temperature gauge question

[Pili380SL]

On my 1983 380SL, with the engine warmed up,I've noticed that the temperature gauge reads quite bit higher with the engine running than with the key on/engine off (second position). Is this normal? I've studied the wiring diagrams and it seems it might be an alternator problem. The fuel gauge reads the same in both situations, btw. Any ideas? Thanks ia.

Paul

[cbdo]

It's an electic gauge, and I would suspect is somewhat sensitive to the operating voltage. Your system voltage when running should be about 14-14.5 due to the alternator putting everything up to charging voltage; when the engine's off, you're down to battery voltage alone, which will run about 12.6 or so IIRC. It's a guess, but a reasonable explanation, that your gauge is calibrated to read correctly at the higher voltage and is a bit off without that. I'd expect MB to have allowed for that and calibrate so that the correct reading is engine-on.

That said, I'll happily defer to a more educated writer!

[Pili380SL]

Excellent deduction, cbdo, although I must admit I was hoping for a different answer. I'm fighting a high operating temperature (110c+), with an occasional pegging of the meter. The strange thing is, the cooling fan does not kick on. I manually grounded the fan switch, and it kicks on perfectly. I've replaced both the fan and gauge sending units, as both of them had the center probes loose (?). Manually kicking the fan on does not change the temperature reading. I should tell you this all started by replacing the thermostat, because on each start up the gauge would slowly increase it's reading to 110c+, at which point the fan would kick on, temp would go down and stay about 90c. I replaced the t-stat, trying a trick that DaimlerChrysler posted about drilling three 4mm holes in the frame of the t-stat, to fix the gauge reading problem. With this set-up, the gauge would slowly increase to peg, fan never kick on, and stay there as long as I was brave enough to let the engine run. I bought another t-stat, installed it un-modified, and same result. Thoroughly frustrated, I took the springs out of the first t-stat, installed it in a permanently open position, and it takes a lot longer (about 20 minutes of driving), but the gauge slowly pegs again. I checked the manual for testing of the temp sending unit or gauge, but couldn't find ohm figures per temp. BTW, fuel mixture was verified o.k. by a local shop with an exhaust anylizer, since I considered a lean mixture to be the culprit. Also, the engine runs perfectly through all of this, and doesn't seem to radiate much heat, as I've seen overheating engines do. Sorry for the long post.

[cbdo]

I'm immediately suspicious if the engine doesn't seem to show the characteristic signs that would match the gauge info. A possibility: Run the car a bit, til the engine's warm but by no means hot; around the block might work. Then take (for example) an electronic oven thermometer that has a metal probe to monitor temp of cooking food (or a darkroom thermometer might work), put the probe in either the expansion tank or the radiator, and compare readings as the engine warms up at idle. If there's a significant inaccuracy in the lower 200s (F), there should also be a disparity in the 150-190 range, where you'll still be safe with the caps off, especially since you've got a permanently open thermostat.

No major mechanical expertise claimed, just a bit of inventiveness in experimenting, and I'll still gladly defer to an actual knowledgeable person!

[Pili380SL]

Good idea, cbdo. I "cooked" the old temp sensor in a pot of water to get aprox ohm readings, to check the sender. With the gauge reading 100c, it checks out about right (about 80 ohms, which is what the sender read as the water starteed to boil). I will check it with an oven thermometer, as I just decided my wife needs one . Thanks for your input; I'm no expert either but two non-expert heads are better than one.

[ctaylor738]

I'm a little fuzzy on this, but isn't the guage measuring resistance, which is independent of voltage? The manual discusses every other guage except temperature.

Most DMM's these days have a temperature function.

[Pili380SL]

Chuck, you might be right, the sending unit is obviously variable resistance to ground, but it might not be based on battery voltage. The "instruments" schematic shows the temp gauge and the fuel gauge hooked to the same source, but I haven't figured out the source. The lead is marked "continued on following page", but it's the last page! An update on my problem: I just took a drive, about twenty minutes in 85 degree temps, stop and go, A/C OFF (stupid me, I hadn't tried leaving the A/C off for an extended amount of time), and the temp gauge registered a steady 90c. Turn on the air, and it begins its slow climb. Earlier today I took the same test drive, A/C on, and it never did peg (though I think continuing the drive it may have), but hovered around 110-115c. I wouldn't think that it should run so hot with the A/C on; based on this my attention turns to the radiator. Thanks for the input, guys, this site is great.

[Pili380SL]

Update; I found the voltage source for the gauges, it is battery voltage; fuse 6 in the fuse block. I reluctantly abandon the electrical problem solution, though voltage drain with the A/C and blower on could have an effect. The twitching of the gauge, even without the turn signals on is what made me suspicious. I've read posts about this (twitching) from others so I assume it's common for these cars.

[ctaylor738]

My 380SL's twitched.

Again, this is not my area, but when the AC is on, isn't heat dissipated through the condensor, which would raise the temperature of the air going through the radiator.

Common problems which would be made worse by this:

- a marginal radiator in terms of flow,
- maybe your fan clutch is not turning the fan fast enough at idle
- aux fan not coming on as it should

[Pili380SL]

I agree with your assessment, and I'm going to pull the radiator and have it checked.

Paul