Overheating Diesel? - Page 2 - PeachParts Mercedes-Benz Forum

rwthomas1 · #16 04-03-2003, 02:00 PM

rmmagow,
Likely your gauge is not accurate. My car runs at 90-95*c all the time. I tried a new 80*c t-stat and it only runs a needle-width cooler, not at 80* like you would expect. If you can grab the upper radiator hose and its hot, but not so hot it burns you, after just shutting the engine off likely you are fine. If it was serious you would be boiling over. At least get a reliable gauge on the motor to get a second opinion rather than base the replacement of some potentially expensive parts on what very well might be an inaccurate gauge. RT

gsxr · #17 04-03-2003, 03:56 PM

Egads. Where to start:

1- No, the electric aux fan on a 1983 300D is not triggered by engine temperature at all (sorry Ken). That applies to later models only with OM60x engines, and temps above 105C. For the OM617's, only A/C refrigerant pressure (high side) kicks on the electric fan. The fan does NOTHING to cool the engine, it's puropse is to provide airflow over the condenser & assist A/C performance.

2- BoostnBenz, your car is running too cold. Either a bad t-stat or a bad temp sender in the head (when the get old, they put out low readings - $10 for a new one). The temp should be over 80C when warmed up or you are wasting fuel and wearing your engine out prematurely.

3- "rmmagow", yes 100C is hot for 75F ambient temps. You most likely have one of the following problems (hard to tell without more info) : loose fan belt, failing fan clutch, dirty radiator fins blocking airflow, partially plugged/dirty radiator, or corroded t-stat housing. I'd start with cleaning the radiator fins - you will be shocked at the amount of crud between the radiator & condenser. It's a big job to remove & replace, though a lot cheaper than buying a new rad/clutch that wasn't needed!

4- Edge, no, what you read was a Band-Aid Rube Goldberg fix for a different problem. Apparently the t-stat housings can corrode over time so the t-stat bypass does not seal against the rear of the housing, preventing full flow to the radiator. The PROPER cure is a new housing, which is ~$90 or so. The drilled hole is (IMO) a lame way to get out of replacing the housing.

For the record, my 1984 300D has almost everything new in the cooling system, and runs 90-95C year round under all conditions... sub-freezing to blistering 110F heat with the AC on max. I've never seen it get to 100C and almost never below 90C unless coasting down a mountain.

HTH,

BoostnBenz · #18 04-03-2003, 09:09 PM

Quote:

2- BoostnBenz, your car is running too cold. Either a bad t-stat or a bad temp sender in the head (when the get old, they put out low readings - $10 for a new one). The temp should be over 80C when warmed up or you are wasting fuel and wearing your engine out prematurely.

Hmm, interesting as both of them do the exact same thing. The temperature seems to vary with speed but I would have to agree on nights like tonight when it was low 30s and my car only hit 50-60*C while driving around 50mph. I still get noticeable, but not hot heat out of this though, so I doubt it is the sensor. I have to change the coolant anyway, works out better this way.

Thanks.

gsxr · #19 04-03-2003, 10:05 PM

Jeff,

When I bought my '84, it ran way too cold as well. Usually between 50-60C on the gauge. I also got weak heat from the heater. Replacing the thermostat helped a lot, and provided hot air, but the gauge still said 60-70C. Replacing the sender popped it up to the 85-95C range, where it has been ever since! Luckily both are cheap to replace...

Good luck,

Ridge · #20 04-05-2003, 03:22 AM

A friend left my house last Sunday to drive over the Sierras back to California, got about 15 miles up the mountains and called that he was overheating. Drove his 300D back to my house, heat under control downhill and we installed a spare clutch fan. He tried the mountains again and everything was fine. Check the fan clutch.

Ben

84300DT · #21 04-05-2003, 06:14 AM

for what it's worth , in my '84 the aux fan does come on even when the a/c is not working, but it's gotta be real hot outside for that to happen...

rmmagow · #22 04-05-2003, 11:39 AM

One thing I'm noticing is that turning on the heater for a bit brings the temp right back to normal where it'll stay if I keep it there. I'm going to change the T-stat, maybe pull the rad to clean the fins and stuff, flush it and put in new coolant. The t-stat housing ecternally loks OK, the fan clutch, when the engine is cold and I try to spin the fan it only goes a quarter of a turn or so, when hot it feels like there is more turning effort needed so it's probably OK. Near the t-stat housing is a black connector that sort of is easy to wiggle, is this the sending unit? How does the connetor remove, just yank it? Don't want to go breaking things if I can avoid it. The car only ever gets to a smidge over 100C so I think it's going to be just some minor tweaking to fix (I hope) Thanks for all the usefull replys.

gsxr · #23 04-05-2003, 12:28 PM

The '84 300D only had an A/C trigger for the aux fan. If it comes on at other times, someone modified the circuitry. See if you can find the sensor that triggers it by engine heat & post a photo.

