Why wont my glow plugs turn on?

glow plugs dont last for ever
have them visually inspected

they need to heat up to spec or motor wont start

what motor or vehicle ?

It's likely a bad glow plug relay found on the left fender well. You may have bad glow plugs as well. You should hear a faint click when you turn the key to glow psn. . Open the hood and have someone glow it. The click should be abvious and the wires will run from the relay to the glow plugs. also obvious.

I copied this from my GM service manual for 1982 6.2 diesel.

1. If the glow plug relay is not clicking on and off then turn the ignition to the RUN position and connect a 12V test light to ground and touch the pink/black wire in the glow plug relay connector.

2. If the light is off repair the open cicuit in the pink/black wire to the splice.If the light is on then touch the light blue at the glow plug relay connector.

3. If the light is off replace the glow plug relay. If the light is on, disconnect the connection at the thermal controller and touch the black wire in the connector.

4. If the light is off, repair the open circuit in the light blue wire from the thermal controller to the glow plug relay. If the light is on repair open circuit in black wire back to a splice. If no trouble is found replace the controller.



You should also get your hands on this Haynes book. Diesel Engine Repair Manual, part # 10330. Excellent coverage on the 6.2 and 6.5. In it, they say that early 82\'s have a hole at the number 2 hole on the pin connector of the wire harness for the thermal controller. this allows dirt in there and corrodes the pins. Remove the connector and clean thoroughly the pin area on the connection and controller. Seal the hole with some silicone. If things are still acting up, replace the wire harness connector.



Hope some of this helps.

not always the golw plugs can be bad and the engine can be low on compression. does it start alot better if you add a gallon of kerosene to 5 gals of desiel? Or a quaart of gas? the glow plugs can be checked with a test light the connectrs are in plain veiw and the wires going to them are big. Follow the wires back to the relay they go straight there, the relay is real close to the battery as it draws alot of power follow the big wire on the battery that doesn't go to the starter and thats the glow plug relay. they last good but the connections get dirty, reconnect all the connectors on itand see with the key on if the glow plug light comes on on the dash that wire that lights the light is the same wire that lights the light fires the glow plugs. You can take the wire off the glow plug( you need to do it anyway it's a yearly service anyway and with a test light connected to power touch the disconnected glow plug with the lights probe and the light should light, not all of them are likly to be bad but if you find two or more just change them all as the ones that aren't out are likly to fail anytime and if you want to drive though the winter you need all the help you can gt. the battery cables and the one going to the glow plug relay are likly to fail at the battery so go over this good It might start fine with just two glow plugs working I'd pick the two center cyclinders or the two outboard ones as once the engine starts it'll keep running long enough to start the other ones. low voltage as your cranking can effect the glow plugs but the relay is suposed to turn the glow plugs off at cranking and time out after they've been on long enough. dirty connections are almost always the problem so clean everthing before you replace anything. Needs cleaned before yu put on new parts anyway and the glow plugs need to be taken out with the right tool or you may end up with a broken off glow plug in your cyclinder head.

glow plug light and glow plug are not on same system
check at gow plug conection (top of valve cover both side
follow from relay front valve vover right side silver box)
power 12 volts on all 4 wires in both conector left and right
if not replace relay if 12 volts relay ok (test light clip on ground)
then check glow plug (test light clip on positive ) touch wires going
to glow plug should light up if ok (quick test ) if any fail replaced
if all seems ok look at the oil injection system most common
problem with these motors I have yet to replaced a glow plug or relay

The glow plug relay is responsible for the duration of glow plugs, this needs to be the starting point. You should get a 12v out for so many seconds when ignition is switched on.
I have taken the cover off relays in the past, cleaned up the internals, sprayed with circuit cleaner and got plenty more years out of them, worth a try.

Yes you have a defective glow plug relay if you have your owners manual you can find it and replace it, the unit also has a timer on it, if you don't have a book look for the glow plugs and follow the wires from the glow plugs to where they terminate there you will find the relay unit and replace it, to test the glow plugs you remove the wires and test each plug by pulling off the wire and with a cheep ohm meter touch one end to the glow plug and one end to ground if it swings almost al the way over it is ok, if it dos not move then that glow plug is bad. if you find 2 or more bad then replace them all, and if a glow plug has shorted and caused this problem you will have to replace the glow plug relay. Good luck.

if you need connectors and terminals for the glow plug connections under the valve cover, Ford sells then so you don't need a whole $600 harness.

info for your use

Section 03-07B: Glow Plug System 1996 F-250, F-350, F-Super Duty Equipped with 7.3L Diesel Engines Workshop Manual
DESCRIPTION AND OPERATION Glow Plug System The 7.3L diesel engine uses a glow plug system which preheats air in the combustion chamber to improve cold engine starting. The system consists of eight glow plugs, the glow plug control module (12B533), powertrain control module (PCM) (12A650), engine oil temperature (EOT) sensor and barometric pressure sensor (BARO sensor) (12A644). The glow plug system is electronically controlled by the powertrain control module. The powertrain control module energizes the glow plugs immediately after the key is placed in the ON position and then determines how long the glow plugs will be on according to the engine oil temperature and the barometric pressure. The required time for the glow plugs to be energized decreases as the engine oil temperature and barometric pressure increase. The glow plugs are self-regulating, and the powertrain control module controls relay ON time. The powertrain control module protects the glow plugs by energizing them for short durations if the battery voltage is abnormally high.

