It's an engine heater but not the original in-the-block heater. It's a lower radiator hose engine heater. There may also be an in-the-block heater above the starter.

__________________
1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

Yes, That's correct Kerry. I think the idea with this aftermarket "lower rad hose" type heater is basically the same as a factory blockheater except for the location. The o.e block heater is right in the freeze plug hole in the engine block as you mention and the aftermarket heater in the lower rad hose. I think the coolant in the lower rad hose is heated with an element (same as o/e block heater) and because it is low in the circuit allows for convection heat to travel up the hose and throughout the block, maybe even throughout the rad itself ?.

Like I said before it did a better job keeping the block warm for me up in the cold last winter than the o.e unit that was in there before.
I should mention that since posting previously I have gone through the service records of that particular car and found it to have had a new block heater installed in 2004 and burnt out in 2009. I am not sure what brand of block heater it was that was installed so I really should'nt use the term o.e for my particular circumstance in hindsight. It could have been a cheaper jobber unit put in to replace the original at that time I don't know.The car in question is an 85' model so the original if replaced in 2004 lasted for 19 years.(pretty good)

I was a little cocerned with the element affecting flow of coolant in the lower rad hose for summer operating temps. I kept close watch of the guage this summer and noticed no difference in operating temps.
I think the newer rad hose heater was quicker to heat up the coolant and therefore more efficient.
I noticed my power consumption bills get pretty expensive in the winter when plugging in 2 of these old diesel block heaters. I will definately be using a timer this winter. The lower rad hose unit I expect to perform better/cheaper with a timer since it is quicker to heat as mentioned.

My experience was the reverse. OE block heater was quicker than lower radiator hose heater.

__________________
1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

Temro brand Lower rad hose heater bought at Canadian Tire last winter in her 85' model. I'll pay more attention this winter as my car still has the original in block style block heater to compare to. May not be a fair comparison if it is the original 26 year old block heater in mine.
I am fairly certain that her block heater would actually make the temp. guage rise just a hair on the guage, I'll check that again this winter and maybe post a pic of it. That was a bit of a cinch for me to like this lower rad hose heater better.

I had my 2.2L engine and it started in the Montana Rockies without a block heater, but during the real cold times (-28 F), I had to double bump the good glow plugs and spend a deal of time getting it running. The rest of the time, it started OK, just took a little longer to heat up.

I bought an aftermarket heater for my 300D at Napa, and took it in for installation to a local shop in Fergus Falls, MN (Near Fargo, ND). I gave them a printout about the sage advice from others in extreme winter enviornments about putting in an OEM block heater (requires a torch applied til bolt is glowing red), but the mechanics would not listen and insisted on installing the OEM, which was available. After I said OK, on the basis that they give me a written estimate on time, they vowed to start it and have it finished in the morning. Well, what do I know . . . am just a woman and just a dumb truck driver. They didn't even want to read the info from here on this forum warning about the OEM black heater on an older car. Well, guess what . . . . they were not happy to see me the next day. They finally had to read the printout and fire up the torch, and I had them stuck on that written estimate. My advice . . . use the one that you have until it burns out. Replace it with OEM if the bolts are not welded on from time, or get the aftermarket replacement if they are as good as welded on.

Every winter that my 300D spent in Fargo area cost me a new set of glow plugs. But NYC was a piece of cake, even on the coldest days.

Torie

__________________
1983 300D, the "Avocado"
1976 240D, 4-spd the "Pumpkin", SOLD to Pierre
1984 190D, 2.2L, 5-spd, my intro to MBZ diesels, crashed into in 2002