The sending unit for the dash gauge is on the cylinder head, roughly between the glow plugs for cylinders 2 and 3. The connector near the t-stat is not for the dash gauge. I'm trying to determine what that t-stat sender is for, I think it may be for the glow plug relay...? I'll post again when I find out.

psfred · #24 04-05-2003, 07:43 PM

If there is a senor with wire in the thermostat housing, that is the fan switch. As far as I know, ALL MB diesels have a temp switch for the aux fan, AC or not (certainly my 220, 300D, 300TE, and my brothers 300D (75) and SDL do.

If the wire is off and grounded, the fan will run all the time, ditto if the switch is shot.

Sounds to me as if the overheating problem is either a plugged rad, bad thermostat, or a bad radiator cap preventing coolant pressurization. This will result in boilovers when it gets hot, especially if you make a highway run and pull into a rest stop.

Peter

84300DT · #25 04-05-2003, 08:33 PM

Quote:

Originally posted by psfred
If there is a senor with wire in the thermostat housing, that is the fan switch. As far as I know, ALL MB diesels have a temp switch for the aux fan, AC or not (certainly my 220, 300D, 300TE, and my brothers 300D (75) and SDL do.

If the wire is off and grounded, the fan will run all the time, ditto if the switch is shot.

Sounds to me as if the overheating problem is either a plugged rad, bad thermostat, or a bad radiator cap preventing coolant pressurization. This will result in boilovers when it gets hot, especially if you make a highway run and pull into a rest stop.

Peter

i know my car has at least one sensor hooked into the t-stat housing.
frankly i was kind of shocked when it went on due to the a/c not working for a while..

gsxr · #26 04-05-2003, 09:26 PM

Quote:

f there is a senor with wire in the thermostat housing, that is the fan switch. As far as I know, ALL MB diesels have a temp switch for the aux fan, AC or not (certainly my 220, 300D, 300TE, and my brothers 300D (75) and SDL do.

If the wire is off and grounded, the fan will run all the time, ditto if the switch is shot.

OK, I just looked this up in the W123 Electrical manual (ETM). The auxiliary fan is controlled by a relay. The ONLY signal that triggers this relay (and the fan) is a temperature switch at the receiver/drier. It closes at 144F. So I was partially wrong, the A/C is a temp switch, not pressure swtich (the 124 switch is pressure based and I (wrongly!) assumed the 123 was the same). However there is still ZERO connection to the engine temperature, at least on the W123.1x3 (1982-85 300D/TD). See page 127 of the ETM.

The bad news is I couldn't find out what the stupid mystery sensor is! My best guess is on page 303 of the ETM (in the ACC section), where there is a "cold engine lockout" temperature switch that is on the "right front of the engine". I believe this prevents the A/C compressor (or something) from engaging until the engine warms up. The switch triggers at 35-40C (engine temp) but I couldn't tell if this is the one on the t-stat housing or not. Oh well...

Regards,

190D22 · #27 04-05-2003, 11:59 PM

Ok well at idle, after the car has been warm for a bit, my engine temperature climbs to 100 +. But when I start moving it goes down really quickly to 85 and sometimes even lower than that. With the heat on it stays at a solid 85. My electric fan works with the AC, but I have never seen the big belt driven fan run, ever. What should I do first? Thanks guys.

gsxr · #28 04-06-2003, 12:54 AM

Sounds like your electromagnetic fan clutch is either bad, or not getting the signal to turn on. I'd read the OM601 engine manual and/or the ETM (electrical manual) for your car, that will tell you how it's supposed to operate, then you can figure out what's wrong. But it sure sounds like something is broken, IMO...! Wouldn't hurt to email Doc Booth from the MBZ.org diesel list, he's the 201/diesel expert...

Regards,

psfred · #29 04-06-2003, 11:54 AM

The easy way to test for the presence of a coolant temp fan switch is to pull that wire off the thermostat housing and ground it with the key switch on. If it is a fan switch, the fan will come on.

Peter

ewstan · #30 04-06-2003, 12:12 PM

According to the manual, the AUX fan only works when the A/C is on. Sounds like your fan clutch is defective.

ewstan