1986 F250 Glow Plug problem. There are a few things that can cause that problem. One is the relay. The other is a temp switch on the front of the motor. One easy way to tell if the glow plugs are no good without taking them out of the vehicle is with a 12 volt test light.
If GP's ck OK then the glow plug controller and relay are next to check. 1) Look for bad relay connections (loose,burnt,dirty), or bad controller ground wire. 2) Check voltage at large relay terminals. GP side (ribbon resistor) voltage should be the same as hot battery side with key "ON", and relay energized (WTS light is on).
1999 F250 Super Duty, CC, LB 7.3L Powerstroke. Occasionally, the 30Amp fuse that protects the glowplug heating function will blow. Usually just after or during engaging the glowplug heating function. If the 30 Amp fuse is blown, the Wait to Start light will not illuminate when you place the key in the ON position. You can ground the signal wire using the test light and you can ground the "Ground Terminal" to activate the Relay. Keep in mind ONLY to use (-). The GPR is ground switched via the PCM. But, that in and of itself does not prove the relay to be functional under a load. The GP Cycle is dependent upon EOT and AIT. 2012. If You need to change the glow plugs on the Ford Focus then You need to follow these steps. First, You will want to locate the glow plugs. They are on top of the cylinder heads on the engine, and there are multiple of them. They look a lot like spark plugs, so they should be easy enough to find.

Turn on the key and you should have about 11.3-11.8 volts if your relay is activating the glow plugs. If you get no reading, then your relay is not working OR your PCM is not asking your relay to pull down. You have already ruled out the latter issue by detecting voltage at the small terminals so your PCM is fine.

Just remove the small wire from the left side of the GPR and run a new wire from that post to a push button switch and from there to ground. The glow plugs will not heat unless you push that button if you set it up that way. See post #8 from Kwikkordead for detailed instructions.
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    • ford focus c max glow plug relay location