June 2015 - Technical Newsletter

This article is a true description of an AECS technical help desk problem and how it was solved.

Vehicle: 2006 Mercedes Actros 3, WDB 934 EDC MR2, V8 Unit pump engine.

Problem presented to the Helpdesk

This V8 truck arrived at the workshop (who owns

AECS diagnostic equipment) running on 4

cylinders.

The diagnostician used the Jaltest truck scan tool

as the check engine light was on with the fault

codes indicating that the injection system on cyl.

2, 4, 5, 6 was faulty.

By clicking on the fault code “unit injector 6” the

below wiring diagram (picture1) appeared on the

scan tool’s laptop with the offending solenoid

valve highlighted (CY11).

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By clicking on the injector, a picture pops up with the component and its location highlighted. Clicking on that picture an in-depth description of the component with its measuring data comes up. The injection system used in this engine is a unit

pump system. The camshaft pushes a plunger in a

single plunger Diesel injection pump. The plunger

Picture1:

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pressurises Diesel, which is pumped to only one injector

fitted in the cylinder head. On the side of each pump is a

high current solenoid valve, which is activated by the

ECU and controls the fuel quantity to the injectors.

Jaltest scan tool database picture

What shall we do?

Replace or Measure

Four pump units failing at the same time?? That is only

theoretically possible, I am sure everyone agrees with

that. It is time to measure. It is worth it as new pump

units are not cheap. Replacing all four units is just irre-

sponsible.

The Jaltest datasheet states that the coil of the pump

should be 0.6 Ohm. The resistance was checked on

the faulty and good valves. All resistances were the

same.

Scope time

Let us look with a scope at the injector patterns on a

non-working cylinder (picture 2) and an operational

cylinder and compare the two (picture 3)

On injector#1, Channel 1 the power supply to the

injector switches up nice and sharp as soon as the

valve needs to be activated. On Channel 2, the current

through the 0.6-Ohm valve gets up very high very soon

so current limiting (fast switching) sets in almost

immediately

Picture 2: ATS 500XM scope recording, Ch1 connected to injector #1 (not faulty) power supply, and Ch2 connected to the injector current control wire.

Picture 3: ATS 500XM scope recording, Ch2 connected to injector #4 (faulty) power supply, and Ch1 connected to the injector current control wire.

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In the recording on injector #4 it

is visible that the power supply

does not switch up to full system

voltage, and that the current

control side does not start con-

trolling the current by fast

switching immediately. The in-

jection duration is also much

shorter (660usec) on the faulty

injector #4, while injector #1 had

1.87msec injection duration.

What is common on the 4 faulty

pump unit solenoids?

Let us look at the Jaltest wiring

diagram again. CY10 (cyl5), CY7

(cyl2), CY9 (cyl4), CY11 (cyl6) all

share the same power supply,

linking back to the ECU pin 9.

This immediately made the pow-

er supply coming from the ECU suspect.

Since ECUs hardly ever fail, we asked the diagnostician to

record with the scope the 2 separate power supplies to

the ECU and see if they were collapsing under the load of

the injectors 5,2,4,and 6.

The power supplies to the ECU were perfectly flat lining.

We asked the diagnostician to jump a new wire from the

ECU pin 9 to one of the faulting pump coils, while all

other coils were disconnected.

This jumper wire made no difference; the pattern was

still as bad as it was before.

As a last test, before we would condemn the ECU, we

asked the diagnostician to disconnect a bad coil and jump

the wiring from the bad coil’s connector to a good coil.

The good injector would now also not produce a proper

signal making the engine run on 3 cylinders out of 8….

ECU

It is time to have the ECU repaired at or replaced, with-

out a shadow of doubt.

After a discussion with the owner, it was decided to send

the ECU away for repair.

A faulty power supply switch was found inside the ECU,

and replaced.

The ECU was plugged in and the truck was on the road

about one hour later. The repaired ECU did not needed

any coding with the Jaltest scanner, saving time. It did

needed resetting all fault codes, introduced and relevant

fault codes.

Picture:4 Jaltest ECU location picture and connector pin numbering

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Conclusion

Hindsight is a beautiful thing, but should we have gone

for the ECU straight away? Alternatively, should we

simply have followed the codes and replaced the 4 pump

units? ECUs fail seldom so that should really be a last

resort.

The equipment this workshop has (Jaltest truck scan tool

and ATS 500XM scope) enabled the diagnostician to

prove without doubt that the fault was in the ECU,

before the ECU was pulled out. Imagine how you would

feel, faced by “is it the ECU or have I missed some-

thing”…..

In addition, this case proved that sometimes ECUs do fail,

despite us telling you in training that they seldom fail.

Please select your equipment and technical support pro-

vider carefully. There are many equipment providers,

but can they really assist you comprehensively when the

going gets tough?

For AECS Ltd

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06 8749 077

www.aecs.net

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Take screen shots, record and play back live data

Individual or Multiple Combined live data Graphing

Automatic vehicle recognition

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