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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Re: I have an 2001 F350 with a 7.3L. The check engine
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
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
First large lug with two yellow wires = incoming power for the glow
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
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.
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After about every 100Kms one or more glow plugs burn out. Think of them like toaster elements which ignite the fuel on start. The recommended solution is to replace all 4 at once as the labor for changing one is the same as for changing all 4. If you change just 1 (and you would have to be able to figure out which one it is - not easy) the other 3 will follow (failing) in a few weeks afterward.
HI. This is an indication of a failing glow plug. It will be necessary to test each plug to isolate the issue, or run a diagnostic scan on the CPU. During the scan, you should be issued a code that will isolate the actual glow plug that is malfunctioning. If you insist on testing each plug, use my procedure below to assist. You can find the resistance specs in your operators manual.
A test light can be used to determine if the glow plugs are working.
1. With the test-light grounded, insert the test light probe into a glow-plug wire to check for continuity.
2. If the light comes on, check the other cylinders in the same manner. If the test light failed to light, check for continuity at the glow-plug relay.
3. If no power is present, check the control circuit of the glow plug relay.
4. If the control circuit is working, the relay may be defective (FYI, manufacturers may have the system fused and fusible links may be located throughout the system).
5. If the glow-plug circuit is working, the glow plugs themselves may be defective.
Checking glow-plug resistance:(Remove the plug for this test)
1. With an ohmmeter, connect one lead to the terminal and the other lead to the body.
2. Read the resistance and compare to specifications.
3. A visual inspection may also be necessary to check for excessive carbon buildup and physical damage to the tip.
4. A glow plug that has excessive carbon deposits on its tip can give the correct ohmmeter reading but still not work. Clean the tip with carbon cleaner.(DO NOT USE A BRUSH TIP CLEANER)
How did you determine there is no power getting to them? If the glow plug light comes on and then turns off immediately, that is the glow plug controller telling you the glows are burnt out.
Test the glow plugs by using a multimeter with an ohm tester. Take the engine cover off and pop the glow plug wires off the glow plugs. They should pop off like a spark plug wire. Find a suitable ground and place the positive lead on the glow plug. It should read approx 1.0 ohm resistance give or take 0.5 ohm. If you find that you have one or more plugs with either high resistance, no resistance, or a large variation between the 4 plugs, then you have likely diagnosed the problem and the plugs need to be replaced. If they are all the same or within .1 or .2 ohms, then the plugs are not the problem.
Beyond that, make sure you have done the obvious and check the fuses. The relay may need to be replaced as well. Good luck.
Your priblem sounds more like the glow plug timer relay than the
exhaust. White smoke after start up is a sign of diesel entering the
exhaust before it has burned completly. This is as a result of the glow
plugs switching off too early, which is usually caused by one or more
faulty glow plugs or a faulty timer.With engine COLD, IGNITION ON...
Check voltage with all 4 plugs connected. You should have a reading of
If there is no power, check in-line fuse on loom coming to plugs.
If power is present, check for How Long. If only for a few seconds, the
problem is with the timer. This unit (the larger of the relays and
usually black in colour) is fitted on the fuse board, and there
are a few different types. They come in 7 pin..8 pin..and even 9 pin
The glow plug light is often NOT accurate (even with a new timer unit
fitted) as to the time of heating, and you will almost certainly find
that power will flow to the plugs for quiet some time after the light
goes out. This is not unusuall with The Jetta Tdi.
I hope this helps you sort out this problem