It’s a cold winter day, and your Power Stroke won’t start. What do you do?

1. As we’ve stated before, the number one failure that will keep you from starting your truck is the glow plug relay (GPR). The first thing you should do is get your spare glow plug relay out and replace the one in the truck. You do have a spare glow plug relay, right? Chances are, this will get you back on the road.

The easiest thing to do is to just replace the GPR and see if it fixes the problem. However, if you want to test the operation of your current glow plug relay, here’s how:

The glow plug relay is located to the left (passenger side) and slightly behind the fuel filter, on the passenger side valve cover. It’s a lot easier to get to if you remove the engine appearance cover. 1999.5 and up truck have two relays in this location, on for the glow plugs and one for the intake air heater. The Ford workshop manuals indicate that the glow plug relay is the one toward the front of the truck, however some people have reported that their relays are reversed. The easiest way to find out is to follow the wire from the intake air heater to the relay. Obviously that relay controls the heater, so the other relay controls the glow plugs.

On 97 and up models, the GPR has four posts on the top with the wires connected with ring terminals. The two small posts are the relay coil, which controls the operation of the relay. One post is connected to the battery and is connected to the PCM.

The post connected to the battery is always hot; the PCM grounds the other post to turn on the relay. The two large posts are the switching part of the relay. The large red wire leads to the battery, and is always hot. The other post has two wires on it, one for each bank of glow plugs. When the relay is energized, the two large posts will be switched together inside the relay and current will flow from the battery to the glow plugs.

On the 95 and 96, the relay has two posts on the top and a connector on the side. The connector on the side contains the control wires for the coil (like the two small posts on the 97-up), and the two posts are equivalent to the two large posts on the 97-up.

First, make sure that the relay is receiving power from the battery. The large red wire leading to the large post on glow plug relay should have battery voltage present (#1 on picture). The end of the wire and the nut on the relay are covered with an insulating sleeve you’ll have to pull back. Measure the voltage between that post on the relay and the battery ground. You should be able to measure battery voltage. If you can not, you will need to repair the open circuit between the relay and the battery.

Next, check to see if the relay is turning on. The opposite terminal from the battery feed goes to the glow plugs (the other large post, with two wires connected to it, #4 on picture). Set up the volt meter to measure between this post on the relay and the battery ground, and then turn the key to the run position. The relay should turn on and you should be able to measure between 9 and 12 volts for somewhere between 10 and 120 seconds. The exact time varies; see the previous article for more information. If you want to get some extra time to make the measurement, you can disconnect the engine oil temperature (EOT) sensor (picture). With the EOT sensor disconnected, the PCM will turn the relay on for the full 120 seconds. If you cannot measure battery voltage on this terminal when the relay is on, the relay is bad.

Occasionally, a GPR will fail in such a way that you will be able to measure voltage on the terminals when the relay is on, but there is not enough current flowing through the relay to turn the glow plugs on. A more definitive test is to use an inductive current meter to measure the current flow through the large red wire coming from the battery to the GPR. When the GPR first turns on, the current will spike extremely high (over 300 amps) and then drop down to about 125 amps after several seconds. If there is no current flowing, or less than 100 amps, the relay is bad.

2. If there is no white smoke coming out of the exhaust while you are cranking, or you cannot smell raw diesel outside the vehicle after cranking it over, then the injectors are probably not firing. As your batteries age, and especially when they are cold, they may still have enough power to turn the truck over, but not supply enough voltage to fire the injectors. Hook up to a battery charger or another vehicle with jumper cables and try starting the vehicle again. If it starts, you may need to replace your batteries or alternator. If your truck is more than 3 years old, your batteries are most likely to be the culprit. Most auto parts stores will test your batteries and alternator for free.

3. If you have the time to wait, plug in your block heater for several hours and then try to start the truck again. If it starts, you may either have a problem with the glow plugs, glow plug wiring, or possibly fuel gelling.

To test the glow plugs and wiring harness:

Unplug the valve cover wiring harness. The outer two pins on each side of the connector go to the glow plugs. On each pin, measure the resistance between the pin and a good ground. The value for a good glow plug will be between 0.1 and 2.0 ohms. If it is greater than that, the glow plug is bad. See a picture
If the glow plugs check out, then you could have a problem with the wiring between the valve cover connector and the glow plug relay. For each glow plug connection in the connector, measure the resistance between the connector and the post on the glow plug relay with two wires on it. The resistance should be between 0 and 2 ohms. If it is greater than that, the wiring is bad.
The final piece of wiring to check is under the valve covers. Measure the resistance between the valve cover connector and the glow plug. The resistance should be between 0 and 2 ohms. If it is greater than 2, then the wiring is bad.
4. If you don’t have time to wait for the block heater, you can try starting the truck with a high powered hair dryer or heat gun blowing into the intake. If it starts, you may have a problem with the glow plugs or glow plug wiring. See #2 for how to test these items.

5. If the engine still won’t start, or starts but runs poorly, won’t run above idle, or dies after a short while, you may have gelled fuel. If you have any winter additives handy, pour some directly into the fuel filer bowl in the V of the engine, and add at least the normal amount to the tank. Some additive makers have a special additive made expressly for this purposes; others recommend a concentrated dose (2 – 3 times) of the normal additive. If you don’t have/can’t get any additives, or if they don’t do enough, you will have to warm the truck up somehow. If possible, move the truck inside a garage or shop. Plug in the block heater, put a space heater nearby, do whatever you can think of to warm up the fuel tank, fuel lines, fuel filter, and engine. Please exercise caution; diesel fuel and diesel fuel vapors are very combustible, and you don’t need to add fire damage to your problems.

I highly recommend against using any type of starting agent such as ether. When the glow plugs/intake air heater come on, they will ignite the starting agent in the cylinders and intake, which could cause extreme damage to your engine. Even if they have been disabled, the starting agents have much higher volatility than diesel fuel, which means that starting the engine using them will cause very high cylinder pressures. While it is unlikely that this will cause an immediate failure (though it is possible), it is definitely not good for your engine’s long term health.

If you still can’t get your truck running, it may be a common problem such as the cam position sensor, or some other failure. If possible, hook up an OBD-II code reader to see of there are any codes set that give you any clues, and work from there.

Good Luck!