View Full Version : Glow plug relay question
Hi,
I've been having starting issues with my '95 Suburban. Thankfully a while back I bought all four books from this website that relate to the 6.5, and have been following the troubleshooting sections. Looks like the issue may well be with the glow system. On a colder day the rig won't start, but on a hot day, or if it has been plugged in overnight, or once the engine is warm due to having been run earlier that day, it starts OK.
I ran the checks for the 1994+ Glow System as described in the Troubleshooting & Repair Guide book.
Single heavy wire terminal (input) has battery voltage.
Double heavy wire terminal (output) has battery voltage when glow plug lamp is ON.
Then I took the connector off and tested the connections inside it.
Pin A has battery voltage in RUN.
Pin B has battery voltage, almost, when the glow plug light is ON. Battery voltage was about 12.3 V and at the pin it was 11.85 V.
Pin C has a resistance of 16.6 Ohms, whereas the book says it should be 0, so I think that's where the problem is.
1) What should I do to make the resistance at Pin C zero? I'm guessing the ground to that system needs to be cleaned up, is this correct? And if so, can someone tell me where the ground to this system is, or indeed where all the grounds are, and I'll clean them all up.
2) The description and the glow system diagram in the book show that there are two glow plug fuses, one is 10 Amps, one is 20 Amps. I found the 10 Amp one in the fuse box under the hood on the drivers side, it's associated with the glow plug light, but can someone tell me where the 20 Amp fuse is?
Additional information: Before I took the glow plug relay our I checked for DTC codes. None were stored, I was getting 12. Out of curiosity, and to see if the DTC system is working, I checked for codes after removing the glow plug solenoid, and got a few, 29, 49, 57,62, so I guess the system is working OK.
Thanks very much for any help!
Don't know if anyone was planning on commenting on my post above, but the situation has changed a bit, here's the latest.
Yesterday I was getting 16.6 Ohms at Pin C. Started troubleshooting again today, was going to clean up all the ground connections I could find, checked the voltage readings again before I started, and this time I was getting around 250 Ohms at Pin C. Was totally stumped.... So I started moving the glow plug relay connector and the wire looms around a bit, to see if the Ohms reading would change. Then I noticed something weird.
A bit of background info. I have this minor untidy area under the hood, the work of a previous owner, which I was going to get around to in due course. There are two wires coming out of the wire loom/bundle at the rear of the engine, in the area of the fuel filter, both have ring connectors on the end, which don't look original. One is a thin wire like most of the wires in the bundle, it's black with a white stripe, and the other is a thicker one, almost the same gauge as the heavier wires attaching to the glow plug relay terminals, and it's solid black. They are connected to another thick black wire which comes from the grounding stud and bolt on the firewall just below the windshield wiper on the passenger side. All three wires have ring terminals on them and they are joined together by a nut and bolt through the ring terminals. This is what I was going to tidy up down the road. Anyway, I noticed that this bolt joining the three wires together, was lightly sparking to the engine block, as I was moving the wire loom around. Again I was stumped, as I assumed these are ground wires. And I think they are ground wires, but now they have voltage in them. The thicker one has 9.8 V and the thinner one has 9.97 V. On testing Pin C now, which is also a ground, it has zero Ohms, as it's supposed to have, but it is also showing 9.84 V, same as one of the wires.
So what's going on? My knowledge of this is not great, but obviously I'm getting volts into my ground wires. I'm guessing a live wire is touching those ground wires somewhere, maybe deep inside one of those wire bundles? Or are there any common places where this seems to happen in these vehicles?
Again, any comments or further troubleshooting tips appreciated. Thanks.
Hi
I am not sure about the oddities you have in the mix here...
When the glow light is on....do you have voltage to the glow plugs.????
Remove the glow plugs and check each one with a set of jumper cables off the battery and see if they get red hot .
60G plugs are the good ones to use.
The Drivers side plugs are easy....
Passenger side....not bad....
