Glow plug relay testing
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Laughing Gas
Joined: Apr 2016
Posts: 850
Likes: 114
From: Virginia
Glow plug relay testing
Hey guys, as with most of the country, it has been brutally cold here in Virginia. Never got below 0, but we did have some nights where it got down around 5-10 degrees. Highs were around 25 at the most. I left my truck unplugged for about 2 days in a row of this weather, went to start it, and it cranked and cranked and cranked, but never started.
So I do a little research and decide to test out the glow plug relay. With key off, the "big" post on the left that always has power going to it read 12.5 volts. The "big" post on the right that gets no power until the key is on read 0 volts. Turn the key on, and the post with constant power reads 12.1 volts. The post on the right (that sends power to the glow plugs) fluctuated quite a bit anywhere from 7-10 volts. 10.8 volts was the highest I saw. I did hear a distinct click from the relay when my buddy turned the key on. From what I've researched, I believe I am in need of a new relay...would y'all agree from what I've stated?
So I do a little research and decide to test out the glow plug relay. With key off, the "big" post on the left that always has power going to it read 12.5 volts. The "big" post on the right that gets no power until the key is on read 0 volts. Turn the key on, and the post with constant power reads 12.1 volts. The post on the right (that sends power to the glow plugs) fluctuated quite a bit anywhere from 7-10 volts. 10.8 volts was the highest I saw. I did hear a distinct click from the relay when my buddy turned the key on. From what I've researched, I believe I am in need of a new relay...would y'all agree from what I've stated?
Postmaster
Joined: Nov 2015
Posts: 4,807
Likes: 125
From: Colorado Springs
The 7-10V was with the truck OFF?
If you need a new relay look into the Stancor, it's a beast! On my E99, I was only able to use one mounting bolt but it's held perfectly.
Monster White-Rodgers Glow Plug Relay
If you need a new relay look into the Stancor, it's a beast! On my E99, I was only able to use one mounting bolt but it's held perfectly.
Monster White-Rodgers Glow Plug Relay
Tuned
Joined: Oct 2006
Posts: 286
Likes: 0
From: Northern Virginia
Yes sounds like a good diagnosis. If you need to get the vehicle going, you can try jumping across the two large posts with either a big screw driver or a good jumper cable. Be aware, you usually get a pretty good arc when you connect it. Allow ample time for the glow plugs to heat up. No more than two minutes. Then try to start it.
If your not in a pinch, I would just replace the relay and go from there.
If your not in a pinch, I would just replace the relay and go from there.
Thread Starter
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Laughing Gas
Joined: Apr 2016
Posts: 850
Likes: 114
From: Virginia
The 7-10 volts was with the key turned on. Also, I only got 7-10 volts for roughly 15 seconds before the meter dropped to 0. From what I've read, I should be getting around 12 volts on the post that sends power to the glow plugs for about a minute or two depending how cold it is right? If I ever do get in a pinch, I will try crossing it with a screwdriver. However, as long as I plug it in overnight, it starts up pretty good. Thanks for the help guys, and I will be looking into that stancor relay for sure.
Tuned
Joined: Mar 2016
Posts: 354
Likes: 0
If you are getting less than 10 volts out of the relay with 12 volts in then yes you need a new relay. Had the truck been plugged in when you only got power to the glow plugs for 15 seconds? If it had then your PCM is sensing that the engine is warm and you don't need the full time of glow plug activation.
Cargo Master
Joined: Nov 2005
Posts: 2,766
Likes: 56
From: East Tawas, MI
Just beware...........
If your relay is weak, it may be doing a lot of duty and retiring.
How old are your glow plugs?
Living in the tropics (i'm northern Michigan) , I tend to see very aged glow plugs south of the line. A bad GPR could be the result of a few other things you don't generally use a lot in the south. Could be UVCH, GP's or it could just be the relay.
Best advise is to start with the stancor. If your plugs are over 5 years, I would look that way and use the correct GP's. Million threads in here on that.
Denny
If your relay is weak, it may be doing a lot of duty and retiring.
How old are your glow plugs?
Living in the tropics (i'm northern Michigan) , I tend to see very aged glow plugs south of the line. A bad GPR could be the result of a few other things you don't generally use a lot in the south. Could be UVCH, GP's or it could just be the relay.
Best advise is to start with the stancor. If your plugs are over 5 years, I would look that way and use the correct GP's. Million threads in here on that.
Denny
Lead Driver
Joined: Sep 2005
Posts: 9,399
Likes: 188
From: Norco,CA
I went threw this years back. The click you here is showing you it is bad. It cuts out way to early. Change it out. I thought that click meant it was good till I started thinking things out.
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Logistics Pro
Joined: Oct 2011
Posts: 3,953
Likes: 120
From: Mcdonough, GA
I'd say you have diagnosed your problem perfectly.
The distinct click you hear at "key on" is the large contacts of the relay closing. When the contacts are clean it's metal to metal contact, electricity should flow right through and the voltage on both posts should be the same.
Problem is that over time the contacts get pitted, corroded and damaged and let less current / voltage through. Attached is a picture of the insides of the last relay I replaced. When the relay is energized the large round washer is raised up to come in contact with the copper colored posts in the top of the housing which are attached to the large posts we are familiar with. As you can see at the end this one was not transferring electricity very efficiently.
The distinct click you hear at "key on" is the large contacts of the relay closing. When the contacts are clean it's metal to metal contact, electricity should flow right through and the voltage on both posts should be the same.
