Curious Voltage at Preglow Relay

[Elroy Jetson]

Greetings, fellow Diesel Heads. The Bismark, a stock 1985 300CD, has been intermittently draining her 3 year old monster Interstate battery. After a week, and at random -- sometimes it's fine -- I go to start, get a glow light on the dash for the appropriate time and get a quarter turn out of the starter before ....nothing. Oh good, an intermittent electrical problem on a German car.... She draws 20 milliamps at rest, so that's not it, and the charging system seems fine, judging by the output voltage across the battery when she's running. I'm suspicious of the Huco preglow relay. With the engine not running, I get 12 volts at the output pins to the glow plugs until the relay clicks off, then zero. So far, so good. With the engine running, though, I get 7.3 volts at each of the 5 pins, until I shut her down. I also get the glow light on the dash when I pull the connector to the plugs to measure the output voltage from the relay. I've let her run for half an hour, and still 7.3V at all 5. I'm thinking this isn't normal. Any ideas? Thanks.

[Jim H]

Originally Posted by Elroy Jetson

...I go to start, get a glow light on the dash for the appropriate time and get a quarter turn out of the starter before ....nothing.

I would focus on the "a quarter turn out of the starter before ....nothing." instead of the glow plug system. I had something similar happen last fall on my Suburban. It turned out to be corrosion on the negative batter terminal. I cleaned both terminals and the problem went away.

[Elroy Jetson]

Thanks for the thought, Jim. The starter turns just fine, and with great vigah, when the battery is charged. The terminals and ground are clean and solid at both ends. I'm thinking that maybe the glow relay is staying semi-energized - which I admit sounds crazy, but the 7.3 Volts is consistent - and is not letting the alternator properly recharge the battery as she runs.

[Elroy Jetson]

Seriously, guys....has anyone every measured the output voltage from the 5 pins on the preglow relay while the engine is running? I'm seeing about 7.3 volts from all 5 even after the engine has been running for 20 minutes. Thinking this might be the source of my electrical problems. Thanks.

[kolnerspieler]

You can check which circuit is drawing the current when the car is shut off. If you have a good ammeter you can check the current across the fuses when the car is off and see which circuit is drawing the current to drain the battery. You do not have to break the circuit meaning you do not have to remove the fuses as long as you have a good ammeter. You can just put it across the terminals of the fuse contact points and see what is drawn.