by Lewis Plumb |

Question: My Volvo 740’s engine management light comes on occasionally. It then idles a little roughly, but drives OK. I find that if I disconnect the battery overnight, all goes back to normal for about a month – but then it happens again. Can you tell me how to cure this? Peter Riggall, Tasmania


Answer: This is a common cause for concern among owners of newer classics, so we’ll make our answer as general as we can.

The warning light may come on to indicate a specific problem detected in one of the engine management systems. It may also illuminate in response to an incorrect level of oxygen in the exhaust gases which can’t be corrected for some reason.

So although a faulty electronic component (eg, a sensor or fuel injector) might be to blame, a simple misfire due to a faulty spark plug or a damaged HT lead can also trigger the light. The poor running may simply be the ECU putting the engine in ‘limp home’ mode. Disconnecting the battery wipes the fault from the ECU’s memory.

There could be many causes. Our first suggestion is to carry out a visual check of vacuum hoses, the engine wiring loom, the HT leads and other basics for obvious defects such as chafing, disconnection, perishing or burning. If nothing looks suspicious, interrogate the on-board diagnostic (OBD) system for fault codes. We’ll describe OBD-1, common to most fuel injected cars up to about 1995.

The OBD system monitors the inputs and outputs of the engine management system. It detects whether a specific voltage input from a sensor is too high or too low. It also monitors the output devices (such as the injectors) and warns if an apparent open circuit or short-circuit exists.

The output of the Lambda (oxygen) sensor in the exhaust should continually switch directly between about 1.5V and 10.5V in normal running. If it sticks at one of these levels for any length of time or hovers uncertainly in-between, it will trigger the warning light.

There should be a diagnostic socket for the system (it’s on one of the inner wings in your case). Turn on the ignition and bridge two terminals in it (see your workshop manual) to set the ECU into diagnostic mode.

A bent paperclip is a perfect tool. The system then flashes fault codes through the engine management warning light. These codes are detailed in the workshop manual. Have a pencil and paper handy to write down each code as it’s flashed up.

Note that the codes are stored, so there may be historical faults present too.

The memory can be reset by disconnecting the battery for a while or, on your Volvo, three successive disconnections with a pause between each.

Remember that many faults are caused by poor wiring and connectors. Before condemning a sensor or other part that has been denounced by the diagnostic system, go through the wires and plugs carefully.

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