Glow plug relay problem? - Ford Powerstroke Diesel Forum
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Old 10-15-2013, 08:17 AM
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Glow plug relay problem?

So for starters I have a 1997 f-250 7.3l auto tranny, 3 inch lift, 4 in mbrp exhaust and downpipe. In the mornings I have been having rough starts. Not too cold here in Oregon but around freezing in the mornings. This morning it took 4 tries to finally start. Each time i would wait 30 seconds after the WTS light goes off and crank it. Nothing but a lot of white smoke. Finally after the 4th time it started right up. Not sure if it is the relay or not. About a month ago I tested it and with the key off, the hot terminal read around 12.5 when i turn the key on it dropped to around 11.3 i want to say. The other one read around 11v. Does this mean that the batteries are going bad and not getting enough juice to the plugs or is the relay bad? Thanks for any help. Im going to test it tomorrow to get more accurate readings.
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Old 10-15-2013, 08:55 AM
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Originally Posted by powerstroke814 View Post
when i turn the key on it dropped to around 11.3 i want to say. The other one read around 11v. Does this mean that the batteries are going bad and not getting enough juice to the plugs or is the relay bad? Thanks for any help. Im going to test it tomorrow to get more accurate readings.
Yes, get these two measurements accurately. If the second ("not always hot") big terminal is at exactly 11.0V when the always-hot one is at 11.3, you're right at the margin. The _difference_ is the key; anything more than 0.3V means the relay is suspect. So you're right at the threshold, apparently.

You should also measure the resistance of the glow plugs; they should all be right around 1.0 ohms.

Not conclusive, but if you're able to crank the engine that many times, and eventually start it, your batteries are _probably_ good. A load test on each battery, separated from one another, would be more conclusive.
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Old 10-16-2013, 10:00 AM
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So I took the measurements and on the always on terminal i got 12.47v and when i turned the key on I heard the relay click. Took measurement of the other big terminal and it read 0.00v. This should indicate that the relay is bad correct?
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Old 10-16-2013, 10:32 AM
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Did you measure from ground to the post or from post to post?

If from ground, I'd say the relay is bad. If from post to post, you wouldn't see much difference since both posts would be about the same voltage.
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Old 10-16-2013, 11:18 AM
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I went from ground to the post. Grounded from both the alternator and the negative of the battery terminal. The hot one read 12.5 and the other one stayed at zero. Going to replace the relay now. any suggestions on replacement? or threads on it? Thanks
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Old 10-16-2013, 11:23 AM
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The Western Plow relay or the Stancor (now White-Rodgers) 586-902. Here's thread on the Western Plow relay: Western Plow GPR Relay

Cheers!
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Old 10-16-2013, 12:16 PM
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what do you guys think of the napa ones? The echlin brand?
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Old 10-16-2013, 12:47 PM
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Junk. The glow plugs draw over 100 amperes, for 1-2 minutes. You need a relay rated for high current, for continuous "on" duty.
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Old 10-16-2013, 04:47 PM
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Well I need one by tomorrow. So I won't be able to order one I need to be able to go pick it up. Any house brands that are better?
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Old 10-16-2013, 06:35 PM
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If you need one today, just get a NAPA ECH GPR109 (don't tell them what its for or they will sell you a more expensive one). It will work fine temporarily (it mounts 90* from the stock orientation but fits fine), so order up a Western Plow or Stancor. Cheers!
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