Burning out DME Relay
09-13-2002, 01:14 PM
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Burning out DME Relay
Can anyone help me, Dealer replaced Burned relay circit board and DME relay 5 mouth ago. Yesterday car would not start, replaced Spare relay and was fine. This morning car would not start, replaced with new relay. After running for 10 mile I checked the relay and it was Hot 
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09-13-2002, 05:17 PM
#3
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Hi Scott,
Here's a breakdown of how the DME relay works and what it supplies:
-The relay has two internal relays.
Terminal #1/86 = 12V directly from the battery.
Terminal #2 = not used
Terminal #3/30 = 12V directly from the battery.
Terminal #4 = not used
Terminal #5/87 = switched 12V supply to DME, via relay #1.
Terminal #6/85 = relay #1 control "signal," from DME.
Terminal #7/87b = switched 12V power to fuel pump, via relay #2.
Terminal #8/85b = relay #2 control "signal," from DME.
-Ignition turned on, DME grounds 85 via DME terminal #36, completing circuit from 86 (12V from battery) to 85, energizing relay #1, .
-Relay #1 switches 12V power from 30 to both 87 and relay #2.
-87 supplies 12V power to DME terminal #37.
-DME grounds 85b via DME terminal #3, energizing relay #2.
-Relay #2 supplies 12V power to the fuel pump via 87b.
Okay ... the above is a mouthful, and probably only makes sense when you have the wiring diagram in front of you. Still, what you can get out of it:
-The DME relay receives its 12V power directly from the battery.
-The DME relay receives "control" signals from the DME (in the form of grounds which complete a circuit).
-The DME relay supplies 12V power to the DME & fuel pump.
Your symptoms suggest either voltage that is too high or a short/low resistant circuit, which is causing the DME relay to draw too much current.
Since the DME relay receives power directly from the battery, a short/low resistance on the "goes-out" side of the relay could result in a significant current.
I don't believe your problem lies with either 85 or 85b, as these terminals should already have a significant, current-limiting resistance between them and the 12V power supply (i.e., the relay coil).
This leaves 87 (DME 12V power supply) & 87b as potential suspects.
I don't believe your problem lies with the fuel pump (87b), as it's fused.
My best guesses:
-You're simply having bad luck with the relay, and your "hot" relay is normal.
-You have a short/low resistance somewhere between DME relay terminal #87b & the fuel pump, or somewhere between DME relay terminal #87 and the DME.
-For some reason, DME terminal #37 is drawing significant amps (there is no fuse between the DME relay and DME #37).
-Aftermarket equipment has been installed on your car, tapping into the output of the DME relay (either 87 or 87b).
Trying to determine how much current is being supplied/delivered by the DME relay can be done, but it's not exactly simple. Also, I'm not sure you're ready to go there. That is, you may not be completely convinced you have a problem just yet.
If you have a multimeter handy, you can at least check the voltage across your battery with the engine idling. Off the top of my head, it should be something close to 14V.
Well ... that's (more than) enough for now
.
Here's a breakdown of how the DME relay works and what it supplies:
-The relay has two internal relays.
Terminal #1/86 = 12V directly from the battery.
Terminal #2 = not used
Terminal #3/30 = 12V directly from the battery.
Terminal #4 = not used
Terminal #5/87 = switched 12V supply to DME, via relay #1.
Terminal #6/85 = relay #1 control "signal," from DME.
Terminal #7/87b = switched 12V power to fuel pump, via relay #2.
Terminal #8/85b = relay #2 control "signal," from DME.
-Ignition turned on, DME grounds 85 via DME terminal #36, completing circuit from 86 (12V from battery) to 85, energizing relay #1, .
-Relay #1 switches 12V power from 30 to both 87 and relay #2.
-87 supplies 12V power to DME terminal #37.
-DME grounds 85b via DME terminal #3, energizing relay #2.
-Relay #2 supplies 12V power to the fuel pump via 87b.
Okay ... the above is a mouthful, and probably only makes sense when you have the wiring diagram in front of you. Still, what you can get out of it:
-The DME relay receives its 12V power directly from the battery.
-The DME relay receives "control" signals from the DME (in the form of grounds which complete a circuit).
-The DME relay supplies 12V power to the DME & fuel pump.
Your symptoms suggest either voltage that is too high or a short/low resistant circuit, which is causing the DME relay to draw too much current.
Since the DME relay receives power directly from the battery, a short/low resistance on the "goes-out" side of the relay could result in a significant current.
I don't believe your problem lies with either 85 or 85b, as these terminals should already have a significant, current-limiting resistance between them and the 12V power supply (i.e., the relay coil).
This leaves 87 (DME 12V power supply) & 87b as potential suspects.
I don't believe your problem lies with the fuel pump (87b), as it's fused.
My best guesses:
-You're simply having bad luck with the relay, and your "hot" relay is normal.
-You have a short/low resistance somewhere between DME relay terminal #87b & the fuel pump, or somewhere between DME relay terminal #87 and the DME.
-For some reason, DME terminal #37 is drawing significant amps (there is no fuse between the DME relay and DME #37).
-Aftermarket equipment has been installed on your car, tapping into the output of the DME relay (either 87 or 87b).
Trying to determine how much current is being supplied/delivered by the DME relay can be done, but it's not exactly simple. Also, I'm not sure you're ready to go there. That is, you may not be completely convinced you have a problem just yet.
If you have a multimeter handy, you can at least check the voltage across your battery with the engine idling. Off the top of my head, it should be something close to 14V.
Well ... that's (more than) enough for now
.