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Serviceman's Log

Electronic and electrical faults in cars can give rise to all sorts of weird faults. Often though, the fault itself is really quite simple although tracking it down can be quite a challenge.

Items Covered This Month

• Limping VR Statesman

• The Pulsar that wouldn’t reverse

• What happened to Nine
Digital?

• Komatsu dump truck Tiptronic gear selector

Back in the July issue, a colleague of mine related some interesting service stories on car electronics. This month, he’s got several more interesting stories on car electronics to tell so I’ll let him tell them in his own words.

The limping Statesman

One of our long-time customers recently brought in a Holden VR Statesman with what appeared to be serious automatic transmission issues. The car had all of a sudden gone into “limp” mode or more correctly, “limp home” mode. What happens is that whenever the control computer (ECU) loses any major inputs or detects a circuit malfunction, the transmission reverts to “third gear only” operation when drive is selected.

This allows the car to be driven, rather than towed, to a repair shop.

To explain this more fully, the VR series (1993 on) was the first Holden to utilise the electronically-controlled 4L60-E transmission behind their V6 and V8 engines. The 4L60-E was similar to the previous model’s hydraulic unit (the 4L60), the difference being that gear shifts were now initiated by a series of 12V solenoid valves rather than the old method of complicated hydraulic hardware.

Click for larger image

The transmission solenoids are operated by the same ECU that controls the engine’s EFI system and other ancillaries. This was easily achieved because the engine and transmission shared many of the inputs required for their operation. The most important data comes from the TPS (throttle position sensor), the MAP (manifold air pressure) sensor and from temperature, road speed and engine RPM sensors. TPS and MAP are important in this instance, as they monitor accelerator position and engine load to enable correctly timed (and smooth) gear-changes – just as the old-style transmissions were controlled by a kick-down cable and vacuum modulator.

This was a perfect opportunity to use our diagnostic scanning tool. You just plug it in, read the codes, diagnose the problem, quote the job, order and fit the parts required and the customer is back on the road. Well, that’s the theory and I wish it was always that simple! More often than not, it just doesn’t pan out that way.

In this case, a fault code had been recorded for just about every device in the system. So how could you believe the readouts? Was every solenoid and sensor really faulty? I don’t think so!

The first thing to check was that the wiring harness to the transmission had not been damaged or become unplugged? This was checked out but everything was intact so we erased all of the codes that had been recorded, then switched on the ignition to see which codes (if any) would log again. And immediately, the same mass of fault codes reappeared.

To coin a phrase: “Houston, we have a problem”.

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