Dustindustrial

I have a 98 ML320, and am hoping someone can help me with identifying some wires at the ECM.

What I'm looking for is the front left and right O2 sensor signal wires (G3/3 & G3/4), the secondary air pump wires (M33), and switchover valve wires (Y32).

I have the diagram below, but when looking in the ECM compartment, nothing appears to be labeled, so I'm having trouble matching up the wires I'm looking for. I think I may have found the O2 sensor wires, but am not 100% sure (second picture below).

Any help or advice would be greatly appreciated.

Maj. Dundee

Here are the connector views. Whatever testing you are doing has to be made at the ECM.

Dustindustrial

Thanks, this is a big help.

Dustindustrial

When the secondary air pump is running, how low does the voltage need to drop on the front o2 sensors, to satisfy the computer, and not display the CEL?

Maj. Dundee

Instead of asking obscure questions, why don't you get to the point and state what your problem is. Are you getting a code P0410?

Dustindustrial

Thanks for replying Major. I am getting a P0410.

I've had the car into Mercedes several times, and after replacing my pump, check valve, and relay, and cleaning the heads with a "special tool" and cleaning solution, they're final conclusion was that the heads need to be removed and boiled clean at a machine shop. Shortly before dealing with P0410, I put $2000 into the car for some other repairs, the majority of that being to replace a bad flap actuator motor. I'm not looking to spend another $2000 right away to have the heads cleaned, but I can't stand staring at the CEL.

What I'm experimenting with are two relays, that switch when the secondary air pump goes on and off. When the pump goes on, I want to divert the O2 sensor signal through a diode, to make sure I get enough of a voltage drop to not trigger a P0410. When the pump is off, the O2 sensors will function as they normally would.

I'm fairly far along with it, but whether I put the diode inline on the O2 sensor low signal wire, or high signal wire, I seem to get about the same results, where the O2 sensor voltage stays right about in the .480 range with little fluctuation.

That leads me to belive that I'm doing something wrong, because with no diodes inline, the O2 sensor voltage is constantly fluctuating, and goes way lower than .480.

Any suggestions or thoughts would be greatly appreciated.

Maj. Dundee

Dust, the Air Pump is commanded to come on any time the coolant temp. is less that 40° C (104° F) along with other prerequisites. The function of the Air Pump is to heat up the O2 sensors quicker to get them to operating temp.

During that time (usually 20-30 secs), which is called Open Loop, the O2 sensors are not active until they are heated, then the system goes into Closed Loop, and it is then that the sensors become active. The Air Pump then is turned on again during every driving cycle as a system check.

Your problem is not with any of the O2 sensors, but it is with one of the two banks or both, being clogged with carbon. To determine which ones are, remove the Phillips screws on both Switchover Valves and remove the top portion on each. Start the engine whereupon you should feel exhaust coming out of the tops of the valves. Whichever one (or both) does not have exhaust is the clogged bank.

Dustindustrial

According to Mercedes, my problem is on the left side.

I think what's causing my CEL and P0410, is that I'm failing that subsequent system check. The CEL always returns when I'm idling. So far it's always been at a stop light. I'll hear the air pump going on and off several times. I'm assuming I'm failing the test, so it's trying again. After so many failures, I get the CEL.

What prompted me to try this "mod", is I've heard of other people with the same problem doing the mod and their CEL not returning. They've done it at the O2 sensor wiring and pump wiring. I thought it would be cleaner to do it at the ECU.

Maj. Dundee

Did you buy the truck from a M/B Dealer?

Dustindustrial

I bought it from a used car dealer, not an authorized MB dealer.

That's part of my problem here... It had P0410 when I bought it, and I didn't even know, because the CEL bulb was removed... . It was an unpleasant surprise when I took it in for inspection.

They are of course saying that they didn't know... that the previous owner must have done it... which is possible I suppose.

I already put so much into other repairs. I'm afraid to put the money into cleaning the heads right away, not knowing yet if other stuff will go wrong down the line. If 11 months from now when I'm due for my next inspection it's still going strong, then I would have the heads cleaned. Right now I just want to get rid of the CEL.

Maj. Dundee

It is not necessary to remove the heads to clean the ports.