On 18 Jan 2015 14:13:29 GMT, Phil K wrote:


I just wanted to thank everyone for the help with my Nisan Tino wiper
problem. I followed Lee's advice and bridged pins 3 & 5 of the relay. The
wipers then worked! (Thaks a million, Lee!)

So I guess this points to the relay being at fault. Now I have what I
suspect is probably a silly question... If it works fine with the relay
pins jumped, why do I need a relay in there at all? Can I just leave it as
is, with the pins 3 & 5 bridged?

Is there anything I can try to restore the relay? Are the contacts in there
clean-uppable?


The relay functions as a repeater switch. The term originated in
early telegraph days where a weak (digital) signal over a long length
of line was used to relay a fresh source of voltage over the next
stretch of line to extend the circuit beyond what a basic telegraph
key could directly send to the receiving sounder at the next station
(the extra voltage otherwise needed would increase the contactor gap
requirements on the telegraph key and raise the risk of electric shock
exposure for the operator - it also allowed the signal to be
regenerated before line capacitance introduced severe timing
distortion).

Aside from its original function, a relay allows safe operation of
high voltage high power electric motors using low voltage low power
circuits at the operator controlled panel. It also allows the control
of multiple high power devices from a single control panel button.

The uses of relays are many and varied but most commonly relate to
isolation of the operator and the low level logic control circuitry
from the higher voltages and currents that need to be switched in
order to control high power devices.

In a vehicle, the relay is used to control high current devices using
low current signals, the voltage usually remains the same safe 12 to
14 volt of the vehicle's supply. Quite often, the relay can provide
basic logic gating and control that would otherwise involve mulitpole
panel switches and many yards of additional wiring expected to carry
several amperes of load current which would generate significant volt
drops without resorting to inordinately heavy guage wiring. A basic
relay or two, placed strategically can save a small fortune's worth of
copper in a typical vehicle's wiring loom.

So, in answer to your question about needing a relay at all, the
answer is simply "No." :-)

Getting the wiper motor to run by shorting contacts 3 & 5 was merely
a test of the circuit between the relay's contacts and the motor. Now
that you know that this part is ok, your next question is "Is it a
faulty relay or a wiring fault to the relay's coil?"

Two basic faults are possible with the relay. One is where the coil
has gone open circuit, the other is where the contacts fail to close,
either because of a mechanical issue that prevents full travel of the
contacts or else the contacts themselves having burnt away
sufficiently to prevent proper electrical contact.

In either case, a replacement relay will restore the circuit to full
functionality (although in the case of contactor trouble, you may be
able to dress the contacts with a suitable file and 'bend' adjust the
contact springs to compensate and effect a cure. However, it still
makes more sense to simply swap it out for a new replacement).

The relay itself may be perfectly fine and is simply failing to
receive the controlling signal from the panel switch due to faulty
wiring or switches. If you can get someone to operate the wiper switch
whilst observing (listening to) the relay, you should be able to
ascertain whether the relay is responding (or trying to respond) to
the control voltage or not (without a meter or simple test lamp. it's
not possible at this stage to determine whether a failure is due to
open circuit coil windings or a lack of the wiper start signal).

A pretty failsafe way to apply test voltage (or even grounds) around
relay connections is to use a simple test lamp where one end is
clipped to the battery positve terminal and the other to an insulated
probe contact that you can apply to the relay coil connections.

Most relays only require a fraction of an amp to operate (circa 0.05
to 0.5 amps) a battery backed 12v 6W test lamp should at least cause
enough movement of the contactor assembly to either be seen or else
heard, if not actually fully operate the relay in question. If you
probe a ground return connection the lamp will simply fully light up
without danger of excessive short circuit current flow through the
wiring.

You can connect the test lamp to the ground return negative terminal
of the battery and test for the appearance of control voltages, in
this case you'd have your helper operate the wiper switch and look for
the lamp to light up in response.

If the lamp lights up ok to this test but the relay fails to react in
any way, it's a pretty safe bet that the relay itself is at fault. If
the lamp fails to light in response to the operation of the wiper
switch, you either have fault in the wiring or the switch.

If the lamp, when connected to the battery positive glows dimly when
applied to the 'hot' end of the relay coil which operates (fully or
partially) and doesn't completely dim out when your helper turns the
wipers on, you're still looking at a wiring or switch fault rather
than a faulty relay. An open circuit coil winding would usually[1]
fail to produce any lamp glow with this test (other end of the test
lamp connected to the ground return negative battery terminal).

[1] There may be other relay coils in parallel and possibly other
circuit elements to confuse this test procedure (using a DVM would be
just as confused by such complications - more so than a simple test
lamp since a DVM, on its voltage range, will quite cheerfully show
full battery voltage even via leakage paths of just a few micro-amps
which are otherwise totally insignificant to the full and proper
operation of the circuits in question).

If a relay develops a short circuited coil fault (a very rare type of
failure), this normally results in a blown fuse feeding the control
switch panel supply and the lamp test when connected to the plus 12v
terminal will show full brightnes whichever end of the coil is probed
(assuming positive logic with the 'cold end' of the relay coils wired
to ground return - in some cases it's the 'cold end that is switched
to a ground return level voltage, typically when some of the
controlling logic relies on relay coil wiring to other relay coils or
contacts, usually applied as 'hard logic' where any other
combinational logic would make absolutely no sense whatsover.
--
J B Good