Possible Causes: Insufficient glow plug ON time will not allow enough heat to accumulate in the combustion chamber to easily facilitate starting. If the glow plug system ON time does not meet any of the specifications in the accompanying chart the problem is most likely a faulty wire harness connection, ground connections or glow plug relay.

If the glow plug resistance to ground is high, the most likely causes are an open UVC (under valve cover) harness or open glow plug.
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Relay Operation
Install a DVOM on the glow plug feed side of the glow plug relay (large stud with two wires connected). Turn the ignition key to the ON position but do not attempt to start. Note the time in seconds from when the key is turned on and the glow plug relay energizes until the glow plug relay de-energizes. The glow plug relay makes a loud click noise which is easily heard when it energizes and de-energizes. The dome light will dim and the dash voltmeter will dip when the glow plugs are drawing current from the battery. Compare the times measured to the table (time will be affected by engine temperature, battery condition and vehicle altitude). The voltage at the glow plug feed terminal may vary from 9 to 12 volts depending upon battery condition.
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If battery voltage not present check for B+ at power supply terminal (terminal with single large wire). Power for glow plug power supply is supplied from starter relay through two fusible links at solenoid.
Glow Plug Operation
Disconnect all of the glow plug/injector harness connectors from the valve cover gaskets. With the pigtail installed, measure glow plug resistance to ground (preferably B-). A resistance measurement of .1 to 6 ohms indicates a good glow plug.



Glow Plug Harness Continuity
Measure for continuity from the connector harness to the glow plug feed terminal on the glow plug relay. Resistance should be less than 5 ohms.
Incorrect measurements will result if all glow plug/injector connectors to valve cover are not disconnected.

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Glow Plug Relay The glow plug relay (12A343) has a plastic base and two mounting bolts. It is operated by the powertrain control module (PCM) (12A650) which senses engine oil temperature, PCM voltage and barometric pressure. When the engine oil temperature or the barometric pressure is low, the PCM activates the glow plug relay and the glow plugs. If the PCM voltage is too high, the powertrain control module deactivates the glow plug relay and the glow plugs immediately energize for a shorter duration, decreasing the modulation of the duty cycle.
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Glow Plug Removal

1. WARNING: THE RED-STRIPED WIRING HARNESS CARRIES 115V DC. SEVERE ELECTRICAL SHOCK MAY BE RECEIVED. DO NOT PIERCE.
   Remove valve cover (6582). Refer to Section 03-01D .
   

1. Disconnect glow plug electrical connector using a pair of pliers.
   

1. Remove glow plug using 10mm socket.
   

1. Clean and inspect glow plug tip for damage. Replace if damaged.
   

Installation

1. Install glow plug and tighten to 19 Nm (14 lb-ft).
   

1. Connect glow plug electrical connector. Make sure glow plug wiring is routed to avoid moving parts.
   

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Glow Plug Control Glow plugs are used to warm the cylinders to improve cold-engine starting. The PCM uses the engine oil temperature (EOT) sensor and barometric pressure (BARO) sensor to determine how long the glow plugs will be on. The PCM energizes the glow plugs longer if the engine is very cold or if the barometric pressure is low at high altitudes.
The PCM controls the relay that supplies power to the glow plugs with the glow plug control (GPC) signal. When the GPC signal is grounded by the PCM the relay closes, turning the glow plugs on.
The glow plugs are controlled by the following sequence:

1. PCM energizes the glow plugs.

1. The PCM energizes the glow plugs immediately after the ignition is placed in the ON position.

1. The PCM then determines how long glow plugs will be on based on EOT sensor and the BARO sensor. The required time to warm up the cylinder decreases the engine oil temperature and barometric pressure increases at low altitudes.

Glow Plug Lamp Control The glow plug lamp (GPL) signal controls the WAIT TO START light located on the instrument panel. This light is used to indicate when to start the engine. The cylinders are being warmed by the glow plugs during and after this light is lit. The PCM energizes the glow plug lamp longer if the engine is very cold or if the barometric pressure is low.
When the GPL signal is brought low, the "Wait to Start" lamp is turned on.
The glow plug lamp is controlled by the following sequence:

1. PCM lights the WAIT TO START light after a key on reset occurs.

The PCM determines the glow plug lamp activation time based on engine oil temperature, barometric pressure and battery voltage.

1. PCM turns off the WAIT TO START light.

The glow plug lamp is turned off when the timer counts to the number of seconds specified by the PCM. The glow plugs will normally remain on longer than the "Wait to Start" lamp.
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Sounds like a faulty cold start assistor. On top of the engine you will find an aluminium housing with four things that look like glow plugs screwed into it. These 'glow plugs' are only available complete with the housing fron Vauxhall. Approx £70 +
Airbag light is probably a fault with the seatbelt pretensioner.
Check the wiring under the seats for damage or perhaps the plugs have pulled apart.

replace the relay but check connections first as often connection get hot and corroded.failing which buy a push button switch use a relay and and some heavy wire to and from relay to the heaters and work them manually.you could try seperating the relay and cleaning the connections inside.