I was not getting 8 sec. out of my glow plug light on a cold starts. More like 5-6 sec. and it seems to be taking to long to start when cold. I ended up replacing the glow plugs with some new motorcrafts and I'm getting the same issue. I was told that this may be the glow plug controller unit needing to be replaced. Does this sound right? Any help would be great.

 

What truck do you have?
There are two glow plug systems that are way different.

 

on my 89 7.3 (it is a different setup on the 6.9, so I don't know if this could happen) the connections to the glow plugs were corroded. The system uses electrical resistance through the plugs to know how long they should stay on, as resistance changes with temperature. I am pretty sure that is how the system works anyways. I thought my plugs were going bad, as it was starting hard. One wire had broken off in the back, and I checked all the plugs with a testlight. (to test glow plugs, just pull the wire off, and run a test light from the positive terminal on the battery to the top of the glow plug. If the light turns on, you're good to go) When I put the wires back on, the wait to start light only stayed on for a second or two at best. Turned out to be bad connections to the plugs. i replaced the connectors on the end of the wires, it was easier to replace them than to clean them, and the system has been trouble free ever since. Also make sure nothing is touching the controller, I had my controller wear through the rubber on my accelerator cable, and it shorted out, causing a similar problem. I would say check your connections, and on one of our older trucks, it probably wouldn't hurt to change them anyway, it takes like 15 minutes and costs 2.00 for the connections.

 

this is on a 1990 7.3 IDI

 

Check out the connections like I said, because dirty or corroded connections will cause the light to not stay on as long as it should. Could be the controller, but I think that part runs in the 190 dollar range, so might as well try to the 2 dollar solution first. Make sure all your terminals on the controller are clean as well. The controller sits right behind the intake if you take off the air filter assembly you can get to it.