My check engine light came on yesterday under acceleration. Nothing else seems wrong. Went home and removed negative lead from battery and CEL was off after starting car. This morning on way to work came on again at about 65mph. Car is not exhibiting any other symptoms.

1. I will be going to local auto parts store to have them read codes on way home (where is the connector located)

2. Is there a listing somewhere for the codes that they will pull?

3. Any other ideas regarding the light coming on without any other symptoms?

Thanks much!!!!!!!
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1993 240, 1992 240GL, 1985 240DL Wagon, 1983 240DL

Your car is not obd 2 so the auto parts store cannot read the code unless they know where the little diagnostic box is under your hood. You can do it yourself. Go to the FAQs on this site and check out the section titled "Engine: obd diagnostic codes.

Checking code on diagnostic unit revealed a 1-2-3 code, which is listed as "coolant temperature sensor signal".

Does anyone know where the sensor is located and how to test it.

Thanks!
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1993 240, 1992 240GL, 1985 240DL Wagon, 1983 240DL

Yes it is buried under the #3 cylinder intake runner. Make sure the double connector is attached and contacts clean first.
Dan

Sorry not familiar with #3 cylinder etc, would this be the area under and around the throttle body?

Thanks
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1993 240, 1992 240GL, 1985 240DL Wagon, 1983 240DL

Yes the front cylinder is #1, the sensor would be easier to remove with the TB removed.
Dan

Before you remove the throttle body to get closer to the
#3 cylinder to get to the sensor you might want to check
the wiring that goes to the sensor, it may just be loose,
dirty or broken.

Joseph in New Mexico

Able to loosen coolant temp sensor without taking off manifold. When I install new one do I need to put anything on the threads to keep it from leaking?

Thanks
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1993 240, 1992 240GL, 1985 240DL Wagon, 1983 240DL

Now that I have completed the replacement of the coolant temp sensor I can explain it so others can benefit.

First of all it is possible to change the sensor without removing the manifold.
Get yourself a deep socket (19mm) and a swivel adapter that goes into the socket along with a 12" extension.

I removed any vacuum hoses that were in the way and was able to loosen the sensor and remove it.

The old sensor had a copper washer on it which I transferred to the new sensor and I also wrapped the new sensor threads with some nylon plumbers tape before installing.

WHen reinstalling I was able to hand tighten as much as possible and then used the 19mm deep socket along with vice clamps which I used to grab the socket from underneath the manifold and really tighten it down

Cleared CEL and all is well.........................
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1993 240, 1992 240GL, 1985 240DL Wagon, 1983 240DL

The check engine light came on again this morning and the code is 1-2-3, which is the coolant temp sensor. I replaced this a month ago and all was fine until today!

Does anyone know what the voltage should read at the connector? Do I just check it with a VOM?

Could the problem be caused by bad thermostat or radiator cap?
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1993 240, 1992 240GL, 1985 240DL Wagon, 1983 240DL

Check under engine sensors in the 700/900 faq for instructions on how to test the sensor.

If you have already changed the temp sensor you may have a ground problem. Check the resistance between the engine and pin 5 of the LH ecu plug (ecu unplugged). If you have resistance, you have a ground problem.

Try cleaning the ground wires on the fuel rail. There is also apparently a crimp in the wiring harness for all the ground wires under the intake manifold that can go bad. I am still having ground issues with my 93 so I am planning to investigate that.

If you have already changed the temp sensor you may have a ground problem.

You bet like I would bet. The part that concerns me is the replacement sensor got wrapped with plumber's tape before threading into the head. The sensor's ground (on the LH2.4 version) is through those threads. The threads should not be taped. The seal is the copper ring.
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Art Benstein near Baltimore

Draw your curves first and then plot your data!

I also don't like the idea of tape, Art, but I do think that Teflon tape PROBABLY gets torn up enough during assembling the joint to make good contact.

However, I have not verified this with trials!

(I wouldn't feel bad about using tape on a joint that is simply meant to complete a circuit, a binary yes-no; e.g. low oil-pressure sender.)

And, a bigger HOWEVER: READING the coolant temperature sensor depends on getting RESISTANCE right, and modifying (increasing) the path of resistance with Teflon tape is a no-no. You are right.

Straight-threaded fittings seal at a shoulder -- e.g. this sensor, spark plugs, oil pan drain plug, banjo fittings at fuel lines, brake bleeders -- and usually take a sealing (deforming) washer. No pipe dope, no Teflon tape. A bit of anti-seize.

Tapered fittings rely on the taper, and can take sealant, such as Teflon tape and/or pipe dope.

All of which you already know. :-)

OK Art, I will be removing the sensor and cleaning off the teflon tape and see what happens. Although that's the instructions I got from another brickboard member (to wrap it with teflon tape?.

And it has not given me any problems for a month and a half after replacement?

Thx
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1993 240, 1992 240GL, 1985 240DL Wagon, 1983 240DL

I don't blame you for being a bit skeptical. I am skeptical myself about the original diagnosis -- those sensors don't really "go bad" without some obvious physical signs, of which you have mentioned nothing. But the teflon has to go, even though perhaps you screwed it in tight enough to cut through it and allow the ground a good connection.

The code does point very specifically to the sensor. I would have approached it the way you first intended -- measuring the voltage at the ECU as directed in one of the FAQ entries. That way you get first hand verification of what the ECU is complaining about. And you are including the harness and connector in your test.

When you remove the sensor to take off the tape, use a good light to inspect the connector's contacts for corrosion.
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Art Benstein near Baltimore

If you pick up a starving dog and make him prosperous, he will not bite you; that is the principal difference between a dog and a man. -Mark Twain

I would take the tape out, but unless you taped the copper crush washer too, the sensor should ground through the washer.

It's easy to check. Check the resistance between the head and the body of the sensor. If there is no resistance the tape is not causing a problem.

I would also check the resistance between pin 5 of the ecu and the body of the sensor. There should be no resistance.

"I would also check the resistance between pin 5 of the ecu and the body of the sensor. There should be no resistance."

Let me make sure I understand this?

Pin 5 on the ECU is ground--correct?

So if I connect an ohm meter between the body of the sensor and pin 5 on the ECU then I should show a near zero reading on the meter, meaning the sensor is properly grounded---correct?

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1993 240, 1992 240GL, 1985 240DL Wagon, 1983 240DL

How do I remove the connector from the ECU? I took a look at it the other day and it wasn't readily apparent.

Thanks
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1993 240, 1992 240GL, 1985 240DL Wagon, 1983 240DL

Look for a bit of springy metal to squeeze.
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Art Benstein near Baltimore

"We made too many wrong mistakes."

"Look for a bit of springy metal to squeeze."

Could you be a bit more specific?

Thanks
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1993 240, 1992 240GL, 1985 240DL Wagon, 1983 240DL