I have change the glow plug relay its brand new bt i didnt change the glow plug will that cause it not to start

Your diesel vehicle is equipped with glow plugs, and also with a Glow Plug Light that will either come on and stay on, or flash or blink when a fault is detected by the ECU (engine management unit). When the Glow Plug Light comes on, the ECU stores information about the condition that has caused it to come on.4 Jan 2016

Is It Safe to Drive With the Glow Plug Light On? ' YourMechanic Advice

https://www.yourmechanic.com/article/is-it-safe-to-drive-with-the-glow-plug-light-on Glow plug light wont go out Google Search

Have you checked the glowplug relay.

P1391 Glow Plug Circuit Low Input (Bank #1)

• DTC P1391 or P1393 will set when the monitor indicates that all four glow plugs are not being powered on that bank. An open in the glow plug circuits, glow plug relay, glow plugs or monitoring circuit could be the possible causes.
  
  Both DTCs P1391 and P1393 together will set when the monitor indicates that all eight glow plugs are not being powered. An open in all glow plug circuits, the glow plug relay, glow plugs, fusible links or all three monitoring circuits could be the possible causes.
• Possible causes:
  • glow plug relay
  • glow plug shunt
  • open in glow plug circuit(s) or monitoring circuit(s)
  • glow plugs
  • PCM

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Sounds like a glowplug problem to me. The most common problem I've had is the relay on the glow plug controller failing. I have had several where one lug actually burned through the plastic case on the relay. The controller is located on top of the engine. The relay I'm referring to looks like a typical ford starter selenod. Does the wait to start light come on when you first turn the ign on? If not you definitley have a controller issue. If it does come on, you may still have a controller issue but it is probably just the relay itself, not the whole controller.

You can test the relay with a test light. With the key off you should have +12v on only one of the big lugs. When the wait to start light is on after you first turn on the key you should have +12V on both of the bug lugs. You should also hear the relay click when you turn on the key. If you don't get +12V on both big lugs you should change the relay. I have found them quite a bit cheaper($30-40) at International truck dealers than anywhere else, and I have not had good luck with anything other than Ford or International name relays(the 7.3 is actually an International engine). You may need to take the old relay with you to get the correct one. If you change the relay but still don't get +12V on both lugs you will probably need to change the whole controller(you can return the relay if you need the whole controller because it will come with a relay already installed).

If you get +12v at both big lugs on the relay and the wait to start light comes on like it should but the engine still doesn't start like it should you mave have some glow plugs burned out. If you look on the intake side of the heads you should see the glow plugs, there will be 4 wired together on each side of the engine. If you can't find them you can trace the wiring harness back from the controller. They should be lined up 4 in a row. To test the glow plugs disconnect the wire from the plug, check continuity with an ohmmeter from the terminal to ground(any where on the steel part of the engine). If the meter gives you an ohm reading the plug is fine. If the meter reads open the plug is bad. I would also buy the glowplugs at either international or ford, and take an old one with you to make sure you get the right one as they made several different plugs for the same engine. (I bought some parts store brand plugs once and they didn't last two days!) Plugs run around $20-25 each, so if you find only one or two bad I would change only the one or two.

One note - take it easy on the ether! If any of the glow plugs are working the ether can cause a hot spot in the cylinder which can burn a hole in a piston! I've seen the results of this and it ain't pretty.
Since you haven't had this happen yet I'm guessing the relay or controller are probably the problem, but you may change one of them and find a couple of bad glow plugs as well. The last time I changed anything on my rig I had to change the controller and all eight plugs(the parts guy said the failed plugs probably caused the controller to fail). It is worth the time to troubleshoot if you can save on a few plugs.

Check for battery to motor ground and battery to body ground for corrosion under mounting bolts. also check motor to body ground strap for corossion and overall condition. Bad ground can cause all sorts of problems. Also check all electrical connectors to glow plugs for corossion.

this problem with g/plugs not operating ? and the operating light not illuminating ? may i suggest you check the fuse and relay normally found in fuse box in engine bay on other vehicles but if you have hand book it will give location ?

Sounds like you have a bad glow plug relay that could cause a power spike and drain your battery,it could also take away power to the fuel pump causing the loss of power.Hope this helps you out some!!

You need to check the glow plug system. Check for power from the glow plug relay.

The relay should cost about $35-45. A common Ford Starter relay can be used in place of the official glow plug relay. All that is required is a relay with terminals that have 5/16 or 3/8 terminals and can carry about 150-200 amps. When all 8 glow plugs turn on, it draws about 150 amps is all. There are two relays for the 7.3 diesel. One energizes the glow plugs and the other energizes a heating element in the air intake. That one is used to help eliminate white smoke while warming up and is turned on and off by the engine computer. It is usually only called for when the temperature is 20 degrees F or lower.
Otherwise only the glow plug relay works.