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Advanced Troubleshooting a SOTF System Fault


 
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C.B.
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PostPosted: Mon Dec 03, 2012 3:25    Post subject: Advanced Troubleshooting a SOTF System Fault Reply with quote

Sorry folks, this is a long one and I admit to having it in the can for ages, but I finally got around to putting it all together…

Background.
Strong smell of burning inside the car, this like burning electronic components & not PVC wiring. No visible sign of smoke & no sign of abnormality in any electrical device, until 4WD was first attempted...  4WD engaged with solid lamp indicator (and did physically engage too), but on pushing the SOTF button again to disengage it left the lamp flashing at 2 times per second. (the lamp now flashed constantly at this rate in 2WD mode). I prepared a full report, advised a local dealer and booked the vehicle for a visit.

The dealer, after first tests, wanted to replace the front axle switch so was re-booked for this to happen.   When the vehicle was returned to the garage the following week, they had it in the workshop for a couple of hours and then advised it wasn't the front axle switch.  I would have to re-book and leave the vehicle with them for a couple of days so as they could strip bits and test the wiring harness.

My Investigation and Repair Process.
Given that the dealer actually stated "we did not read your long-winded report - we will carry out our own fault-finding on this", and my concerns over costs, I decided to investigate myself.  hmmm
This was done in the following order:
1 Removed the front axle switch and bench tested - ok
2 Removed a vacuum pipe from each front axle VSV and tested vacuum - ok
3 Removed each front axle VSV and bench tested as per workshop manual - apparently ok
See the images below for this process:




4 At this stage putting together the evidence of the burning smell INSIDE the car and a bit of manual reading, I removed the centre console and lower dash to get access to the 4WD control module.
See the following pictures:










The front cover was unclipped and the printed circuit board removed to reveal the source of the burning.
This was at the transistor TR4, which on further investigation drives the VSV's via 2 wires exiting on pins 14 & 15 on the module.
See the following pictures:






5 At this stage I tested the wiring from the 22 pin module connector (C-94) to the front axle devices, there was continuity present and no shorts apparent.  Because of this and the fact that the VSV's passed the recommended bench test, I decided it could be simply random component failure.  On this basis another 4WD module was sourced and fitted.

6 When the replacement module was fitted everything worked correctly, but only for a few minutes… Then the new circuit burnt out in the same position - TR4. GGgggrrrr violent1  Embarassed

Discovering this I called IM for some technical support and in particular details of the circuit of the 4WD ECU. They said they could not assist and they did not have a circuit diagram for this module.
I then decided to reverse engineer the printed circuit board further than I had done for stage 4 above and on looking at the result of that exercise the conclusion I came to was that the fault had to lie in the wiring to the VSV’s or the VSV's themselves.
See my partial schematic below:-



7 I removed the VSV's and re-tested as per the workshop manual, they passed.  I then however measured the current each was taking and found that the top (grey) was taking 300mA and the bottom (blue) was taking 1.9Amps.  A resistance check confirmed top valve at 40 Ohms and the bottom at 6 Ohms. This confirms the reason for TR4 burning out (partial short in the valve coil winding) – here’s my calculations:

Resistance of Vacuum Servo Valve is normally 40 Ohms each, given that they are in parallel to the output of the 4WD module then 20 Ohms is presented to the circuit.
So at 14.4 Volts the current drawn would be 14.4/20 = 720mA total, (360mA for each valve).

In this case.
Grey valve = 40 Ohms and Blue Valve = 6 Ohms
Total resistance presented to circuit 1/R = 1/40 + 1/6 = 5.2 Ohms
So at 14.4 Volts the current draw would be 14.4 Volts / 5.2 Ohms = 2.8 Amps.

The transistor TR4 (part number 2SB1261-Z) on the control module is rated at maximum of 3 Amps collector current (Ic Max) and more particularly it is rated at 2.5 Watts maximum total power dissipation WITH A 7.5cm2 HEATSINK.    In the fault condition described above the transistor was attempting to dissipate 5 Watts WITHOUT A HEATSINK.   This calculation is based on Power = 14.4 Volts2 / 8 x 5.2 Ohms = 4.98 Watts.
The device in normal conditions would handle power of 14.4 Volts2 / 8 x 20 Ohms = 1.29 Watts.


8 A second replacement module was purchased along with a pair of VSV's, these were fitted to the vehicle and I now had 100% operation of the 4WD system.  Laughing  Razz  
However, looking at the fragility of this part of the circuit (there is absolutely NO protection for this output device), and realising that the two wires are spliced together to travel through the harness as one wire, then re-splice back to two wires near the VSV's. I then cut both wires from pins 14 and 15 of the module about 2 inches from the connector, spliced them together at both sides of the cut and fitted a 20mm in-line fuse holder carrying a 1 Amp anti-surge fuse, (see following pictures).
Now the very expensive circuit module is protected from a simple VSV or wiring harness short. Cool
See the pictures below:





In my opinion this has a design fault and could well have resulted in a vehicle fire. Also, had I not taken control of the situation, it could have resulted in a bill for well into the 4-figure bracket.

