Voltage drop

Hi guys

I’m a newbie in electrical stuffs

I am getting a boost solenoid (Y77) DTC code. I’ve used a DMM to measure voltage on the positive side and negative side

Battery voltage is 13.46 engine running

Solenoid positive side engine running is 10.92 V (Benz wants this to be between 11-14v)
Solenoid negative side engine running is 2.42v

If I add the voltage on the negative side to the one on the positive side then the circuit will be just fine at 13.34

Where exactly do I attack
Positive side low voltage OR
Negative side high voltage

DTC only says Short to ground
Measurements were carried out on the plug coming from the ECU with the solenoid unplugged to eliminate a shorr to ground inside the solenoid

First pix attached is just for illustration purpose

Second pix is the wiring of the solenoid
N3/21 is the ECU
Y77 is the solenoid

Any help pls

Key-on, Solenoid un-plugged you measured 10.92v on Pos side and 2.42v on the NEG side, Correct?

What are the same voltages Key - on and plugged in on the solenoid?

If you are unplugged the POS seems low (you could have unwanted resistance) or a short. And the ground voltage is probably a bias voltage the PCM uses to "see" if the soloenoid is open, or the circuit itself is open.

While unplugged you could put a test lamp to BAT POS and touch the ground side, lamp shouldn't light (depending if its power side control or ground side control) and the bias 2.4 volts should disappear. (If it is a bias)

While unplugged put a test lamp to BAT POS and connect to the POWER side, if the lamp lights, you've a short to ground, and if it lights momentarily then turns off you have got a voltage drop on the POWER side.

What happens if you substitute a 4 amp bulb as the soloenoid? Do the codes clear? Is the PCM happy, or are the voltages differ plugged in as compared to the solenoid?

Thanks for your detailed answer

The ECU already identified that I have a short to ground

My question is since the voltage drop on the POS side of the harness 2.42v is seen on the NEG side of the harness, which of the wire is shorted

I looped another wire to the ground wire and strapped it on the car body, and voltage on the POS side was restored and voltage on the NEG was dropped

I got back my 13.34v, solenoid was able to actuate the wastegate by opening it when the vehicle is started, but Mercedes software has refused to do this actuation itself using F3 and F6 buttons

The problem is the short to ground code is still there, the ECU has refused to accept my NEG LOOP repair

The ECU is giving a fixed fail safe figure of 800hPa, and when the engine is rev the boost remains the same and DTC for a short to ground remains.

I guess the engineers designed where they want the vehicle to be grounded

My question is giving the bias voltage on the NEG side and voltage drop on the POS side, which of the two wires is shorted to ground (POS or NEG)
Which wire should I specifically go after

A higher resistance is definitely the cause. Why is the missing voltage on the POS side finding it’s way to the NEG side

Can a bad ground be the cause
Can a ground ever be shorted to the car body at a point the engineers did not design it to be grounded
Can a POS side be shorted to its own ground wire
Will the wrong amperage of a fuse cause all these problems


I just want to be able to streamline my troubleshooting

EDIT: so this is what I found on the internet

A circuit has a hot, neutral and ground. The hot wire brings the current to the appliance. The neutral line creates a return path, while the ground ground provides an alternate route in case of a malfunction.

1. Stray voltage is leaking from the hot wire to the ground

2. You can blame voltage on neutral and ground wires on VOLTAGE DROP

******so can I conclude that voltage on my ground wire is as a result of voltage drop on my POS wire caused by a short to car body, wrong fuse amperage etc*****

Is my unwanted resistance on the NEG side or on the POS side

Is the ECU seeing unwanted resistance due to wrong amperage of a fuse, and POS wire short to the car body as the same problem

Do I have a bad ground

All the three grounds on my ECU came back with 0.00v

It will be interesting to see the fix for this problem as I would have learnt vital part of car electrical troubleshooting

Warm regards

My question is since the voltage drop on the POS side of the harness 2.42v is seen on the NEG side of the harness

No, it is not. You are assuming that the 2.42v seen on the ground side is the missing voltage from the power side voltage drop. It is not.

As Paul mentioned, this is, likely, a biased voltage that the computer uses for self-diagnostic checks. When the solenoid is plugged in, it will pull this 2.43v [strike]to ground[/strike] up, or down, depending on circuit design. When the computer sees [strike]near 0 volts[/strike] a higer voltage or a ground (depending on circuit design), that tells the computer that the solenoid is there. With the solenoid disconnected, the computer sees 2.42v (bias) and it knows that there is a problem.

To test this theory, with the solenoid disconnected, read the 2.42v with a volt meter. Connect an incandescent test-light to battery (-). Touch the ground wire (2.42v) with the test light. The 2.42v should disappear. If it does, it IS a bias voltage and SHOULD be there when the solenoid is disconnect. No problems on the ground side.

Solenoid positive side engine running is 10.92 V (Benz wants this to be between 11-14v)

This is where I would be concentrating.

I'm not sure what vehicle this is, but I would find the schematics and trace the path of voltage, from the battery (+), to the solenoid connector. Somewhere, voltage goes from a system Battery voltage of 13.34, to 10.34v. You need to identify where this is happening.

I looped another wire to the ground wire and strapped it on the car body, and voltage on the POS side was restored and voltage on the NEG was dropped

I would connect the negative lead of my volt meter to battery (-). With the engine running and headlights turned on, touch the engine block with the positive lead, . If you read any voltage above 100-200mv, then you have a bad engine/block ground. Now, touch the positive lead to the vehicle body. If you read any voltage, above 100-200mv, then you have a bad body ground. Now, touch the positive lead to the PCM/Computer case. If you read any voltage above 100 mv, then you have a bad computer ground.

Thanks Paul and Chad, you guys are great.

I will use scannerdanner video below as a guide, and get back with the fix of this vehicle



I guess without repairing the short to ground, nothing else matters

I also need help with the very troublesome accelerator pedal position sensor P1222 voltage is too low

I have tested 6 pedals so far with the same part number and that didn’t fix it

Part number
A906 300 04 04

Vehicle is a Mercedes Benz sprinter 2007 (W906) OM646 diesel engine CDI4 Euro 4

Update

No idea how I managed to fix the Y77 solenoid

Voltage is now 11.4v engine running and it’s not throwing any DTC

I will still keep an eye on that

Now I’m getting boost pressure sensor DTC and the pedal

It’s saying the atmospheric pressure between the BPS and ECU is implausible

Will the faulty accelerator pedal sensor cause this
Values of the pedal sensor below
At WOT sensor 2 reads 100%
At WOT sensor 1 read 0%

How do i proceed with this repair
The mechanics have that I’ve been to are stomped

At least I’m making little progress so far

Do I attack the pedal first or the BPS

Is the boost pressure sensor the same thing as the manifold absolute pressure sensor MAP?

Also any idea why I’m measuring 0.01v on the white wire written 5v of the APPS

Or is the computer designed to pull it ground

Mercedes Benz test procedure only requires pin 1 and pin 3 tested for 5v

My readings below

1. Yellow/red. 5v
2. Yellow. Signal 1
3. Yellow/brown. Sensor ground 0.03v
4. Brown/white. Ground 2 0.01v
5. Pink/white Signal 2
6. White 0.01v

Pix attached