Learning about the 603 3.5 - PeachParts Mercedes-Benz Forum

JHZR2 · #1 10-09-2018, 11:40 PM

Looking for verification in some items associated with my 3.5L OM603. I'm a 616/617 guy, so this is my first experience with this engine. Ive read a lot of threads, but want to verify my proficiency.

These are the temperature sensors on the metal housing that connects to the upper radiator hose. For some model years, one is for the temp gauge needle, another is for a low/hi aux fan engagement. However, some theories are that in the later models, the temp sensor for the aux fan is in the head behind the IP, and that the other sensor (blue?) is for the electric diesel system, Klima, and/or other idle controls.

This is the other temperature sensor, mentioned above, that's in the head. This one is for the aux fan??

This is the electric diesel control connector that somehow adjusts idle and affects the IP:

There's also this electrical connector in the IP. Not entirely sure why...

There's supposed to be a variable resistor of some sort by the brake booster, that can be set to affect idle speed. My car does not have one. Just this silver box with the fuse in it. Not sure what that's for - ideas??

This red sensor is the overpressure switch for the aux fan. Removal of the red switch will cause refrigerant loss. However, a standard blade fuse as a jumper between the two white electrical connections should force the aux fan on??

I don't believe my aux fans are working, causing a slight temperature creep in slow traffic. The fuse looks good, though the metal strip is a bit soft. I guess I need to check the relay as well, but jumpering is easy. I'd like to verify that both switches work, and depending upon the color of fan switch in the coolant path, change to a more conservative setting.

Thanks!!

sixto · #2 10-10-2018, 02:22 AM

IIRC there is a 3-pin temp switch on the upper hose fitting for the aux fan and AC compressor cutoff. There is a single(?) post sensor beside it for EDS. There is a sender between the head and IP for the temp gauge. There might also be a temp sender for the glow relay just aft of the gauge sender. If the car has a BFS, there’ll be a 50*C temp switch at the water pump.

The ELR actuator that controls idle is on the back end of the IP. The control rod sensor connector is on top of the IP. The control rod sensor connector is about the only oil leak point that requires the IP to come apart to reseal.

Only the 603.96 (3.0) has the idle trim box.

I think you’re correct about the AC sensor with pigtails. No need for a fuse, just jump the wires on the car side of the harness. Most likely the AC circuit should be active for the fan to engage. This is the low speed fan switch. The high speed fan switch is part of the 3-post switch at the upper radiator hose fitting. If you have high speed but not low speed, check the ballast resistor behind the driver headlight. The coil can burn itself to an open circuit.

Sixto
98 E320s sedan and wagon
02 C320 wagon

Mxfrank · #3 10-10-2018, 10:57 AM

This varies by engine, year and model, but here's how it works on the earlier cars. The fan switch on the top hose fitting serves two functions:

1) It turns on the fan at HIGH speed only, usually around 105C.
2) It turns off the A/C compressor at very high temps, usually around 110C

There should be two switches on the reciever/dryer. The first is a high/low pressure switch, which allows the compressor to engage only if refrigerant pressure is above some limit (to prevent running with low freon), and also prevents it from engaging above some high limit (around 300 psi), to prevent damage to system components.

The other switch is a pressure switch for the LOW speed fan function. It will engage the fans at low speed only if refrigerant pressure exceeds some limit. The fans DO NOT come on automatically with the compressor.

wwii · #4 10-10-2018, 11:16 AM

Is it model W126.135 and engine OM603.971? I think I had to play around with the connector at the upper radiator hose (your first picture) to test/check my ECT gauge - when I get home, I'll look at my notes.

JHZR2 · #5 10-10-2018, 01:18 PM

Quote:

Originally Posted by wwii

Is it model W126.135 and engine OM603.971? I think I had to play around with the connector at the upper radiator hose (your first picture) to test/check my ECT gauge - when I get home, I'll look at my notes.

IIRC 603.970 in a w126.

