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Old 12-19-2005, 07:20 PM   #1 (permalink)
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Glow Plug Relay Test

I've been reading most of the cold start complaints before I chimed in with mine. Which 2 poles on the GPR do you check? I have 12+V going in the battery side(big ones)but nothing on the other. The little poles are about the same. I can leave the key on for up to 2 min. and it still wont fire the first time. My GPR is about 2yrs old and I just replaced the glow plugs last winter. Made no change after that. Most of the time I do plug it in but sometimes I dont have that luxury. Thanks guys. Any help will be appriciated!!

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Old 12-19-2005, 07:24 PM   #2 (permalink)
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Re: Glow Plug Relay Test

its the two big ones, one should always have power and both should get power when key is turned on, if only power at one side when key on then probly bad.
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Old 12-19-2005, 07:31 PM   #3 (permalink)
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Re: Glow Plug Relay Test

Should it have power all the time(while plugs are cycling) or just when the key is turned?
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Old 12-19-2005, 09:10 PM   #4 (permalink)
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Re: Glow Plug Relay Test

reread my original post, i edited to answer
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Old 12-19-2005, 10:00 PM   #5 (permalink)
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Re: Glow Plug Relay Test

You should have battery voltage to both 'Big' posts when you first turn the key to the "Wait to Start" position and for approximately 2 minutes after that when the engine oil is cold...depending on your PCM code also.
The way I usually test the GPR is to measure across the two big posts when the relay is energized. You should see very little difference with a good relay. If you are seeing 12 volts on one side and nothing on the other...than either your relay is bad or your PCM isn't sending a ground signal to energize the relay.
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Old 12-20-2005, 08:42 PM   #6 (permalink)
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Re: Glow Plug Relay Test

[ QUOTE ]
one should always have power and both should get power

[/ QUOTE ]

Yes, and a problem I had was with the key on only one side had power so I thought it was the GPR. Well...there is a plug on my 94 in the power line going to the GPR, its purpose I guess so you dont have to unscrew the wire on the GPR just unplug, I dont know didnt make sense to me anyways it was bad, cut it out and made it a solid power line coming in, problem gone, just my .02.
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Old 12-21-2005, 12:44 AM   #7 (permalink)
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Re: Glow Plug Relay Test

Yes, you need voltage on both large lugs when the plugs are turned on to get voltage to the plugs, but the way the relay is switched on is that one small terminal is at 12v whenever the key is on, the other is at 0 volts when the PCM grounds the relay to turn it on for the first couple minutes during cold starts. The 2 small terminals are an electromagnet in the relay, when 12v is applied across them, the magnet draws the contacts together in the relay shorting the 2 large lugs together internally so 12v power flows to the plugs. When the PCM turns off the relay, ground is released from the second small terminal, so you now have 12v on both small terminals which gives 0 volts across the magnet and the contacts are released leaving you with 12v on only one large terminal. So the main thing to check here is if you really have the same voltage on both small terminals, because if so, no way the relay is going to ever energize.
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Old 03-25-2009, 07:45 PM   #8 (permalink)
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I am having the same problem...I have power to the big wire with the rubber boot on it all the time. When I turn on the key I feel the relay click and the small wire becomes hot. But no other wires are doing nothing. Now I am only checking them with a test light. But still it should have some type of power to the other big wire somehow right? Also should the small wires be hot too? Maybe someone can help me too. I just replaced the GP's yesterday so I know they are good but still hard to start in cold. HELP ME PLEASE!!!!!!!!
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Old 03-25-2009, 08:30 PM   #9 (permalink)
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check out this thread with pics
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Old 03-25-2009, 08:31 PM   #10 (permalink)
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Copy and paste of a post I did some time ago on the glow plug relay.

Quote:
To do a quick test of your glow plug system locate the glow plug relay under the engine cover (plastic hood looking thing on top of the fuel filter, draw a line from center of filter to right headlight and the relay is about 6” from the filter housing under that line). When the truck is having a hard time starting use a jumper cable and jump between the two large terminals for 90 seconds or so. This bypasses the relay - the glowplugs draw a bit of current so be ready for a spark or two. Disconnect the jumper and try to start the truck. If it starts fine then your problem is either the relay or what controls it.

