Wait To Start & glow plugs on during cranking
 

I have a question that I thought I knew the answer to, but my truck's behavior is making me wonder...

Does a '97 PSD shut off its glow plugs when the Wait to Start light goes out, or when the key moves to the cranking position?

Reason I ask is that my truck as recently started getting miserable about cold starts. Since it was new it has always jumped to life even in cold Michigan winters and has never been plugged in. But now it pretty much won't start at all, with one exception...if I cycle the key and let the Wait light come on and go off, then cycle the key again and start cranking right away while the Wait is still on. I don't understand why, but it won't start if the light goes off and it will start if I try while the light is on.

Also, I have always used my battery gauge to determine when the glow plugs were on or off, but now I'm wondering if this was in error also because the gauge always drops to around 10v both when the Wait light is on and after it goes off.

Background, when this problem started a month ago I figured glow plugs were due to be replaced, so I installed new Motorcraft parts. After they were out I tested the old ones and they get hot but are not burnt or broken. I know the relay is pulling in, but I haven't voltage tested the relay yet. I have connected a 200amp starter/charger when playing with it to try and rule out a weak battery. Even if I let the glow plugs burn a full 30 seconds and the charger has the starter spinning full speed, it still won't start or even try to fire. But when I cranked it once while the Wait light was still on it fired up, and that's worked twice since. I'm glad to get it started, but I am feeling like there is still something going wrong.

I would appreciate any insight from those more experienced with this engine than I am. Thanks.

The light is a dummy light... It has little to nothing to do with the actual "on time" of the glow plugs... If you go out to your truck, turn the key to the "on" position and leave it there, the glow plugs will be on for upwards of 90 seconds or so, regardless of the light being on or off, which it will be off after 5-10 seconds...

If you want to see this for yourself, there are a few ways to do it... Go out to your truck, turn the key to the on position, watch the ammeter. You will see it drop down a bit. That is the glow plugs turning on. Sit and wait... When the ammeter jumps back up after about 60-90 seconds, your glow plugs are shut off.

Another way you could see this is by using a mulitmeter on the relay. Just hold the meter on the large lug that sends power to the plugs. Turn the key to on and it should have power, sit and wait til it goes off. That's the length of time that your glow plugs are on during a cold start.

So the answer to your question is neither. They are independent of the WTS light and the key being turned to the "run" position. They are timed independently of that and will even continue to be on after you turn the key to the run position in some instances...

Good link here to help with your glow plug questions...

https://www.dieselbombers.com/ford-p...tml#post735911

https://www.dieselbombers.com/ford-p...tml#post537967

Thanks for the helpful advice.

Just as follow-up for those who are reading, I bridged the glow plug relay last night and it started right up so for now I'm assuming that is the problem and ordered a new relay.

Some of my confusion came from the fact that the old relay actually was pulling in, and glow plugs were drawing current, but apparently not enough to really heat up. If nothing else I'm pleased to have learned that if it happens again I can get back running with an old screwdriver.

Go down to NAPA and pick up a GPR-109. Do not ask for a glow plug relay for a powerstroke as they will sell you a $75 one that you dont need.

GPR109 is about $30 bucks depending on the napa.

i may have got a dud, but i got that napa relay and one from fishers and within 2 mths, both didnt work. a buddy told me to get one directly from ford. so far so good after about a year going.