The GP light has been acting up lately. When I turn the key to the 'run' position for the pre-glow, the light does not come on, so I figure there are bad plugs. I'll try to glow again with the same result. Then, if I try to start it, it starts up, but runs roughly for a minute. It is the classic case of bad glow plugs. But it gets interesting. The light will come on if I crank the engine once with the starter (but not start the engine), then turn the key to the 'run' position again. After a few seconds it goes off and the car will start right up. Other times the light will come on the first time I turn the key to the 'run' position and the car will start up. All the GPs have .4 Ohms resistance, the battery is fine, and the fuse on the relay is OK.

Why are the plugs glowing randomly? Works sometimes, not others.

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Andrew

1989 Volvo 745- 202K

You're confusing the illumination of the lamp with the operation of the glow plugs. In reality, those are two discrete functions and they have no relationship to each other.

When the lamp doesn't illuminate in any discernable fashion, one or more than one glow plug is open. Perform a test at the plug to the relay. Remove the plug and measure resistance on the five sockets. Anything that's open indicates a bad glow plug.

The light coming on in the car always coincides with an easy start. When the light does not come on in the car, the car starts hard. When the light does come on, it starts right up, so I'm thinking that they are related. I'll check at the relay too.

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Andrew

1989 Volvo 745- 202K

Yes, you are correct.

However, there are probably three or four plugs that are functioning properly although the light won't illuminate whatsoever.

Furthermore, when the system functions properly, the light may extinguish after 10 seconds, but the glow plugs remain on for about 35 seconds.

While they are remotely related, the function of the light is "somewhat" independent of the plugs.

Ah- I get it now. I'll check it out in the morning.

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Andrew

1989 Volvo 745- 202K