Checking relays

[john_h]

hi folks,

i was just wondering if anyone knows how to check a relay...safely of course.

that if im calling it the right thing...they sit in your engine bay fuse box and are cube shaped, black or grey and have small circuit diagrams on the side.

the reason i ask is because would a knackered relay bring up the battery light in general, if it is located on a battery charge circuit. had the alternator and battery changed and checked harness (which seems ok).

thanks in advance

[davidireland]

What car have you?

[davidireland]

On the small wire on the alternator there should be 12volts with the ignition on. This 12v supply is used to excite the alternator to start charging.
Check for 12volts there.
When the engine is running put a volt meter across the battery poles, what do you get? 13.8-14.3 v are exceptable readings.

[john_h]

not a peugeot, unfortunately, ford foci (as in sig). im just asking as i thought there would be a generic way of testing.

and im hating the idea of having my car go to a dealers, for them to bend me over a table for something so simple. if you know what i mean

[davidireland]

To check a relay. The coil of wire between 85 and 86 is the low current side of a relay. This coil is powered up by say an ignition switch. This in turn allows a larger current from 30 to flow to 87 or 87a. The larger current might be a 12v supply from a battery and 87 might be wired to a set of foglamps.
The relay basically sends larger current from one point to another and is operated by a small current.

To test them you can check for continuity between 85 and 86. if a cooil of wire is broken within it you won't get current flowing.
You can power the relay up with say attaching 85 to battery positive and 86 to negive and then see if you have a circuit flowing through 30 and 87 or 87a by putting your meter on it and setting it to buzzer.

[john_h]

Originally Posted by davidireland

On the small wire on the alternator there should be 12volts with the ignition on. This 12v supply is used to excite the alternator to start charging.
Check for 12volts there.
When the engine is running put a volt meter across the battery poles, what do you get? 13.8-14.3 v are exceptable readings.

yeah ive done all the alternator checks and battery checks, they are within acceptable tolerances, the red wire on the "pig tail" has no resistance and is pushing the voltage across. brand new alternator and battery have been installed.

[john_h]

Originally Posted by davidireland

To check a relay. The coil of wire between 85 and 86 is the low current side of a relay. This coil is powered up by say an ignition switch. This in turn allows a larger current from 30 to flow to 87 or 87a. The larger current might be a 12v supply from a battery and 87 might be wired to a set of foglamps.
The relay basically sends larger current from one point to another and is operated by a small current.

To test them you can check for continuity between 85 and 86. if a cooil of wire is broken within it you won't get current flowing.
You can power the relay up with say attaching 85 to battery positive and 86 to negive and then see if you have a circuit flowing through 30 and 87 or 87a by putting your meter on it and setting it to buzzer.

thanks david , i will follow this when its is daylight outside and i have the time. will keep you posted.