Remove the RH front tire.....block the rig...Remove the Rubber inner fender flap....
The plugs for cylinders 2-4-6 are easy to get to.....#8 is a pain......Must come up from under the truck....
Myself I would leave #8 alone.....
This access is also handy for getting to the starter wires and the front starter support too.
See what's up and go from there.
If the plugs all glow good.....The likely culprit is the controller itself......These do fail....
Good luck....keep us in the loop
Thanks for the reply. At the moment I'm not concerned about the glow plug relay/controller, I'll check it when I have the correct inputs going into it. I'm checking the voltages and ohms at the connector plug when it's disconnected from the relay, and the relay is out of the engine at this time anyway. I'm getting the correct readings at pin position A and B, but not pin position C on the connector, which is the ground. It should not be reading volts there, but it is. I'm also reading volts on two black wires coming out of the thick wire bundle in that general area. So I'm wondering what might be causing this. Wondering if anybody else has had volts going into wires which are supposed to be ground and what was the culprit....
On older rigs there always exists the possibility that previous owners have mucked around with things ....and it sounds like this may be the case with what you describe ......
Have any add on stereo sound equipment or other items been wired in ????
I agree...."C" pin is the ground on the controller.
If there is voltage at the pin....Certainly could be back feeding from elsewhere.....
Butttttttttttttttttt.....Many times ground circuits are controlled through the ECM and the presence of voltage is normal.......
Have you been getting an unexplained battery drain ????
Are the two ground wires that bolt to the intake manifold on the RH rear of the engine in place ???? These are fair size wires......
Another possible.....Check the amperage flow from those "Ground wires" to a known good ground.
Many grounds are from equipment that is functioning all the time and will show voltage...
But the amperage is milliamps.......Many of the newer rigs will drain the battery over the course of several weeks if left sitting..
Check this out.
A bare ring connector would certainly indicate that the wire was meant to be bolted to a stud.
Can you get some pics of those wires and post them.......
Many times people remove stuff and then it does not get back where it should be.
Ground issues can be a real issue on these rigs....Can cause a bunch of troubles.....
Let us know
If those wires are grounds, they need to be grounded. If you have a circuit that is powered, and you remove the ground connection from ground, then the ground side of the circuit will always show voltage. If that ground wire is connected to other ground wires (but not to ground) then you may get odd interactions between the circuits.
Since the wires have ring terminals, and a bolt through them, it sounds like they should be on a ground stud, not a bolt floating in the air.
Since they're not grounded, and you say the previous owner left them that way, I'm guessing one of the circuits is shorted and it will blow a fuse. So, you need to figure out where those wires all go, what circuits they are part of, and which of those circuits is faulty.
When you say
I was getting 16.6 Ohms at Pin C you need to explain where the other probe of the Ohm meter was in order for anyone to assess the reading.
As mentioned.....Grounds or LACK OF can do weird things.
I had a 1986 GMC 3500 4X4 that developed a strange anomaly.....Driving along steady and then accelerating would make the windshield wipers turn on for one swipe.....
Spooks...Nope....A broken ground in the HEI distributor (From the coil in the cap to the plug)
That would have been a real pain.....But I had just done a TUNE UP and replaced the cap, wires, rotor, plugs and such.....So I had been in there.
Removed the cap and took the top off....there it was...The wire had broken off at the male spade terminal....Insulation was covering the break.....
Electricity will search for a path to ground...always.
The electronics on these 6.5 diesels DO NOT LIKE ground faults...or any other electrical anomalies.....This stuff can screw up the PMD and the ECM...
The electrical connections all need to be in good order....Grounds at the battery, frame, engine and such
Inter connect between the batteries...
Battery ground cables at the block and battery.
The factory crimped cable ends have a habit of corroding in the wire strand /crimp area..
Can't be seen easily....
The 1995 rigs have the alternator feeding the RH battery and IIRC the main power box feeds off the LH battery..
Any goofy grounding or positive connections can cause issues.
