In the place where I park my car, I have to go up a rather steep part of my driveway. I have never noticed until today, but when I went up that part of the driveway, I went a little slower then accelerated more on the hill than usual and noticed that for a couple of seconds, the low coolant light illuminated, then went off. It may have been coming on all along on that spot, but the way I turn when I am at that grade, the steering wheel hub would block my view of the light... its just this time I went on past my turn spot.

I then moved my Lexus in another part of the driveway and had to take the Mercedes back down and up again... again, it illuminated, but when I leveled back out where I park, it stayed on. I raised the hood and looked and the reservoir is 1/2 way up, which is where it always is, which is why I was puzzled when the light illuminated. Also, on my old 380SE, I had seen the coolant level way low in the tank and do not recall the light being on, but since it was a 1985, it may not have had that light. I cant recall.

Now, I am wondering if the sensor is in the reservoir itself, or inside the radiator and could this coming on on a very steep hill be normal?

Thanks in advance!

BTW - its a 1988 W126

In the side of the reservoir, held in with a circlip. It has a plug, with two wires clipped on it. It's just a simple float - if the incline is really that steep, it's no biggie. Add a cup or two of water, and see what happens. Now that it's a little bit colder out, the level might have dropped enough to make a difference.

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09 Jetta TDI
1985 300D

Thanks Larry... oh yes, it was much colder out today. Our high was only like 37° (which is like 20° BELOW average for our high this time of year) and it was about 34° when I was driving and this occured. I do not think its been this cold before since I have owned the car.

I want to get the coolant changed soon anyway, since it has the green stuff in it.

I guess its not a big deal, since there was really no other replies of it being a potential issue.

I will add that water though and before long get that coolant changed out to MB coolant.

Thanks again!

86560SEL, there are several potential causes for the low coolant light: 1) coolant level is low due to a leak in the cooling system, 2) a bad low level coolant switch that needs replacement. After you add coolant check the level in the expansion tank regularly to see if you're losing coolant. If so, it could be a worn out water pump, leaking hose connections, slightly blown head gasket, etc. If the coolant level stays constant and the low level coolant light comes on then it's likely the switch is bad.

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Fred Hoelzle

Well look at the reservoir and see if the coolant is where it should be. There should be an arrow or line on it somewhere. On the SDL's you fill them to the seam.

When it gets colder they can just come on. The SDL's comes on at random times on the highway, will stay on for 10 minutes than go off. Replacing the switch didn't fix it. German electronic's.

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1999 SL500
1969 280SE
2023 Ram 1500
2007 Tiara 3200

Thanks. Well, when I raised the hood, the coolant level in the tanks is where it always is... about 1/2 way up (or 1/2 way down ). Its just where this was the first time I had seen this light come on.

With this being a M103 engine (or whatever the 300 I6 engine is), I know they are prone to HG leaks... I just hope this is not the case with this car... not yet anyway. I do not think so, because like I said, the level is the same. I only drive this car about 50-80 miles per week, so I am hoping it last me for a few more 10,000s of miles before any major issues need attention.

I can relate with the bad coolant level sensor, because my 1991 Lexus LS400 has a faulty sensor, which is a common problem on those. My low coolant light stays illuminated, despite the coolant tank/radiator being full. The new sensor is $83 from the dealer, but I know a guy thats gonna give me one from a parts LS400 he has.

86560SEL, I'd say you need to add coolant mixture to the expansion tank. The level should be higher than halfway ... on my E-Class cars the full-level is almost up to the horizontal seam of the expansion tank.

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Fred Hoelzle

Thanks. Well, the coolant level never did go below the 1/2 way mark, but then it started coming on on my first trip out of the day after the engine was warm like if I would pull from a stop on a slight grade, but later in the trip, even on the steep part of my driveway, it would not come back on. If I would take the car out on a second trip, the light would not come back on at all.