Conclusions & Suggestions (made to IM).
(A) The workshop manual (section 4B1) does not cover how to fully test the VSV's and the faultfinding flowchart is incorrect.
(B) The circuit should have simple protection for this output to avoid the potential for a vehicle to catch fire through such a fault.
(C) An information bulletin or manual amendment should be issued to dealers to amend the fault-finding flowchart & VSV testing routine.
I further suggested that if through time the MTBF of the VSV's shows an increase in this failure mode, then a simple solution should be considered that a retrofit fuse-holder wiring modification containing a 1Amp anti-surge fuse could be made available to avoid the potential of vehicles catching fire.


I hope this post will be of assistance to anyone finding the same fault has appeared in their Trooper.   hello2
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mossey
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PostPosted: Mon Dec 03, 2012 8:17    Post subject: Reply with quote

Stunningly superb post well done
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Rhanagar
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PostPosted: Mon Dec 03, 2012 8:29    Post subject: Reply with quote

Well done. Brilliant posting and great attention to detail. Credit to the forum.
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DS9
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PostPosted: Tue Dec 11, 2012 8:45    Post subject: Reply with quote

Good job.  Like they say: If you want it done right you need to do it your self!
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Philip A.
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PostPosted: Tue Oct 01, 2013 21:56    Post subject: Reply with quote

Happened on this post while doing a SOTF search. Technically excellent, deserves to be a sticky - no idea how frequent this fault is, but the approach and explanations are exemplary!
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d12
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PostPosted: Tue Oct 01, 2013 22:44    Post subject: Reply with quote

Great post ,and if you want it done right ! Do it yourself
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PostPosted: Wed Oct 02, 2013 19:42    Post subject: Reply with quote

First time I've seen this  Embarassed

Mods...........Sticky  Question

Be good alongside Rhanagars How to  Wink
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DYLAN
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PostPosted: Wed Oct 02, 2013 19:45    Post subject: Reply with quote

Red Robbo wrote:
First time I've seen this  Embarassed

Mods...........Sticky  Question

Be good alongside Rhanagars How to  Wink


Wink
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C.B.
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PostPosted: Tue Oct 08, 2013 20:17    Post subject: Reply with quote

Thanks for the kind comments folks.  wave

Good to see it's of interest  Very Happy   It should compliment Rhanagars excellent contribution.

BTW, I never did find a UK source for the burnt out surface mount transistor TR4 (2SB1261-Z) or Zenner Diode ZD3 (51 ??), but if I had... the burnt out modules could have been fully repaired instead of being replaced.
Now the fuses are fitted I should never see that happen again!!!
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PostPosted: Thu Oct 10, 2013 20:02    Post subject: Reply with quote

C.B. wrote:
Thanks for the kind comments folks.  wave

Good to see it's of interest  Very Happy   It should compliment Rhanagars excellent contribution.

BTW, I never did find a UK source for the burnt out surface mount transistor TR4 (2SB1261-Z) or Zenner Diode ZD3 (51 ??), but if I had... the burnt out modules could have been fully repaired instead of being replaced.
Now the fuses are fitted I should never see that happen again!!!


TR4 (2SB1261-Z) Here

Zenner Diode ZD3 (51 ??) - pass, sorry  Smile
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PostPosted: Thu Oct 10, 2013 20:05    Post subject: Reply with quote

kennymac wrote:
C.B. wrote:
Thanks for the kind comments folks.  wave

Good to see it's of interest  Very Happy   It should compliment Rhanagars excellent contribution.

BTW, I never did find a UK source for the burnt out surface mount transistor TR4 (2SB1261-Z) or Zenner Diode ZD3 (51 ??), but if I had... the burnt out modules could have been fully repaired instead of being replaced.
Now the fuses are fitted I should never see that happen again!!!


TR4 (2SB1261-Z) Here

Zenner Diode ZD3 (51 ??) - pass, sorry  Smile


Although you could have a look [url=http://uk.farnell.com/jsp/search/browse.jsp?N=2031+204243&Ntk=gensearch&Ntt=zener+diode+51**&Ntx=mode+matchallpartial]Here[/url]
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PostPosted: Wed Sep 03, 2014 8:12    Post subject: Reply with quote

I have just acquired an isuzu trooper commercial which is displaying the same warning light in 2wd.  It appears to engage 4wd correctly and sounds like it is engaging and also appears to disengage.  is it a bad idea to drive it in 2wd with the light flashing.  and if i am to replace the faulty circuit board and vsv's etc how much do they cost?  I will do the work myself..

Thanks  now the happy owner of a LWB diesel commercial trooper.
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stevielong
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PostPosted: Wed Sep 03, 2014 15:02    Post subject: Reply with quote

Check your wiring first, mine was perished just above the front actuator
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