I made some major progress yesterday and today, largely due to JohnC and Robyn - thank you!
I connected those two wires to ground, properly this time, and after that my readings at all three pins at the glow plug relay connector were as they were supposed to be. Zero Ohms at pin position C, and no volts either. The mysteries of electricity... And the annoyance of previous owners doing some crappy things...
The relay seemed to be working fine, it was giving battery voltage at the output terminal when the glow plug light was on, but the rig did not start. I had bought a new relay, so I put it in anyway, since the one in there looked original. I was hoping that might do the trick, but no luck, still no start. So I took out a couple of glow plugs, grounded them at the threads to the engine block, and turned the key to RUN. Nothing, no glow, no heat, zero. So I checked the voltages at the glow plug connectors, and was getting battery voltage. I couldn't believe it, but it looked like my glow plugs were bad! They were fairly new glow pugs, I put them in just over two years ago, less than 8,000 miles ago. AC 11G. I took out six, four from the left side, and two from the right side that I could get to reasonably quickly from the front of the rig. I have a box of 8 new AC 11C plugs, that I bought at the same time as these, so I ran some comparison tests. Connected some jumper wires to a battery, and tested all the new ones and the six from the engine. None of the six from the engine even got warm let alone glowed, just completely dead. All of the new ones glowed real quick as expected. I noticed that when touching the new ones with my jumper wires, there was a little sparking, and when touching the six from the engine, no sparking at all.
Anyhow, for a while I thought that tomorrow I'll put in the eight new 11G plugs I have, but then on second thought I decided to buy new AC 60G plugs, and throw these ones away. I seem to remember that when I was buying them a couple of years ago, they were a killer deal on Ebay, just a couple bucks each. Now I'm thinking that maybe they were a known bad batch, hence the giveaway price??
Has anyone experienced bad glow plugs, like I described? I really am in disbelief right now. Surely this isn't common? These are genuine AC Delco plugs, with not many miles on them. What the hell happened to them? Were they bad when I put them in two years ago, or did something happen in my engine that made them go bad?
Anyhow, I'm happy I seem to have gotten to the bottom of this, and I'll be happy to have AC 60Gs in the engine, hopefully the rig will start fine when I put them in tomorrow, we'll see....
Glad things are coming along well.
60G plugs are what's needed
The older 11G and a few others have bad habits....Some of which is real bad. (Failing with big blisters that prevent them from being removed)
The 11G plugs heat much quicker.....Hence they do not last as long....They do just what you have had happen.....
Get a set of 60G plugs and you should be good to go....
In real cold weather it may take a couple cycles to get started....but the 60G plugs will not fail in bad ways like the 11G
Have fun
I noticed that when touching the new ones with my jumper wires, there was a little sparking, and when touching the six from the engine, no sparking at all.
The new ones were drawing current, hence the sparking. The old ones were burnt out. You would get the same result touching the wire to a block of wood: no sparks.
Glow plugs are easy to test with your Ohm meter. One probe on the threads and the other on the terminal. Expect near zero Ohms, like 0.1 Ohms, if your meter is that sensitive. Keep in mind, "0" is not the same as "0L". The second is the same as infinity (open circuit) or "out of range" (very high resistance) and is the opposite of zero. Zero means current can flow easily through the device. "0L" means current can't flow, or is met with very high resistance to flow.
DmaxMaverick
07-14-2022, 10:26
The 11G plugs were an improvement over the previous 9G plugs (sort of). Both heated very quickly. The 9G plugs would often swell when they fail, leading to the 11G update (adding a thermistor feature). They didn't swell, but didn't last long, either. Both of them were 6V plugs, the reason they heated (and died) quickly. 12V plugs of the time simply took too long to heat, considering the impatient nature of the planet occupants of the era. A 20-30 second "wait" time (of 12V plugs) was out of the question. The AC60 plugs are the go-between, heating relatively faster, while lasting longer without destructive failure issues. A drawback is, in some cases, the OEM glow cycle isn't enough to heat them for a start (cured with a simple mod).
