I have a 1985 Mercedes 300D and I have a couple questions on plugging it in. I would like to point out that I have used the search button but I found very few relevent threads and the ones I did had many opinions on when to plug in but neither helped me with question 2 and there were just too many answers for question 1. I am basically looking for a straight answer, what it would say in the manual.


1. At what temperatures should I start plugging the car in and for how long?

2. If I decide that I need my car plugged in for an hour before I leave for work but I only get up 15 minutes before I leave, is it alright to leave the car plugged in overnight?

It depends on how cold it gets where you live but I believe that temps near or below freezing warrant plugging in your block hear. Standard MB block heaters are 400 watts. After market heaters are often 500 watts. Put it on a timer and experiment a bit. Your block should not be cold to the touch after your warm up time.

I have seen posts here from people living up in the frozen North that leave them on over night and others that only heat for a few hours before their morning drive. I have rarely seen one burned out even if left on for weeks on end.

I live in darn near tropical weather so the only time I benefit from a block heater is when I am burning veggie oil. It starts and idles like I only parked it five minutes before. Heater works right away too.

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You can get farther with a smile and a gun than you can with just a smile.

1984 300D
1985 300CD
1980 300SD
1993 SL500

It depends on what temp you have trouble starting your engine. It is OK to leave on overnight. If you want to be picky about it just buy a timer at your local Walmart and have it turn on 1-2 hours before start time.

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'85 300SD (formerly california emissions)
'08 Chevy Tahoe
'93 Ducati 900 SS
'79 Kawasaki KZ 650
'86 Kawasaki KX 250
'88 Kawasaki KDX200
'71 Hodaka Ace 100
'72 Triumph T100R

i plug mine in when the temperature is below +15 deg F. plug in 3 hours before you need to start the engine,otherwise it wastes electricity imo.
this assumes good starter/battery/glowplugs

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1984 300D Turbo - 231k....totalled 11/30/07 RIP

You don't want to spend more for electricity than you save driving a diesel LOL

I have mine on a timer, turns on the heater 2hours before I leave in the morning, heater is toasty-warm as I back out of the garage and little smoke. Starting problems weren't the issue, just better for the engine and my cold fingers in the morning.

You can leave it plugged in all of the time, I used to with my CAT diesels, just takes electricity but it's ready any time and warm.

If it's about preventing starting problems I'd imagine that depends on the condition of your engine, the oil used, your starter and battery, vs the air temp. I'd think that any M-B diesel that wouldn't start at 20F or above without pre-heating the engine has other issues to be dealt with.

- Jeff Miller
190DT in Michigan

I usually plug mine in when it get around freezing. Mostly so I can have heat quicker. But the car is much happier upon start up too. I usually set the timer or plug it in 1-2 hours before I plan on going somewhere. 15 minutes probably isn't going to do much.
It's cheap to have to block heater plugged in so leaving in on all night isn't going to cost much. But you are kind of wasting electricity, IMO. Just play with it a bit and see what fits your needs and wants.

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I have a receptacle at work and leave my block heater plugged in all day. When I get home I plug it in again. My 85 300SD is pretty much plugged in 24-7 in the winter. I know it may waste energy....but think how much my engine loves me! The instant heat is a nice plus. Never had a problem with the regimen and I've been doing it this way for years. My car loves it!

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Orland Park, IL
1985 300SD 215,000 miles

Is there a concern on the amp capacity of the timer. What have people purchsed from 'Wally World'. Would a typical timer that is intended to light an interior lamp in the house be sufficient?

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'82 300SD 206K miles Anthracite Grey received 250,000 Km badge
'93 GMC Turbo Diesel 1-Ton Dually 113K miles Stolen 17 Jul 05
2005 GMC 2500HD Crew Cab D-max 22K miles Love it!
'68 Corvair Monza 110 Coupe 26K Sold
'66 Corvair Monza 110 Convertible 123K
'52 Ford 8N Sold
'66 Ford 3000 Diesel (204 hours) For Sale
'86 White 2-65 MFWD Iseki Diesel

timer is key. mine start 1.5 hrs before leaving for work. this weekend is supposed to be single digits so i will have them plugged direct and going all weekend. electric bill be damned!!! i pay on the budget!! there is a lot to be said for a warm heater at start.