Problem is that over time the contacts get pitted, corroded and damaged and let less current / voltage through. Attached is a picture of the insides of the last relay I replaced. When the relay is energized the large round washer is raised up to come in contact with the copper colored posts in the top of the housing which are attached to the large posts we are familiar with. As you can see at the end this one was not transferring electricity very efficiently.
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Posting Guru
Joined: Jul 2017
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The hot post broke off of my GPR when I was checking a wrench for size. Knew was bad as only about 7V on the switched post.
I probably also have GP issues as I have 5 ohms to ground from the switched post, but the truck now starts at 4 degrees, so not going to fix that yet.
I replaced my relay with the GPR-109 from Napa; the low price combined with how easy it is to change made it difficult to go for the Stancor part.
I probably also have GP issues as I have 5 ohms to ground from the switched post, but the truck now starts at 4 degrees, so not going to fix that yet.
I replaced my relay with the GPR-109 from Napa; the low price combined with how easy it is to change made it difficult to go for the Stancor part.
Thread Starter
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Laughing Gas
Joined: Apr 2016
Posts: 850
Likes: 114
From: Virginia
Ok, slight update. I just went out today to test it again after it sat overnight. Low was about 20, high today is about 45. Anyways, first I checked the batteries, each battery read 12.2 volts (little low). With key off, post with power to it at all times read 12.2. Post that supplies power to the glow plugs read 0 with key off. With key on, post that’s hot at all times read 11.8 volts. The post that supplies power to glow plugs dropped to 10.7-10.8. The relay did stay on for about a minute this time until I heard it click off. Tried to start the truck, and it didn’t fire, just cranked and cranked. Going to Napa today to pick up a relay due possibly to me needing the truck tomorrow. If the relay doesn’t help, I’ll be ohming out the glow plugs.
FTE Leadership Emeritus
Joined: May 2014
Posts: 27,344
Likes: 5,941
From: Lake Hartwell, GA
Ok, slight update. I just went out today to test it again after it sat overnight. Low was about 20, high today is about 45. Anyways, first I checked the batteries, each battery read 12.2 volts (little low). With key off, post with power to it at all times read 12.2. Post that supplies power to the glow plugs read 0 with key off. With key on, post that’s hot at all times read 11.8 volts. The post that supplies power to glow plugs dropped to 10.7-10.8. The relay did stay on for about a minute this time until I heard it click off. Tried to start the truck, and it didn’t fire, just cranked and cranked. Going to Napa today to pick up a relay due possibly to me needing the truck tomorrow. If the relay doesn’t help, I’ll be ohming out the glow plugs.
Postmaster
Joined: Nov 2015
Posts: 4,807
Likes: 125
From: Colorado Springs
Ok, slight update. I just went out today to test it again after it sat overnight. Low was about 20, high today is about 45. Anyways, first I checked the batteries, each battery read 12.2 volts (little low). With key off, post with power to it at all times read 12.2. Post that supplies power to the glow plugs read 0 with key off. With key on, post that’s hot at all times read 11.8 volts. The post that supplies power to glow plugs dropped to 10.7-10.8. The relay did stay on for about a minute this time until I heard it click off. Tried to start the truck, and it didn’t fire, just cranked and cranked. Going to Napa today to pick up a relay due possibly to me needing the truck tomorrow. If the relay doesn’t help, I’ll be ohming out the glow plugs.
Did you isolate the batteries when you tested them? Keeping the hooked together will defeat testing them. I would get them individually load tested...so you know what you're dealing with.
Cargo Master
Joined: Oct 2007
Posts: 2,712
Likes: 107
From: BoCoMo
GPR may have some wear on it.... but.... I would look somewhere else right now.
20 degrees .... how does that song go....? "Baby it's cold outside."....
Huuuge drain on your bats...
How old are they?
Can you check the Cells (bubble tester)?
Do you have a Load Tester.... (around 25.00 on sale at Haber Freight)?
imo... don't care if it does meter out at 12.2 starting out. It can go 'Down' fast on a load... (and then jump back up with no load)
Test Cells (bubble float tester) if any bad... buy two new batteries...
if 'ok'.... put a hard charge on them and then load test.... if either one fails, buy two new batteries.
(NEVER replace only one)
if pass....
you can use the other relay on your engine as a 'temp' to replace your GPR and 'see' what happens with that other relay when put on the Glow Plugs 'line'.
(Your truck will NOT start if the battery voltage falls below 10.v.)
(I am not writing this for any individual, but also to anyone who may read this in the years to come)
20 degrees .... how does that song go....? "Baby it's cold outside."....
Huuuge drain on your bats...
How old are they?
Can you check the Cells (bubble tester)?
Do you have a Load Tester.... (around 25.00 on sale at Haber Freight)?
imo... don't care if it does meter out at 12.2 starting out. It can go 'Down' fast on a load... (and then jump back up with no load)
Test Cells (bubble float tester) if any bad... buy two new batteries...
if 'ok'.... put a hard charge on them and then load test.... if either one fails, buy two new batteries.
(NEVER replace only one)
if pass....
you can use the other relay on your engine as a 'temp' to replace your GPR and 'see' what happens with that other relay when put on the Glow Plugs 'line'.
(Your truck will NOT start if the battery voltage falls below 10.v.)
(I am not writing this for any individual, but also to anyone who may read this in the years to come)
Lead Driver
Joined: Sep 2005
Posts: 9,399
Likes: 188
From: Norco,CA
If it's turning over like crazy, then batteries are okay. Relay is cheap, and is usually the problem. If you have more then two bad glow plugs that is a problem to. When you do change the glow plugs do them all.