When you say “cured with a simple mod” to extend the glow time, you mean one of the procedures as described in the articles “Improving cold starts for electronic 6.5L diesels” or “Manual glow control for the electronic 6.5 TD” in Vol II 6.5L Turbo Diesel book, written and sold on this website, or is there any other newer and/or better procedure you can recommend?
DmaxMaverick
07-14-2022, 17:03
Correct. It isn't necessary to complicate it. The 84 to 93 models are less simple. The temp switch is in the (all-in-one) controller, IIRC. A more simple solution for the before and after years, is to simply interrupt the temp sensor (switch for 82-83). PCM models will see -40°, forcing the longest glow cycle. A mod isn't always necessary. Most engines play nice with the AC60G's without any help, with all but the coldest starts.
What Maverick mentioned is spot on.......I have started the 6.5 at cold temps....AT LEAST FOR HERE....3-5 F I just switch the key off when the start light goes out ...then back on for a second go at it if need be.
The last cold start I did on my 6.5 (My Son in law has it now) was real easy with one cycle of the plugs....this was at around 15 F That critter came to life with a rattle that would make an early Power Stroke Blush.....
Unless the engine is really tired and needs every little bit of help it can get...the 60G plugs will work fine....
One thing though NO MATTER WHAT...."DO NOT BE TEMPTED TO USE MA STARTLY" (ETHER) In cold temps.....THIS WILL CAUSE SEVERE ENGINE DAMAGE
Suburban has been starting great with the 60Gs, thanks again for your comments. I read a while back about the great idea to cut an access hole beneath the radiator drain plug, so I thought why not cut an access hole abeam plug #8. Made two cuts, bent up the tin, changed the plug, bent down the tin. Maybe this is a known and common thing to do anyway, not sure, but sure did work great, highly recommend it, made it all rather easy, and I was quite pleased with myself.
Since it’s just been mentioned, I’ll take the opportunity to ask, why does the 6.5 have placards here and there warning against the use of starting fluid? I thought it is, or used to be, common to use this in other diesel engines. What’s different about the 6.5 that makes it a no no? Have been wondering about this for ages. Thanks.
Glad your rig is working OK now....
The Addition of "Mouse holes" to access the #8 Glow plug and the radiator drain have been done before...I added a nice hole in my 95 Dually to access the #8 plug and made a little removable cover to fit over the hole....This hole also gives access to the heater hose connections.....Same same on the radiator drain..
I always asked "Why have a radiator drain if you can't reach it" ??????
As far as the use of Starting fluid "Ma Startly"
The indirect injected type diesels (6.2/6.5 and many others) have a pre combustion chamber with the glow plug in it.....The small space of the pre chamber and the presence of the glow plug create a very good possibility of a very violent detonation of the mixture when starting fluid is used.
The glow plug when active WILL ignite the starting fluid long before the diesel is injected ..
Anyway.....The use of starting fluid on direct injected engines (No pre chambers and no glow plugs) is not nearly as hard on the engine and was/is common in very cold temperatures .....Still the use of such products needs to be IN MODERATION.
With the engine turning over the starting aid is added into the intake air stream in small amounts....JUST ENOUGH TO GET THE FIRE LIT.
Many class 8 truck come fitted with " Starting aids " And have a Button or a control of some kind on the dash panel to actuate it....Again even these arrangements only add a small amount of Starting fluid at any one time.
Back in my heavy trucking days I had a 500 Cat in a 1995 Western Star....One cold morning 5F it was a tad balky about starting....I grabbed a big hair drier and stuffed it in the intake pipe and let it run for a bit to get warm air in the air intake.....Helped a lot and got the beast to firing on a couple holes and in a very short time it was running on all 6 cylinders......HOT AIR WILL NOT HURT THE 6.5.
You can opened the air cleaner and stuff a hair drier into the pipe and let it heat things up...
Using starting fluid on the 6.5 can literally lift the heads......No joke......