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currently
[1981 300 td tdidi 165500 dark brown/palamino-Brownie-mine-3k miles of ownership
1983 240d 162+++ Anthricite grey w/ henna red interior and hella lights-wifes car-Red

the above two cars are for sale
and can be seen on the cars for sale thread here. pix also available.


240d-144+ Manilla Yellow w/ palmino interior-greasecar kit-Blondie-the college kids car

23" gt 21 speed still on original tires-still got the nubs
21" khs tandem

You're going to have to experiment as it really depends on your car.

I have one 300SD that can start WITHOUT glow plugs at 30 deg and another 300SD that won't start at 30 deg without cycling the glow plugs 3 times and a lot of cranking.

I think that less than 45 minutes won't do much other than accelerate your warmup time. It may let it crank faster, which would be a big help. More than 3 hours is a waste.

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Michael LaFleur

'05 E320 CDI - 86,000 miles
'86 300SDL - 360,000 miles
'85 300SD - 150,000 miles (sold)
'89 190D - 120,000 miles (sold)
'85 300SD - 317,000 miles (sold)
'98 ML320 - 270,000 miles (sold)
'75 300D - 170,000 miles (sold)
'83 Harley Davidson FLTC (Broken again) :-(
'61 Plymouth Valiant - 60k mikes
2004 Papillon (Oliver)
2005 Tzitzu (Griffon)
2009 Welsh Corgi (Buba)

Do any block heaters have a thermostat? I was thinking it would be cheeper to plug it in when the engine is warm after driving it. Then you would just be keeping the engine warm instead of warming up a cold engine.

Block heaters do not have thermostats (unless you scored something fancy )

I used a 10amp rated timer from fred meyers or something from the sort....its just one of those little cube shaped ones and seems to be working pretty good

Block heater comes on about 10-11pm and stays on until about sunrise....I have non-antifreeze water in my intercooler at the moment so I can't risk a freeze. I'll eventually put some antifreeze in there but I still have work to do

I am using this exact timer: http://www.homedepot.com/prel80/HDUS/EN_US/diy_main/pg_diy.jsp?CNTTYPE=PROD_META&CNTKEY=misc%2fsearchResults.jsp&BV_SessionID=@@@@1012405421.1140112402@@@@&BV_EngineID=ccgkaddhddlkfeicgelceffdfgidglo.0&MID=9876 Its rated at 15amps and 1750watts

I can tell the difference in my overall fuel milage if I plug the car in when it's under about 50F out, so that's what I do. It's on a timer, and unless it's very cold out, I give it about three hours.

A back-of-the-envelope computation showed that running the heater for three hours costs me less than eight cents. That won't buy much diesel fuel. However, with a marginal rate of $.059 per KWh, I probably pay less for my electricity than most people.

I bought a timer with a three-wire cord and outlet, which significantly raised its price, to about $20. Standard lamp timers are usually good for at least 1000 watts, so that's not an issue.

Many utility companies have a "time of use rate". I've been on it for many years now. Basically you pay a much cheaper rate ($.03/KwH) from 7pm to 7am, and a slightly higher than usual rate($.12/KwH) from 7am to 7pm. The idea is to encourage people to use electricity at night during off peak times. I would think that this would be a good fit with a block heater also. As far as timers go, usually around Christmas time you can find regular and heavy duty timers. Most other times all I've been able to find easily are the regular duty ones. I even have my freezer on a timer, heavy duty of course.

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'83 300D (220000 miles and counting)

It depends on the conditon of your car really. I always plugged the SDL in when it was in the 20's because I wanted heat faster and didn't want to put the extra wear on the engine. This winter when it drops below 20 I plug the SD in because it gets a little cranky if temps fall into the mid to low teens.

I usually set the timer so it comes on about 3 hours before I need to go anywhere.

Buy a high quality grounded timer and a good quality extension cord. I actually have the same timer Brandon posted, works great. Also make sure the block heater cord itself is in good shape, I have had to replace both of mine they don't last long. (dirt cheap though only $10 for a generic one)

These things draw a few amps so make sure you don't use a crap extension cord our you are asking for trouble.

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1999 SL500
1969 280SE
2023 Ram 1500
2007 Tiara 3200