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Multi-relay check (HOW TO) by Norman

How to do all those tricky XJ40 repairs
j_r
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Multi-relay check (HOW TO) by Norman

Postby j_r » Thu Jul 11, 2013 9:15 pm

All our motors are fitted with a number of multi-relays blocks, like DBC 10010, DBC 10008, and DBC 10009 etc. etc. all these are, in actually fact, are modules which contain four individual relays mounted on a common connector foot and this in turn connects to the main hiring harness of the motor.

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The DBC 10010, for example, controls the following 4 circuits:- 1. Heated rear window, 2. Aerial up, 3. Aerial down, 4. Ride levelling. So if you are experiencing a problem with one of these circuits, it is more than likely that this module is the problem. Check the fuse feeding this module first, of course, this could be the common problem.
A relay is nothing more than an interface between a low power source on the input and a higher power on the output. This output from the relay can be used to drive a lamp, a blower, a solenoid, a fan etc. and the input is, in the majority of cases, from an ECU (Electronic Control Unit) The relay also gives us galvanic separation between the input and the output (this means that they are isolated and not mechanically connected). All the electrical circuits on an automobile are DC (direct Current) and as such they are polarity sensitive and components must be connected to respect this polarity and most cannot be switched around. A relay is basically a coil of copper wire wound around a ferrous core, when current flows through this coil a magnetic field is set up in the ferrous core and this in turn can be used to attract a beam which in turn is connected to the output contacts of that relay. So when there is 12 Volts present on the relay coil the contacts will be closed and an output current can flow, actuating the output device and then when the input voltage is removed the contacts open, the output current is removed and the device is deactivated.
One great disadvantage of using DC relays is an effect called “reverse emf (ElectroMagneticForce) When the 12 volts is removed, then the magnetic field will collapse, this collapsing field causes an enormous current of very short duration but in the reverse direction, that could severely damage other components in the circuit. I order to combat this reverse emf a diode (Quench diode) is used. This diode allows the conduction of current, but in one direction only, in the other direction the current flow is essentially blocked. The quench diode is connected in parallel across the relay coil and in series with a hefty resistor, when the relay is energised all works fine, but when the voltage is removed from the coil the collapsing field will cause the reverse current to flow, but now it is free to flow through the quench diode which is in effect short-circuited.

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In the DBC 10010 module there are four relays, three are the same and one Relay No. 1 (the one for the rear screen heater) is much more robust, this is because the output contacts must be able to handle a higher current than the other three, but otherwise the circuits are basically the same.
To test the relay circuits we need a 12 V power supply (or battery) and a multi-meter.

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With the circuit diagram in hand, proceed as follows: taking very good care to respect the polarity of positive and negative, inject 12 volts directly to the relay coil. (The little bar on the diode in the schematic corresponds to the white line on the actual diode on the printed circuit board and is the positive end, anode) You should hear the relay pulling in. Now take the multi-meter on Ohms range and connect across points 1 and 2 (these could be different numbers depending on the actual module) Again inject 12 volts on the coil and the multi-meter should show continuity when the relay is energised and open circuit when the relay is de-energised. Conversely when the multi-meter is connected across 1 and 3, it will show short-circuit when the relay is de-energised and open circuit when the relay is energised. In order to check a diode, connect the multi-meter on Ohms range and the black lead on the end of the diode with the white band and the red lead on the end without the white line and the meter should show continuity. Reserve the leads and there should very little or no flow of current. (Open circuit) Replacement diodes can be found in any electronic shop and only cost a few pennies. Relays are mechanical devices and as such are subject to fatigue, a relay on one of our cars may have been operating for many thousands of times during its working life. They too can fail and the solution is either a complete new module or a new individual relay. The technology has changed and improved over the years and most modern cars do not employ relays at all, they use something called thyristors, basically the same as relays but without moving parts (Solid state devices) I am looking into a method to clone a relay module, taking out the relays and substituting thyristors. (Silicon Controlled Rectifiers) SCRs. Anything to keep these old girls on the road. Hope this post helps in trouble shooting relay module failures.
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