I had a 6.2 one time that the glow system failed out on a winter camping trip...COLD
WE USED A TINY BIT OF GASOLINE SPRAYED INTO THE INTAKE AT THE AIR CLEANER....
Just enough to help get the thing lit off.......
Hope this answers your questions
Yukon6.2
07-25-2022, 09:55
I have had the Starting Aid on most of the commercial trucks i have had.
Lousiville with a 671T,Western star with a 8v71,Western star with a 3406B Kenworth T800 with a 3406B
My electronic controlled M11 in a T800 Kenworth which is my fuel delivery truck that i use all winter long dosn't have it.But with the electronic controlled injection and synthetic oil throughout it will start at -25C without being plugged in.Not something that i regularly do,but sometimes there is an emergency fill that has to be done.Sounded horrible like a bunch of hammers trying to get out.
Yes, that answers it nicely, thanks very much. Now I know!
I’ve been using the Suburban every day for the last two weeks and it’s been starting well and running great. I’ve been in the Las Vegas area, and even though it’s hot hot hot there, for the first start of the day the Wait To Start light has been coming on and the glow plugs have been heating up. That was also the case for the last few days back here in the Los Angeles area. However this morning the light did not come on, and the voltage did not go down, indicating the glow plugs were not heating up, and it did not start. What is it that tells the glow plug relay to start working, and the light to come on? Where might I start to troubleshoot? Thanks.
DmaxMaverick
08-01-2022, 20:37
The PCM controls the relay and the indicator (via the IPC logic board). If you need to start and it doesn't because the plugs won't cycle, unplug the temp sensor. This fools the PCM into thinking it's -40. If that doesn't work, the PCM, relay, or harness, in no specific order, has failed.
Great, that’s just what I needed to know. If you could point me to where the temp sensor is, that would be extra helpful! Otherwise I’ll start reading up in the books and online. Thanks!
DmaxMaverick
08-02-2022, 07:05
The PCM temp sensor is in the thermostat housing, and is the only ECT (Engine Coolant Temperature) value the PCM "sees". If this sensor fails, it will mess with the GP cycles and cold start behavior (like timing advance and high idle staying on, or not activating at all). Disconnecting it will "show" the PCM a value of -40°, and a short between the polls will indicate +340°F. A short should trigger the SES and set a code for the sensor. The resistance between the sensor polls can be measured for a linear value of the coolant temp. Very simple test with a VOM (Volt/ohm Meter). I don't recall the range of values, but it should be easy enough to find.
The temp sensor in the head is for instrumentation only, so disconnecting this will only change what you see on the gauge.
Great, I’ll check that out. Thanks very much.
Thanks DmaxMaverick, you saved me. It wasn’t the PCM, as I had a spare that I tried, and it wasn’t the temp sensor, unplugging it didn’t help, I sure hoped it wasn’t the harness, it turned out to be the glow plug relay. I replaced it very recently when I was troubleshooting the initial starting issue, but things went back to normal when I put the old one in just now, which was still working fine. The new one was from Auto Zone, amazing it failed so quickly, what a p.o.s.! Anyway, burb is starting fine and running great again. Thanks!
DmaxMaverick
08-06-2022, 23:30
That's great! It's always nice when we get stuff figured out.
I went through something similar on my 2001 Duramax a few months back. There's no difference between the old and new function, but it's more complicated and the parts are a lot more costly. I bought a high-dollar NAPA GP controller, only to find I had a ground issue. I like spare parts, but I'd rather not have $250 parts in the glove box if I can help it. I'd have just bypassed it, but I have to pass a smog inspection every 2 years, and a bypass won't turn off the SES lamp. The Duramax will start w/o plugs down to -8F (tested, albeit rough), but the EPA and CARB isn't happy with it, and they don't trust me with a button.
Yukon6.2
08-07-2022, 08:05
The EPA might start to have a better disposition...
Since the their hands got whacked by the supreme court.
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