Question about 2001 Ford F350 Super Duty SuperCab

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I have an 2001 F350 with a 7.3L. The check engine

I have an 2001 F350 with a 7.3L. The check engine light is on and the codes read all 4 glow plug problems on the passenger side (cylinders 2, 4, 6, and 8). Could this be the relay or all four glow plugs? How many glow plug relays are there?

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  • Brian Schmidt Apr 27, 2010

    It does start and run normal. A little rough and some smoke at first, but after a few minutes it runs normal. Since the code read for all four glow plugs on one side, I thought it could be the relay. I have changed them before so I am familiar with the proceedure. What are your thoughts, Brian.

  • Brian Schmidt Apr 27, 2010

    I will pull the glow plugs and test. I will let you know. Thanks, Brian.

  • Brian Schmidt Apr 27, 2010

    Your ideas are very appreciated. I will follow your thoughts and let you know the results.

  • Brian Schmidt Apr 27, 2010

    The good point is that I have owned the truck for six years and everythng is just as it was in 2001. I also prefer to fix the problem than to jerry-rig it. I thought it was strange that all plugs on one side went bad. Thats why i thought it might be the relay. But if you say there is only one relay then it can't be that. I will check wiring and connections then the plugs themselves. Thanks, Brian.

  • Brian Schmidt May 05, 2010

    Turns out that a crappy connection at the glow plug relay could cause this problem. Cleaned the connections and the problem has been resolved. Your correct, only one relay. Thanks for your wisdom and expertise. Brian.

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  • Master
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The question I have is does it start? Does it start normally or with a lot of smoke and rough?

It is unlikely, though not impossible, that all the glowplugs are bad.
Some people like to hack their system up but it really isnt necessary. The details below can help you understand the hows and why's of your system...
The glow plug controller (black box that which relay is bolted on top of) is quite the smart little bugger in that it can tell when even one (yes even just one) glow plug is burned out. Now the problem with this is if you have bad wiring it will fool the controller into thinking that a glow plug is burned out.

What you need to do is pull all connections on the relay and clean them up (make 'em so you can just dang near see yourself in them) then reattach them.

How the system works is it measures the voltage drop across the big zig zag resistor if the voltage goes down that means the glow plugs are dropping more voltage across them - this is because we have 12 volts applied to the entire circuit and when you first fire up the system the glow plugs will appear to be a dead short to the system which means they will be dropping very little if any voltage across them so the big zig zag resistor will be dropping the voltage. Then as the glow plugs heat up their ohmic valve increases meaning that they will start sharing the voltage dropping which means since we only have 12 to start with that the big zig zag resistor will start dropping less. Now when the big zig zag resistor voltage gets down to a magical number (we really don't care what it is - just explaining it all out) the controller "knows" that the glow plugs are up to temp, so the controller then shuts off the relay stopping the current flow allowing the glow plugs to cool off some, then the controller turns the circuit back on for a little bit to keep them up to temp (regulates the temp of the glow plugs). It (controller) will keep turning on and off the relay (the clicking you normally hear after the wait to start light turns off) for around 20-30 seconds or so (depends on air temp).

The wait to start lamp will be lit until the controller "sees" that the glow plugs are up to temp.

Now the reason why the connections are so important is that a bad connection will drop voltage across it which will in effect cause the big zig zag resistor to drop less voltage which then the controller views this as the glow plugs must be up to temp when in effect they are not so this causes "short cycling" which leads to either a hard start or no start at all.

Here is the wire lay out from my memory...

Small lug with two red wires on them = Switched power from key switch.

Small lug with one wire wire = leads to controller - this is the wire that the controller connects to ground in order to fire/latch in the relay.

First large lug with two yellow wires = incoming power for the glow plugs.

Second large lug with zig zag resistor and a green wire if I remember correctly - the green wire is the wire that the controller uses for measuring the voltage drop across the resistor.

Third large terminal with two brown wires and another small wire - Brown wires lead to the glow plugs themselves the other small wire is for measuring the voltage drop across the resistor.

Blue wire from controller to weather proof plug = this is the wire that provides a ground for the Wait to start light. The WTS light on the dash is connected to 12v positive and the other side of the lamp is connected to this blue wire.

Now keep in mind that one of the red wires with the green stripe is where the injection pump fuel shut off solenoid gets it's key switch power to tell it to turn on so you must make sure that you keep this connected (most important).

Now if any of the connections concerning the zig zag resistor are dirty or bad then the controller will assume that one of the glow plugs are burned out.

But since you mention that you tried to bypass this earlier with bad results I say check ALL your glow plugs again. You might be surprised to find that one (or more) is burned out.

I like to actually fix the system vs jerry rigging it and getting yourself into a point where nothing matches prints or you let someone else drive the truck and they burn out the glow plugs (at $8-10 each) which can get very expensive in the long run.

It will probably take you less time to actually fix the problem vs just wiring around it then it's easier to fix later on down the road.

Now there are two schools of thought on replacing glow plugs - 1st is replace all them at once and toss the good ones you find. Me I never did it this way because if one is weaker then the rest it will not add additional strain on the others because of the way it's wired (parallel).

The other school of thought which just happens to be the one I like to follow and always did. Only replace the bad ones and enjoy life.
The short of it is you need to check your connections and your plugs.

Posted on Apr 27, 2010

  • 1 more comment 
  • Bryan Fullerton
    Bryan Fullerton Apr 27, 2010

    Id pull them and check them. it should not run rough or smoke on a cold start. If the plugs test good on an ohm meter then id really look at that wiring where they terminate per my comments above.

    I'm trying to remember if that truck has one or two relays. I think it only has one relay which means none of them would work if the relay was bad.



  • Bryan Fullerton
    Bryan Fullerton Apr 27, 2010

    Yes please let me know. I resent not being able to get my hands dirty on many of these projects. lol. I never repaired a vehicle that I didnt learn something.



  • Bryan Fullerton
    Bryan Fullerton May 05, 2010

    Wheee. lol glad you got it fixed. and thanks for letting us know what the actual problem was and how you fixed it. It adds to our experiance when we get it right and wrong if people let us know.
    Thanks :):):)


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