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Old 02-14-2007, 12:26 AM
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Join Date: Jan 2006
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Lightbulb Block Heat Resets Breaker & Flashing Rear Defrost Light

About 2 weeks ago, I notcied that when I plug my block heater into the recepticle it would resets the breaker at my electrical pannel at home. So I discontiued using it.

Is it a simple fix of putting in a new cord and heater? Or could it be something more complicated?



Secondly with this -20 oC weather, I experienced a drained battery today when trying to start the vehicle after leaving it off for 20 min. while shopping. I got a boost and made it home. While driving home I notcied the rear defrost light flashing for about 10sec. and doing this every minute or so.

I left the vehicle idling on the road while I cleared out the garage and then I drove it in. While the vehicle was idling on the road I notcied the rear tail lights DIMMING, inside the vehicle the radio had turned off and the blow motor for the heat was slowing down. I turned off the headlight and the radio came on and the blow motor went back to full speed. I quickly drove the vehicle into the garage and turned if off. It won't start, and I'm wondering if this could be more than just an putting in a new battery.

I had the alternator rebuilt in February 2006. But the battery has been in the vehicle since I bought it in November 2005, so who knows how old it is. I going to purchase a new one tommorow, but I'm thinking it will not solve this problem.

Any ideas would be apprechiated. Thanks.

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Old 02-14-2007, 08:24 AM
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Join Date: Aug 2006
Location: Houston, TX
Posts: 450
You didn't indicate what car you have, but on my '92 300SE, if you turn on the rear defroster and the battery voltage drops too low, the defroster turns itself off and blinks the indicator on the switch.

Turning off your headlamps and having the radio and blower resume more proper operation is a true indication of insufficient voltage in the charging system. Likely your alternator is dead or dying and you're running the car off the battery. With a new, fully charged battery and all accessories off, you should see 14.2 or so volts at the battery at practically any engine speed above idle, and maybe even at idle. Of course, with temps like you're indicating (brrrrrr) that may be a pipe-dream.

Note that only certain failure conditions on the alternator will turn on the charging light. The alternator can be toast and you never see the red light.

If your block heater is popping the breaker, it's shorted out. The heater is similar in design to a electric water heater element. If it's tripping breakers, then there's an internal short due to water infiltration. I don't want to think about what an alternating current short-circuit into the engine block would do to sensitive electrical items like the regulator on your alternator. If the short is not to the housing of the heater, but rather inside the element, then the current leakage is likely into the coolant of the motor, and therefore the current flow will be limited - but still more than capable of messing stuff up.

First warning - don't ever use a block heater on a circuit that's not protected by a GFCI (ground-fault circuit interruptor). Those devices can detect minute shorts to ground and will disconnect the power instantly. You can literally grab an open 120v wire on such a circuit and not even feel a buzz before the circuit opens. A breaker, on the other hand, must experience current draw in excess of the rating on the breaker for a "long" time (electrically speaking) before the circuit will trip - lots of damage can occur during this interval, both to people and to things like cars with faulty block heaters.

Very likely your problems are related.
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Old 02-14-2007, 11:21 AM
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Thanks. It a 1990 300D 2.5 Turbo.

So would you recommend pulling the alternator and having it checked and then if that doen't fix the block heater problem, then to replace the block heater.

Thanks for the tip on GFCI, I will install one today.
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Old 02-14-2007, 11:55 AM
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Join Date: Nov 2002
Location: Milford, CT
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Replace the cord!!!!!! On every one of these cars I have come across the original block heater cord has been shot! Usualy there is exposed wires, and those exposed wires are frequintly covered in dried oil! Talk about a fire hazard, I won't use a block heater in one of these cars until I inspect/replace the cord.

New aftermarket cords are $10, replace it every couple of years.

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