1985-1998 [Subscribe to Daily Digest] |
 | [Main 9000 Bulletin Board | BBFAQ |
Prev by Date | Next by Date | Post Followup ]
Member Login / Signup - Members see fewer ads. - Latest Member Gallery Photos
Relay testing Posted by Ari [Email]  (#2847) [Profile/Gallery]
(more from Ari) on Tue, 2 Nov 2004 05:59:27
In Reply to: Thats true!!, Paul Broeckx, Tue, 2 Nov 2004 02:52:49 Members do not see ads below this line. - Help Keep This Site Online - Signup |
You can do some basic relay testing. As you'll see later on, these tests aren't perfect.
Lots of information can be found at:
http://www.installdr.com/TechDocs/999404.pdf
A basic test can be found at
http://www.installdr.com/TechDocs/999403.pdf
A volt/ohm meter, some wire, and a 12 volt battery will come in handy.
With the meter set to Ohms, measure across pins 86 and 85. There should be some continuity - low ohms. How low? Depends on the rating of the relay. Anything less than about 30 ohms or so should be good.
The basic current path with the relay closed is pins 30 to 87. You need power to make that happen.
Connect ground to pin 85, 12 volts to 86. Hear a click? Good. You should measure continuity between pins 30 and 87. It should be an open with no power. If there is a pin 87A, it should be connected electrically to pin 30 with no power on the relay, and be open with the relay energized.
That's for basic relays - there are other relays with more contacts. There are relays with electronics inside for time delays, or to toggle on and off (e.g. high beam relays).
What this doesn't address is some of the more common failure modes of relays. The most common is a carbon buildup on the contacts. For very small currents, like the meter would use, the contacts pass current fine. For the high current that the relay switches, like the A/C clutch or headlights, that little bit of resistance makes for a big voltage drop, and the relay won't pass current. OK, why not measure that with the ohmmeter? Because most common ohmmeters can't measure that low. Most ohmmeters can only measure an ohm or so. With a 30 amp load through 1 ohm, that would be 30 volts - given the power supply is only 12 volts, that's no current. A good electrical contact for a large electrical load could be less than 0.1 ohms - no matter what your ohmmeter may read, that takes a special type of meter. So unless you can put a large electrical load on the connections, you can't really check that.
The other failure mode I see is microcracks in the large solder pads. Many relays use the solder pads to mechanically mount the relay to the circuit board. With temperature shifts and vibration, this causes micro cracks in the solder pads - intermittent contacts. Very tough to test for. It may work just fine in your hand, but act up when warm or shaking.
If I'm not trusting a relay, I just pop it open and carefully clean the contacts, usually with some very fine sandpaper. Don't polish them too clean - the plating is pretty thing. Just get the gunk off. I then inspect the solder pads with a high-power magnifying glass, and touch up any I suspect with a soldering iron.
posted by 192.249....
, Mon, 1 Nov 2004 05:27:59, Mon, 1 Nov 2004 13:39:39, Tue, 2 Nov 2004 05:59:27 <-- Viewing This Message, Tue, 2 Nov 2004 05:40:02
No Site Registration is Required to Post - Site Membership is optional (Member Features List), but helps to keep the site online
for all Saabers. If the site helps you, please consider helping the site by becoming a member.
 |
 |
 |
 |
 |
