2001 7.3L F-350 crew w/147K
I've tested the glow plug relay by testing voltage across terminals after turning ignition on.
Initially I get 10.7V and it gradually climbs to just over 11V. Then after about 30 seconds, drops to zero. The truck was kind of warm at the time of the test. (Sat for 3 hours). Voltage at the batteries is 12.55V. Also at "left" GP terminal.
So is the GPR not right? How long should current go through and how much voltage?
Problem has been hard or no COLD starts and was about to replace glow plugs/valve cover gaskets.
So . . . the glow plugs are not getting 12V.

 

With the key off, the outer terminal should read the same as the batteries. When the key is turned on, the relay should stay energized for up to 2 minutes, depending on temperatures. When you turn the key on, check the voltage at both large terminals of the relay. They should be pretty close to each other. If there's a sizable difference, the relay is on its way out.

If you want to check the resistance of the each glow plug, you can do so at the connectors on the valve cover gaskets. The front and back two pins are for the glow plugs, and should show somewhere in the neighborhood of 0.6 to 2.1 ohms from the pin to the ground. A little higher is OK, but open (infinite) means it isn't working at all.

 

The difference is 12.55V in and 10.7 - 11.04V out. And staying on for only 30 seconds.
Checking resistance is next. If needed.

 

Here's the no ****ter. If you have not replaced glow plugs in a few years, read up on it. It's an easy job, just takes a few hours. Change your relay as well. Do the 50 cent mod while your there and check the uvch's (inspect them carefully). I was fighting with mine for a year and took the time to do all of the above. Mine starts at -20 without being plugged in.

Long and the short, if your trying to start that rock longer than you should, this is your answer to saving your starter and batteries as well as it being a great time for you to learn and love your truck.

I hated mine before work in the morning, (kinda like looking at your wife in divorce court every morning). Now it's like walking into slippers and prime rib at 5am in Northern Michigan.

170 bucks and your effort. BUT READ UP!!!

Hope it hits home, it'll change your whole perspective on what reliable really means.

Denny

 

So do you think 11V is enough to light up the glow plugs?

 

Your question is hard to answer for this reason. If your batteries are not in good shape, the 11 volts you are measuring could mean that there are some glow plugs that are working and some not working. The battery voltage by itself is only part of the puzzle. As mentioned above, using a multimeter in the ohms position to measure the resistance of the plugs tells you if they are all operational. If they measure ok, then yes, 11 volts is enough to heat the plugs.

Also, if you are measuring 12.55 volts at the input and 11 volts at the output of the glow plug relay, then the glow plug relay is faulty and needs replacement. Voltage drop across the glow plug relay means that the glow plugs will be less efficient. They will get less electrical power from the relay due to voltage drop.

 

Don't know a lot about Diesels but 12.55 is pretty weak for a maintenance free battery. Maybe 65% charged, depending on temperature, maybe 75%, etc.

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A fully charged lead-acid battery is about 2.1 volts per cell, so 12.6 volts.

 

Make sure you take both readings when the glow relay is energized.

 

A maintenance free should be 12.8 at 80F - a tenth of a volt here and there doesn't sound like much, but on a percentage basis it's huge. It might be indicating 12.5 without the surface charge removed, too, so the actual open circuit voltage may be even less. I didn't believe it either till I started looking at the charts.

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12.5 at the terminal is good (GPR off). 11V at the terminal is good (GPR on). The GPs draw heavy current and suck the batteries down.

Try this: Attach the red meter lead on the battery side of the GPR, and the black lead on the Glow Plug side of the GPR. You will see 12.5 volts (or whatever your battery charge is). Turn on the key, and the voltage should drop to Zero or a tiny voltage. If you get a real voltage reading across the GPR, the contacts are fried and it's time for a new one.