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Servicemans Log

Can a standard 4-pin 12V relay be all that difficult to diagnose? Well, not really but you can really be caught out if you assume that all relays conform to the same standard, particularly in the automotive trade.

Items Covered This Month

• Relay standards – no such thing

• Resurrecting a PC

My first story this month is from my friend in the automotive trade and concerns an interesting relay fault in an old Ford. I’ll let him tell it in his own words . . .

We recently had an old car into the workshop with a “battery drain” problem. The symptoms were simple enough. If the car was left unused for more than 36 hours, the battery didn’t have enough charge remaining to start the car.

The car in question was a 1976 XB Fairmont. These old girls are now so ancient that they qualify for HCRS (Historic Concessional Registration Scheme, in NSW) or in plain English, “Vintage Rego”. Now I really do feel old!

This was a classic case of an older technician passing a supposedly simple job to a junior “new-age” technician, thinking that he could see firsthand how simple cars once were. The only electronics on board a 1976 Ford are inside the (AM) car radio and the 35A alternator. There was no EFI, ABS, ECU, sat-nav or burglar alarm. In fact, this car didn’t even have air-conditioning or power steering. And while it was light-years ahead of a 1953 FJ Holden, this 1976 Ford illustrates just how far we have come in the last 33 years.

Anyway back to the problem at hand. With a car this simple, how easy would it be for our young tech (let’s call him John) to solve a problem as mundane as the battery going flat? Admittedly, this was on an older car that he wasn’t familiar with but it should all be routine.

Well, at least it started out well. He first carried out a battery load test and then checked the alternator charge rate to verify that that side of the system was all good. John then quickly found that with all the Ford’s electrical devices switched off, his multimeter indicated a constant current drain of more than 180mA from the battery.

From there, his next step was to check that all the usual suspects were definitely off. By “usual suspects” I mean those little convenience items such as boot lights, glovebox lights and other “courtesy” features that cause so much grief in draining car batteries if they’re not working properly.

In fact, in my experience, these “convenience” items can be very inconvenient at times.

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