a problem in the power distribution system that has lingered for some time. The original junction box melted and compromised the insulation of the four wires that terminate on the rear-most post. Here is that post:
Wire ID

The cable that comes from the starter is fine.

I replaced the box and also cut the wires past where the insulation had melted. Since then, the 'new' wires seem to be ok - not carrying too much heat, though it does appear some of the plastic on the new box has warped a bit.

There has been no fire, just heat.

I think I mentioned that this is not a VR problem as I replaced the original one, and have since used a rebuilt alt. with a second new unit.


Can't be sure the bulb is not burned out at this point, but it was working while this problem was ongoing.

The only wire I can SEE that appears compromised is that brake sensor wire.

[zeke]

The current on the sensor wire was 0.08 at the 10A setting on both sides. After cleaning the leads, it seems I restored continuity, as the brake light came on again in the run position. But, the brake light is red, not yellow?

It pulsed (dimmed) when I revved the engine, but didn't go out. Maybe this was the light I thought was the alt light, b/c I was thrown by the red color?

May be a problem at the cluster?

Also wanted to add that the parking brake light does work. Always has.

[Yak]

Red brake light is for brake fluid and park brake and says "brake". Wear sensor is the circle with dashed lines and is yellow. The color is determined by the film strip, so it'd be really odd to have a red-tinted wear sensor strip. Possible with sunfade or something, but odd.

0.08 sounds okay for a 1 (ish) Watt bulb, so you shouldn't be getting too much current through the pad wear sensor line to melt the connector.


Alt light is a little battery pic so should be hard to confuse.

Left to right: high beam (blue w/bulb pic - probably faded to clear-ish); alt or batt (red w/ batt pic); wear sensor (yellow/amber w/ circles) park brake and fluid (red w/ "brake"). These are hard slots in the left cluster, not like the loose wires and sockets on the other side so they shouldn't be able to be moved or swapped.

[zeke]

OK, pulled the cluster and everything seems to be in order. Ground is secure. No obvious burned wires. Checked the bulbs for both the wear sensor and battery lights and they are fine. But, they do not come on at all, ever.

I am so at the end of my rope with this wagon. I think we are about to break up

I am waiting for a warranty replacement alternator to arrive, and currently drive around with a spare battery and recharge the one in the car with a charger every night.

Any help would be so very much appreciated!