Relay testing (heater blower motor)

[BrokeMyCar]

Hi folks,

I'm repairing the heating system in my 82 SC and may have narrowed the problem down to the round relay in the engine bay:

http://www.pelicanparts.com/cgi-bin/smart/imgdsply.cgi?pn=91161510901-M58

I don't know what the lead numbers on the bottom (30, 86, 87, 87a, 85) mean though - anyone know a way to bench-test it? Also, are there non-OEM replacements? $21 is steep; that's how much I paid for the whole motor replacement! (I went with the McMaster-Carr adaption)

Thanks,

Babak

[fintstone]

The relay acts like remote switch. It is used to allow a small switch (physical size) to control a large current.

Pin 30 is the hot lead where the 12 volts is connected that is intended to power whatever device the relay controls...ie motor, light, etc. It should originate at a fuse rated appropriately for the device (may be switched or unswitched...depending on application). Normally (as drawn) when the relay is not energized, 30 is connected to 87a (The output at 87a is usually not used).

Pins 85 and 86 are connected to a small electro magnet. 85 is the ground connection and 86 the positive voltage. Generally 86 is hard wired hot and the ground to 85 is turned off and on with a switch...but it can just as easily be wired where 85 is hard wired to ground and the switch controls the positive voltage to 86. When power is applied to 86 and ground to 85, the electromagnet is energixed and pulls the contact down so that 30 is connected to 87. This provides the power to 87 and subsequently the device the relay is designed to control.

To test in circuit, check that 85 is ground, turn on circuit and check for 12 volts at 30, 12 volts at 86, 12 volts at 87 output.
Out of circuit, use a jumper to apply ground to 85 and 12 volts to 30 and 86. Check for 12 volts at 87. Remove jumper (12 volts) from 86 and the voltage at 87 should drop to 0 volts.

[BrokeMyCar]

Awesome, awesome write-up Fintstone - thanks so much! The figure was particularly helpful-

Thanks,

Babak

[fintstone]

No problem...just drew it up on PowerPoint. Most people...even pros.. don't know really know how to test them. They just swap with a known good one and see if it works.