leojbourne

Hello, I'm not sure on which forum to post this, so I hope someone can tell me. It's an old car I inherited from my father, but the engine is still good at 127k miles. I have had a lot of work put into this car over the past years: new wiring harness, CPS, brakes and brake lines. I would like to be able to drive it some more without problems.

Recently the ambient air temperature sensor started reading about 40 degrees high. There are two types of sensors for this year and model. I have the one that is on a long wire with a probe at the end that looks like the jack on a set of old stereo headphones. The probe is mounted below the driver side bumper on some plastic air louvers.

I would really appreciate some help on this, because I have a quote of nearly $400 P&L to replace the sensor, and that is a lot. Also, the mechanic said that it might be more, if he had to put in more time.

So please tell me:

1. Where does the sensor wire plug in? Maybe it's just a loose connector at the plug-in side instead of a failure at the probe side. But I can't find where it plugs in. The mechanic asked the local dealer, who didn't seem to be sure, but said it plugged into the instrument cluster, which means the cluster would have to be removed (expensive) to work on the sensor wire or replace the sensor, plus figuring out the routing of the cable.

2. Is the probe a passive thermistor or is there an active element in it that sends a signal of some kind up the cable?

3. Can I just cut the cable a foot or so from the probe, expose the interior wires, solder in a new probe from a new cable assembly, and waterproof the splice? This would save all the trouble of removing the cluster. As I recall, this part went bad about five years ago, but the cost of replacement wasn't nearly as much back then for some reason.

Any suggestions would be appreciated. I don't want to mess up the car computer with overly high outside temperature readings (over 115 degrees) when it's actually only around 70 degrees. The temperature difference (from actual) seems to vary by day, but is always high.

MJ50

moved to W202 forum...

ptcruiser

Why not just forget it...If it's for outside temperature, it's just telling you it's hot or cold outside or whatever. Common sense will tell you what the weather is outside.
I doubt you will be driving across the Gobi desert, or Alaska in the dead of winter, so what do you really want it for? That is it's function, so the car will be fine

leojbourne

Thanks for your reply. I have seen differing views online about whether the ambient temperature sensor ties into the on board computer and may have an impact on other things, such as air conditioning or air mixture. If the temperature indicator is purely cosmetic, then I will not worry about fixing it, but if there is a computer-related effect, then presumably this would cause problems in the long run if left unrepaired. I cannot find a wiring diagram of showing this sensor. Any help finding one would be appreciated.

I don't know what's inside the sensor cable. If it is just two wires in a waterproof casing, then maybe I can get the other kind of sensor (a little pointy cone, not very expensive) and its connector assembly (I don't know the part number -- does anyone?) and just solder in the other type of sensor, which is plug-in and could easily be fixed next time.

ptcruiser

Read this http://en.wikipedia.org/wiki/Mass_flow_sensor and it indicates that the MAF will do the work for the ambient temperature.
If you don't have a MAF, then I have no idea, buy I doubt it would or has any real effect.

ptcruiser

You car has a MAF, now that I see the year

leojbourne

Unless I can find a Mercedes mechanic who has worked on this model to advise me, I don't know how many assumptions I can make. I saw a BMW youtube video that says the ambient temperature sensor will inhibit the air conditioner if it is an open circuit. Mine seems to be a partial short (high temperature), so it won't be a problem showing too low, but I don't want the computer impeding the heater, either.

Anyway, looks like I have other problems as well. The battery just went dead. An an hour at idle after a jump start didn't charge it up. So either the battery is going bad, or maybe I have a hard short somewhere that is draining it rapidly, or else the alternator is going bad (but I have no dashboard light showing that).

ptcruiser

I guess it is an exercise in frustration. Go to a dealer and look for a mechanic who is older than dirt, and ask him.
I just replaced my battery, and same thing. Charged it and nothing. New battery, and all is well. Stupid thing was, I couldn't get in the car to open the trunk, as I had no Valet key.
I was a lesson learned. There is a trick to do it if that should happen in the future.