On 2020/02/09 4:31 p.m., Adam wrote:

My car's blower motor relay may be bad (or the cause of noisy fan). So I
found the following video...

How to Test a Relay the Correct Way
https://www.youtube.com/watch?v=ire7vyDqelg

However, I don't have a cordless drill battery (used in the video).
Will the following battery charger...

UPG D1724 Sealed Lead Acid Battery Charger (6V/12V Switchable Single-
Stage With Alligator Clips)
https://www.amazon.com/gp/product/B0...c_act_title_1?
smid=A10QFO4IXVZNRN&psc=1

work ? Other frugal ideas ?


OK, the video is fine as far as it goes, but it really is not the last
word in properly testing a relay as the real test is do the contacts
work under load.

So, what I would do if I was suspicious of this relay, and I didn't want
to remove the cover to visually inspect the contacts, is I would first
hook the relay to 12VDC with the +12VDC source connected to terminal 86,
and the negative side of the 12VDC supply to terminal 87. The diode
shown in the wiring diagram on the relay shows that the relay will not
operated if these are wired backwards as the diode would block the
current flow.

Next I would find a load of a couple of amps - perhaps a car light bulb.
You want something that is 24Watts - which is roughly 2 amps at 12VDC.
You then wire up the lamp so that the switch inside the relay - contacts
30 & 87 - are in series with the light and the 12VDC source. And you
also hook up a voltmeter across terminals 30 & 87 - to measure voltage
across the contacts.

Now power/energize the relay - the 12V lamp should light and the voltage
across the terminals 30 & 87 should drop to as close to zero VDC as
possible. Anything under 0.100VDC would probably be OK, with actually
0.00VDC being the ideal. If you see around 0.50 or more VDC across those
two terminals then the relay contacts are corroded and the relay is
unlikely to operate anything drawing heavier current well at all. It
should be replaced.

Without testing relay contacts under a reasonable load you really don't
know if the contacts will operate well enough to do the job that is
required. These relays are usually rated at around 25A, so a 2A load is
roughly ten percent and will give you some idea of the contact condition
if you measure the voltage drop across them.

John :-#)#