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RV8 alarm ECU - suspected vulnerability
Brian Moyse (BRG 1714) from Surrey posted a caution on the V8 Bulletin Board concerning the risk of damage to the alarm ECU if the battery runs down if the car is not used, typically during the Winter months. (Oct 03)

October is the time of year when many RV8 owners may be having thoughts about laying their cars up for the Winter months. This posting is to remind members that it is important to either disconnect the battery on the car during a long period of lay-up or to ensure that the battery is kept fully charged because there are electronic systems on the RV8 which will slowly discharge the battery if it is left connected for
long periods. The note is also a warning to RV8 owners of the potential damage which can occur to the original equipment alarm electronic control unit (ECU) if the battery is allowed to discharge and then attempts are made to start the car with insufficient voltage from the battery.

At least two RV8 members have suffered fatal alarm ECU failures which appear to have occurred immediately after attempts to start the car were made with low battery voltage/capacity. In one instance the problem was a discharged battery, in the other the battery connections were found to be loose. In both cases attempting to start the car resulted in the starter motor solenoid "pumping". This occurs when there are sufficient volts to pull in the solenoid, but insufficient volts to turn the starter

motor. The current draw of the starter motor winding results in a further drop in the voltage which then causes the solenoid to drop out. The voltage then recovers, whereupon the solenoid pulls in again and the cycle repeats for as long as the starter motor/ignition switch is operated.

A possible explanation for the ECU failures is an inherent vulnerability to voltage transients (spikes) on the electrical supply system. Voltage transients could occur as a result of the solenoid "pumping" - that is rapid switching of an inductance (the solenoid coil plus the starter motor winding) in a direct current circuit, similar to the voltage transients that cause arcing at the contact breaker points in a distributor when switching the ignition coil circuit. The ECU failure is total in that no communication is possible with the remote "plips" or the MG Rover "Testbook" diagnostic system and the alarm system will not set or un-set.

Footnote from Victor Smith (Harvest Gold V8 1089) who reminds readers that a good way of keeping the battery on an RV8 or MGBGTV8 in good condition and well charged during a lay-up is to connect a battery management unit to the battery so a regular trickle charge is maintained. The advantage of the battery management units is they cut off when the battery is fully charged thereby avoiding the risk of the electrolyte being boiled. See RV8 Workshop Note 56 for more information on battery management units.

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