I did go ahead and pour more coolant in it to get it above the 1/2 way mark. As mentioned, it has green coolant in it so I am soon going to change it out - probably not until spring though. The coolant I added was Prestone. On the bottle it read, "add to any color coolant, all makes, all years, all models". Its more like a yellow or very light green color. I thought it was safe to add it in. Strange that coolant such as this claims to be for ALL makes and ALL models, yet many say not to use it in a Mercedes.

I have read and been instructed to only use MB coolant or Zerex.

My coolant tank never registered more than about an inch below the reference mark at the seam: hot or cold, for about 3 years. Coolant (I use G-05) temp gauge acted normally, as far as I was concerned, never moving above 110 (or so) except in summer traffic. I was advised by a knowledgeable MB mechanic to "just add a quart, or so..." & I put in a quart of pre-mix and the level hadn't moved the next time I checked it.

Added yet another quart and the level had moved up about an inch (in the clear tank) : now the temp gauge stays right at the 9:00 position once the car is warmed up.

What this indicates to me is that there is a pretty good FUDGE-factor as far as coolant level in the radiator is concerned. It is, after all, a 4 gallon system...

Cheers!

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'91 420 SEL @ 199K, '92 SVX @ 181K, '93 SC400 @ 86K, '93 Kaw ZX-11 @ 30K, '87 F250 @ 181K , 2001 Valkyrie Interstate @ 6K, Y2K Honda NightHawk 250 with 1.5K, '88 420SEL I.@ 179K & the 2nd latest, an '88 420SEL II.@ 210K runnin' parts car, '85 F150 300/NP435

Thanks... well everytime I look at it when I get home and raise the hood, the level is always at the seam. As mentioned, I added some, but I have not driven the car since. I only drive this car a couple of times per week. I do not think its leaking, because there are no drips at all of anything under the car. I have also noticed that at some point the radiator has been replaced and its not leaking either. I know these engines have reputations for head-gasket issues, but I think when they fail, its different than when other cars tend to leak. When I think of headgasket issues, issues I see on most cars is oil in the coolant, coolant in the oil, overheating, steam out of the exhaust, etc, but on these cars, I have never heard of that being the problem when the HG fails. It seems on these that they either leak coolant at the head or they burn coolant, etc, but I would think it would steam then out of the exhaust all of the time. My car has nearly 155K, so I am hoping its already been replaced. My car does not overheat at all either. It normally always runs at 80°c or slight above or below. No coolant smell from the exhaust, no oil in coolant, no coolant in oil, etc. Also I do not think the coolant level is going down, because as mentioned, it stays at that seam area, or just slightly below.

I am just hoping its not a leaking heater core or something. I do not see any coolant under the hood leaking or and coming in the cars floor, but I am not sure where the coolant leaks on these when the heater core is bad.

As mentioned, the level is not really going down, but the light comes on sometimes when pulling out, but later even on a steep grade, it dont come on. Maybe like someone else said, it could be a faulty coolant level switch. I have a 1991 Lexus LS400 and the coolant level sensors have been known to fail on these cars. My car is no exception. My low coolant light (its a little yellow light on the lower left instrument cluster that looks like the Washington Monument, that is supposed to be a radiator ) that is always on, despite the coolant being full and always being full. The new sensor is $81 from Lexus, so I will probably replace it soon.

I will just drive the Benz and watch the level again. I am not sure if the coolant level always stays at that 1/2 way mark even when the radiator itself is low... does it?

Put a quart in the clear tank and drive the car, let it cool down: if the level returns to the 1/2 way mark, put another quart in...

Eventually the level in the tank will rise, probably when the actual radiator is full, eh?

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'91 420 SEL @ 199K, '92 SVX @ 181K, '93 SC400 @ 86K, '93 Kaw ZX-11 @ 30K, '87 F250 @ 181K , 2001 Valkyrie Interstate @ 6K, Y2K Honda NightHawk 250 with 1.5K, '88 420SEL I.@ 179K & the 2nd latest, an '88 420SEL II.@ 210K runnin' parts car, '85 F150 300/NP435