ECT gauge, you mean the gauge in the cluster on the dash?

Above is noted that the one in the head goes to the gauge. Should be easy enough to pull with one and decide which makes the gauge drop to full cold, right??

I do need to check aux fans. I also tend to see idle between 450-550 rpm depending upon conditions, so I’d like to determine if that is accurate and if the EDS is operating.

JHZR2 · #6 10-10-2018, 01:40 PM

Quote:

Originally Posted by Mxfrank

There should be two switches on the reciever/dryer. The first is a high/low pressure switch, which allows the compressor to engage only if refrigerant pressure is above some limit (to prevent running with low freon), and also prevents it from engaging above some high limit (around 300 psi), to prevent damage to system components.

The other switch is a pressure switch for the LOW speed fan function. It will engage the fans at low speed only if refrigerant pressure exceeds some limit. The fans DO NOT come on automatically with the compressor.

As you can see, the R/D has only one sensor. Not sure why. All works perfect wrt cooling. Only the aux fan is suspect.

Not sure which role that red sensor plays.

Quote:

Originally Posted by sixto

Only the 603.96 (3.0) has the idle trim box.

Any idea what this silver box with the red fuse is then??

wwii · #7 10-10-2018, 03:15 PM

Lukily the W126 Startek manual is online

At the end of the Electrical Troubleshooting Manual (pdf link) (page 201 on the printed page) some of the coolant temperature sensor designations and locations are shown - If you can figure out the sensor designation from the wiring diagrams in the ETM then you can use this section to give you hints to where they are - later in the manual there are location pictures.

Code:

B10/8 Coolant temperature sensor (climate control)
B11/2 Coolant temperature sensor (EZL/AKR)
B11/4 Coolant temperature sensor (EDS)
B11/8 Coolant temperature sensor (coolant temperature gauge/preglow system)

Sensor B10/8 looks like it controls the aux fans and the manual says its location is "Top of the engine"

Mxfrank · #8 10-10-2018, 03:21 PM

The red switch is the aux fan switch. Box with the fuse is the overvoltage protection relay (OVP). Certain key circuits receive power through this relay, which incorporates a zener diode to snub voltage spikes.

wwii · #9 10-10-2018, 11:25 PM

Quote:

I'm pretty sure the one with two wires is B10/8 which feeds the N22 AC control module which will turn on the Aux fan(s).
.
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dieselbenz1 · #10 10-11-2018, 09:00 AM

I'm pretty sure the one with two wires is B10/8 which feeds the N22 AC control module which will turn on the Aux fan(s)

Yup thats the sensor! Is the Climate Control working as it should?

JHZR2 · #11 10-11-2018, 09:10 AM

Quote:

Originally Posted by dieselbenz1

I'm pretty sure the one with two wires is B10/8 which feeds the N22 AC control module which will turn on the Aux fan(s)

Yup thats the sensor! Is the Climate Control working as it should?

AC works beautifully as does heat.

The ACC acts a little funny when near its setpoint but it’s not a concern imo.

dieselbenz1 · #12 10-11-2018, 10:33 AM

Does the fan speed also change in automatic?

JHZR2 · #13 10-11-2018, 11:13 AM

Quote:

Originally Posted by dieselbenz1

Does the fan speed also change in automatic?

Absolutely.

dieselbenz1 · #14 10-11-2018, 05:54 PM

I would expect the fans to start once water jacket reaches the setpoint of (105 degs C? I think).
The climate control unit also controls the aux fans so you are looking good.

JHZR2 · #15 10-11-2018, 06:00 PM

Quote:

Originally Posted by dieselbenz1

I would expect the fans to start once water jacket reaches the setpoint of (105 degs C? I think).
The climate control unit also controls the aux fans so you are looking good.

Interesting point about ACC.

Ive read that on w140/w210 perhaps, holding auto 10s forces on the aux fans. Should this happen on a w126?

I’m thinking something is not right with my aux fans. Can’t quite figure what just yet.