To diagnose the relay further you will need a digital voltmeter. With key off you should have battery voltage to one large terminal and nothing to the other large or either of the small terminals.
With key on for the first two minutes or so you should have battery voltage to both large and one small terminal. The other small terminal should show ground. This shows the relay active.
With the key on and after the glow plug cycle has finished you should have battery voltage to one large and both small terminals. The computer activates the relay by grounding the second small terminal thus activating the relay.

It is possible to have battery voltage to both large terminals when the relay is active and still have a bad relay. If the contacts inside are pitted or corroded you may have enough voltage passing through it to light a test light but not enough to actually run the glow plugs. Generally you will lose voltage across the relay if this has happened. This is why you need a meter (preferably digital so you can test the glow plugs if you need to) rather than a test light to check the relay. With the glow plugs on (first minute or so after turning the key on in cooler weather) and the engine not running check the voltage at both of the large posts on the relay. There should be no more than .3 volts difference (less is better). You also should not have significant voltage loss from the battery + terminal to the always hot terminal on the right inner fender mounted starter relay, and again at the always hot glow plug relay terminal. Check this voltage loss with the key off, glow plug relay not active.
Hope this summarizes things for you. Sounds to me like you have a bad relay if you do not have power to both big lugs when you first turn the key on.
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Old 12-28-2009, 01:32 AM   #11 (permalink)
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I found an abundance of glow plug relays on eBay for $25.90 with free shipping. Takes up to 7 days to receive but the parts store wants around $70. Easy to replace...since they go out every other year you should know how.
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Old 12-28-2009, 09:15 AM   #12 (permalink)
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My 96 7.3 is also having problems starting when it's cold. I can hear my GPR click off after about a minute or two. I have unpluged my wires going to the glow plugs and I have power to them the first and last terminal on each plug, the power goes off when the GPR click off, so I think I have the right wires and the GPR is working OK. I have change the glow plugs a few years ago, could they be bad again?

PS: When I plug my truck in, it starts right up.

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Old 12-28-2009, 07:22 PM   #13 (permalink)
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checked voltage across both large lugs , it drops from 11.5 down to 10.5 volts and small lug has good voltage. is this a bad relay?
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Old 12-28-2009, 08:46 PM   #14 (permalink)
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I would think so. This is for the F450 right? Does it have the shunt on the glow plug side of the relay? Just curious.
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Old 12-28-2009, 10:48 PM   #15 (permalink)
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Quote:
Originally Posted by jspd234 View Post
checked voltage across both large lugs , it drops from 11.5 down to 10.5 volts and small lug has good voltage. is this a bad relay?
If this means you're checking voltage with the key in RUN, and "supply" side big lug is 11.5 and "load" side big lug is 10.5, then YES, your GPR is done. That means voltage drop is a full 1.0V, and the "bedtime" limit (see above) is 0.3V. A verification test is to put the voltmeter between the two lugs (red probe on the supply side, black probe on the load side) with the key in RUN, and it should show the actual voltage drop; in your case, it should read 1.0V.

Quote:
Originally Posted by cobra428 View Post
My 96 7.3 is also having problems starting when it's cold. I can hear my GPR click off after about a minute or two. I have unpluged my wires going to the glow plugs and I have power to them the first and last terminal on each plug, the power goes off when the GPR click off, so I think I have the right wires and the GPR is working OK. I have change the glow plugs a few years ago, could they be bad again?

PS: When I plug my truck in, it starts right up.
Measure the voltage drop as described by jsp. Also, where you unplugged the UVC connectors, you can measure the resistance of each glow plug. On the gasket connectors themselves, connect an ohmmeter between each of the outermost terminals (corresponding to the ones you tested for power, but on the gasket side) and a good ground, one at a time. Each one should read about 1.0 ohms. Much higher or lower means the corresponding GP is suspect. "Open" (infinite resistance) means that GP is shot.

What's the outside temp where you are? (Presuming the truck is outside.) We're getting down to the teens (F) at night, and even with strong batteries and a Stancor GPR, it still helps to plug it in at night. If you put it on a timer, 2-3 hours should do it. The heater draws just about 1kW, so whatever your rate per kWh is on your electric bill, it costs exactly that much per hour to run. Our rate is about 12 cents / kWh, so 3 hours is less than 40 cents.
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