So in the quest to figure out why my 1993 190E 2.6 is so slow, ive recently checked the voltage coming from the o2 sensor and saw that it was a constant 0.489v at idle, while engine is revved to 3000rpm, and at cold.

i figured that maybe the o2 sensor for the ford mustang that i bought last year was broken so I'll just grab another one.

i went and bought another one (part number 13942), spliced the wires and installed it again. i took another look at the voltage and wtf, its still 0.489v constant, no fluctuation at all.

anyone have ideas why this is happening?

this just occurred to me, am i supposed to be reading DC or AC voltage from the o2? i've been reading the DC voltage

The Mustang O2 seensor trick works fine, sounds like you have another issue. It might not be realated to the slow issue, but if it is always reading about .5v DC it is a problem. The .5 volts is the center baseline for the computer to receive until the o2 Sensor warms up tp temperature and begins to relay accurate readings to the ECU computer. The O2 sensor needs to warm to about 300-600 I think before It is providing accurate readings, hence the two white wires from the Mustang o2 sensor. They are the heater wires and I dont think they matter which is pos or neg, they are reversable and work correctly as far as I can tell, I have used the sensor myself.
I guess my thought for your issue is that it might not be coming out of the "wait for the o2 sensor to warm up to temp and until then send a constant .5 volts" mode. My thoughts would be to first check to see that the MB o2 wire connections are providing a ground at the brown wire and voltage at the other wire in the two contact connection that went to the OEM o2 sensor, without a ground and the voltage, the o2 sensor will not heat up...as fast. Now, eventually, once you are driving, the sensor, if the trouble is just the heater circuit, the sensor would warm to operating temp just from the heat of the exhaust and should begin to give the computer readings. If the heater wires are woring correctly and the signal wire ( black) is connected correctly to the OEM harness green wire, You might have an ECU problem, but that is beyond my diagnosis abilities

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Christopher Henkel
1990 190E 2.6 - Arctic white SOLD
1986 190E-16v - Blauswartze
1993 300CE - SOLD
2003 W208 CLK 320 Cabriolet - Magma Red

Are you getting battery voltage to the heater wires?? With the O2 sensor connected?

For that to go into closed loop the heater must work.

__________________
MERCEDES Benz Master Guild Technician (6 TIMES)
ASE Master Technician
Mercedes Benz Star Technician (2 times)
44 years foreign automotive repair
27 Years M.B. Shop foreman (dealer)
MB technical information Specialist (15 years)
190E 2.3 16V ITS SCCA race car (sold)
1986 190E 2.3 16V 2.5 (sold)
Retired Moderator

mb doc you are a genius, the heater wires do not give any voltage at all. i tested my dad's W124 and checked his heater wires, they give off 13.37 DC volts.

i hadnt come online to check this topic yet, but i came to that conclusion myself after some more testing. I drove back from work on the freeway, went about 70mph most of the way, then parked the car in my driveway without turning the engine off. went inside to grab the multimeter, unplugged the O2 sensor and tried to read it. it WAS giving me voltage and WAS fluctuating even when I gave it throttle and let it off, but just was very slow.

i turned my engine off, went back to my dads W124 to do the same test, and his engine had been turned off for a couple hours. i unplugged the O2 sensor and read the voltage from there, was jumping around alot faster than mine was.

went back to test mine again and the O2 sensor didnt give anymore voltage. after that, i decided to test the heater wires and saw that it wasn't giving me any voltage.

so its been found, heater wires to o2 sensor are dead, now any ideas how to tackle this seemingly huge problem?

mb doc, do you think my MAS relay could be fried?

Yes, that was the reason for my ?.

You can by-pass the MAS with a fused wire from the power wire from the fuel pump.

In other words make the heater operate any time the fuel pump runs.

__________________
MERCEDES Benz Master Guild Technician (6 TIMES)
ASE Master Technician
Mercedes Benz Star Technician (2 times)
44 years foreign automotive repair
27 Years M.B. Shop foreman (dealer)
MB technical information Specialist (15 years)
190E 2.3 16V ITS SCCA race car (sold)
1986 190E 2.3 16V 2.5 (sold)
Retired Moderator

ill try switching my MAS relay out with another one from another 190E i have access to and see what happens.

on the other hand, could this be related to why my check engine light doesnt appear when I have the o2 sensor unplugged and leave the car running for a few minutes? on my dads W124, the check engine light came on when I left the O2 sensor unplugged for about 5 minutes, but on my W201 it never came on. i could probably even drive all the way to work with the O2 sensor unplugged and it wouldnt make a difference!

if my MAS relay is broken, would the other things that it control also be dead? such as my fuel pump, A/C, etc... or is that not necessarily the case?

i did a search on this site for MAS relay, and it looks like it has alot to do with the A/C compressor, but my A/C is working perfectly fine.

A MAS relay preforms 6-8 functions depending on the car & application...any 1 of those functions can fail w/o affecting the other functions!
Loss of the O2 heater power is a common failure.

__________________
MERCEDES Benz Master Guild Technician (6 TIMES)
ASE Master Technician
Mercedes Benz Star Technician (2 times)
44 years foreign automotive repair
27 Years M.B. Shop foreman (dealer)
MB technical information Specialist (15 years)
190E 2.3 16V ITS SCCA race car (sold)
1986 190E 2.3 16V 2.5 (sold)
Retired Moderator

What exactly is this heater we are discuissing here? Does
the O2 sensor have heating elements running inside of it? Or
are you guys referring to the heater core that the coolants
runs through to keep the passenger warm in the winter??

thanks, youve been extremely helpful and knowledgable. ill swap my MAS relay out with one from a 92 190E 2.6 (ive confirmed that they should be the same part) and see if the heater wires receive any voltage. if so, time for me to goto the junkyard and find myself a new MAS relay!

there are two white wires that goes into the O2 sensor to heat it up, and on my car they do not do anything.

First generation O2 sensors had a single wire ( sensor signal only ).
This would require the exhaust temperature to be a minimum of 5-600 degrees, for the sensor to provide a good signal.
During extended idle periods the sensor would cool off and not generate a signal to the ECM anymore.
Second generation had 3 wires ( 1 signal, 1 heater power & 1 ground ).
The heater will keep the sensor at operating temperature ( even at idle, and thus keep sending a signal for the ECM to control fuel-air for better exhaust emissions ).
Third generation has 4 wires ( 1 signal, 1 signal ground, 1 heater power & 1 heaterground ).
The purpose is to provide more reliable grounds.

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2007 C 230 Sport.

mb doc, you are a genius once again, the MAS relay was replaced and now everything works. thanks so much!

actually mb doc, another question for you:

do you know what are all the things that the MAS unit controls? i want to make sure this unit is 100% working

all i know is that it controls A/C compressor, fuel pump, O2 sensor heater.