Monday, 20 June 2016

Checking power supplies

We are going to do a bit of revision, and what better than an everyday example:

A vehicle comes into our workshop that does not start or does so with difficulty.

We verify that it does not start, and then the first thing we do is check the existing or stored errors using the diagnostic apparatus.

 On the diagnostic machine we see that there is an injection relay error. Next, and due to the ease of testing another relay, we replace this relay to see if it is indeed malfunctioning (this is a common 20 amp relay, and there are bound to be some in the workshop), we fit the new relay and test it.
The car continues to have the fault and does not start, there is no other option but to start checking the installation.

For this, we use the vehicle's diagram and first check the power supplies and grounds.

How do you proceed in case of a relay failure?
We all know that a common relay (4 pin) is configured as:

Pin 85 – Relay coil power supply (+)
Pin 86 – Relay coil ground (-)
Pin 30 – Power supply (+30)
Pin 87 – Positive output to the component

To speed up the check and to not have to look in the relay box or in the car installation to see which wire corresponds to each one of the relay pins, what is wrongly usually done is to remove the relay from its base, clamp a multimeter on the three pins (85, 86 and 30) to verify that the power supply or the ground is connected that corresponds to each one of them, in order to see if indeed there is battery voltage at 85 and 30, and ground at 86.

With this check, you can only verify or not if there is a battery voltage or ground, without being able to check the power supply current value at this point.


After fitting the relay back onto its base, we realise that although the negative and positive power supplies are arriving at it (as shown in the above test) in the ignition phase the relay operates correctly, but in the starting phase, the relay ceases to function.

This leads us to the conclusion that although the test was carried out with the relay fitted and with the ignition on and it all functioned correctly, one final test must be carried out, which is:

to verify these power supplies in the starting phase.
At this point we have:
A drop in voltage due to the consumption of the starter motor itself.
A normal operating consumption of the engine control unit of about 10 amps.

This consumption results in the power in the power supply cable to the relay coil dropping below the acceptable parameter for activating this coil and actuating the relay's internal electromagnet, and therefore this relay ceases to be switched on and the engine control unit does not have the necessary power supply for it to operate.

The actual cause of this specific fault (more than sufficient reason to describe it in this blog for mechanics) that resulted in this car coming to the workshop was a defective fuse. 

This was a 30 A fuse of poor quality which generated a resistance when under load that caused a drop in current in the positive line (pin 85 of the relay), preventing its operation. A simple look at the fuse in question did not reveal a problem, and the cause of the defect was the material of which it was made.

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