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This ar­ti­cle is a true de­scrip­tion of an AECS tech­ni­cal help desk prob­lem and how it was­solved. By HP Lei­jen, AECS.


2006 Mercedes Ac­tros 3, WDB 934 EDC MR2 V8 Unit pumpengine.

Prob­lem pre­sented to the Helpdesk

This V8 truck ar­rived at the work­shop (which owns AECS di­ag­nos­tic equip­ment) run­ning on four 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 cylin­ders 2,4, 5, 6 was faulty.

By click­ing on the fault code “unit in­jec­tor 6” the wiring di­a­gram in Photo 1 ap­peared on the scan tool’s lap­top with the of­fend­ing so­le­noid valve high­lighted (CY11).

By click­ing on the in­jec­tor a pic­ture pops up with the com­po­nent and its lo­ca­tion high­lighted.

Click­ing on that pic­ture brings up an in-depth de­scrip­tion of the com­po­nent with its mea­sur­ing data.

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 pres­surises diesel, which gets 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. 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’s worth it as new pump units are not cheap. Re­plac­ing all four units is just ir­re­spon­si­ble.

The Jal­test data sheet 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’s look with a scope at the in­jec­tor pat­terns on a non-work­ing cylin­der and an op­er­a­tional Cylin­der, and com­pare the two.

On in­jec­tor no.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.

In the record­ing on in­jec­tor no. 4 it can be seen that the power sup­ply does not switch up to full sys­tem volt­age, and that the cur­rent con­trol side does not start con­trol­ling the cur­rent by fast switch­ing im­me­di­ately. The in­jec­tion du­ra­tion is also much shorter (660usec) on the faulty in­jec­tor no.4, while in­jec­tor no.1 had 1.87msec in­jec­tion du­ra­tion.

What is com­mon on the four faulty pump unit so­le­noids?

Let’s 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 power sup­ply com­ing from the ECU sus­pect.

Since ECUs hardly ever fail we asked the di­ag­nos­ti­cian to record with the scope the two 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 three cylin­ders out of eight.

ECU It is time to have the ECU re­paired at or re­placed, with­out 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 need to have all fault codes, in­tro­duced and rel­e­vant, re­set.

Con­clu­sion Hind­sight is a beau­ti­ful thing, but should we have gone for the ECU straight away? Or should we sim­ply have fol­lowed the codes and re­placed the four pump units? ECUs fail sel­dom so that should re­ally be a last re­sort.

The equip­ment this work­shop has (Jal­test truck scant­ool 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 with “is it the ECU or have I missed some­thing?”

Also 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 provider care­fully. There are many equip­ment providers, but can they re­ally as­sist you com­pre­hen­sively when the go­ing gets tough?

READER RE­PLY 01506036

Jal­test scan tool data­base pic­ture

Jal­test ECU lo­ca­tion pic­ture and con­nec­tor pin num­ber­ing.

ATS 500XM scope record­ing, Ch1 con­nected to in­jec­tor no.1 (not faulty) power sup­ply, and Ch2 con­nected to the in­jec­tor cur­rent con­trol

ATS 500XM scope record­ing, Ch2 con­nected to in­jec­tor no.4 (faulty) power sup­ply, and Ch1­con­nected to the in­jec­tor cur­rent con­trol wire.

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