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7.3L IDI Diesels (Not Power Strokes) Technical discussion of topics related to vehicles powered by the 7.3 Liter In-Direct Injection Navistar engines.

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Old 01-20-2012, 01:33 AM   #1 (permalink)
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GP glow plug controller system basic diagnostics

I've been looking for info and reading a lot on the early '90's 7.3 IDI glow plug system and thought it would be nice to have a few basics in one place with an easily searchable title.
I haven't found my answers yet so this post is to help myself as well as all other TDS users in solving their glow plug system issues without having to search thru scores of threads on users specific glow plug problems.
It will not be an all inclusive complete diagnostic and repair guide to all aspects of the glow plug system, but I will try to go a little beyond just the very basics. Sort of a text only flow chart.
Lets start with this handy reference link from cdnsarguy
GP controller "How it works"

"My truck is hard to start, is it the glow plugs?"
To decide if your GP system is working correctly or not (and the hard start is not fuel related) we should start by answering these questions.
1. Are my batteries fully charged, and supplying adequate amps for cranking/starting.
If not, first check alternator output, then have batteries load tested.

2. At temperature "X" my WaitToStart light stays on for "Y" seconds for a cold start [without block heater].
.........(need table of values here)...........
If your WTS light times are not within a couple seconds of the approximate times at a given temp.
a) Check for bad connections at glow plugs. May be broken, rotted or just disconnected.
b) With a cold engine remove wires at glow plugs and check resistance. They should all be 1 ohm or less. If not replace as needed, or remove and inspect all for burnt/swollen tips (replace all if they're autolites )

If GP's ck OK then the glow plug controller and relay are next to check.
1) Look for bad relay connections (loose,burnt,dirty), or bad controller ground wire.
2) Check voltage at large relay terminals. GP side (ribbon resistor) voltage should be the same as hot battery side with key "ON", and relay energized (WTS light is on). Lower voltage =weak/bad relay. No voltage =bad relay or relay trigger wire (white wire on small terminal) circuit issue, check for battery voltage there when key is turned to "ON".
If relay is bad, it can be purchased by itself, or with the controller as a unit.
If relay ck's OK the controller can be faulty, but is not really testable separately, either it cycles GP's or it doesn't.
3)The wiring harness to the glow plugs is another possible issue and can be checked by 1st disconnecting wires at all GP's, Using an ohmmeter check for continuity from each wire end to terminal at bottom of ribbon resistor on relay where harness connects. Or by connecting a jumper from "hot" side of relay to bottom of ribbon resistor harness connector, then using a test light, or volt meter from each GP wire end to ground.(The WTS light will come while this method is used.)





{Use of a battery charger me be needed for extended testing of the GP system to maintain appropriate battery voltage levels.}.




This thread is a work in progress, sort of a "wiki" or possibly a potential "sticky" on the subject I suppose. As additional info is posted I will edit this original post without quoted attribution for ease of future viewing. All replies will obviously be in the thread below.
Thanks in advance for any contributions.

Some questions for completing this post.
1 A short table of approximate seconds of glow time @ various temps.
2 What sensor does the GP controller use to determine temp? integral or?
3 What voltage should be reaching GP's, 9-10?
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Last edited by jga2z; 01-22-2012 at 10:25 PM.
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Old 01-20-2012, 06:12 AM   #2 (permalink)
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If the truck is plugged in it starts right off. If I let the truck sit and cold soak I have to cycle through the wait to start multiple times up to 6. The connections appear to be good. After the wait to start light goes off I do not hear the solenoid cycle through the three additional times. Could it be the solenoid? power to the solenoid? I have checked the glow plugs to ground and they appear to be good. Any ideas?
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Old 01-20-2012, 06:39 AM   #3 (permalink)
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have you tested you glow plugs?
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Old 01-20-2012, 07:28 AM   #4 (permalink)
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Yes and there is continuity from the tip where the power wires hook up to ground.
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Old 01-20-2012, 09:53 AM   #5 (permalink)
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did you test the gp with an ohm meter or did you just do a continuity test.if you dont have an ohm meter you can test the gp with a test light.
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Old 01-20-2012, 10:59 AM   #6 (permalink)
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I used an ohm meter. I did not check to see what the resistance was I just looked to see that there was continuity.
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Old 01-20-2012, 02:03 PM   #7 (permalink)
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i had the same problem and it was 1 bad glow plug.i tested the gp and replaced the bad plug and it solved my problem.
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Old 01-20-2012, 02:46 PM   #8 (permalink)
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puddlejumper67
I fail to see how your post is relevant to the topic of this thread as titled as it asks a question about your particular problem and does not contribute to the basic diagnostics PROCESS topic.
If you review the original post you should find answers to all your questions thus far.

Perhaps you would be better served by starting your own thread dealing with your issue if you do not find answers here regarding how to diagnose the problem.

I started this thread for the sole purpose of providing a simple diagnostic procedure to diagnose GP system problems and hopefully when completed reduce the amount of duplicate threads on resolving these issues.
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Old 01-20-2012, 10:28 PM   #9 (permalink)
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Quote:
If you want it done right, do it yourself
Your quote.

Probably a thousand and then some post's have been written about GLOW PLUGS and GLOWPLUG CONTROLLERS.
Even if there was/is a sticky on the subject, damn few would even look at it no matter how detailed it would be. Most are just to damn lazy or ignorant to use the search. Maybe they just like to start a thread to increase there number of posts!
CDNSARGUY has the most comprehensive in detail subject on them.
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Old 11-29-2012, 09:01 PM   #10 (permalink)
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Not to beat a dead horse, but I'd really like to know the answer to question 2 above:


Quote:
2 What sensor does the GP controller use to determine temp? integral or?
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Old 12-02-2012, 01:38 PM   #11 (permalink)
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I would like to see a chart of actual glow plug time vs temp chart. It would be handy for guys running manual gp switch
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Old 12-02-2012, 09:07 PM   #12 (permalink)
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No such information exists in the factory service manual. Presumably whoever makes GPCs is who's got that information. I imagine the GPC is actually very simple, but I've never looked at once since mine is working fine (knockonwood.) Here's some stuff that's in there, though.
Glow Plug Controller
The solid-state Glow Plug Controller, attached to the top of the engine block, controls the glow plug pre-glow and after-glow time. It also controls the circuit's operation by sensing engine temperature, glow plug voltage and after-glow voltage from the start/run circuit.

When the Ignition Switch is turned to RUN, voltage from Maxi-fuse K is applied through the Glow Plug Controller to the Wait-To-Start Indicator Lamp.

The Glow Plugs heat up in zero to fifteen seconds, depending on engine coolant temperature. After the Glow Plugs heat up, the controller cycling switch opens and turns the Wait-To-Start Indicator Lamp off. The Glow Plugs are now warm enough for the engine to be started.

At the same time the Ignition Switch is turned to RUN, voltage from Maxi-fuse K is applied to the after-glow timer (located inside the Glow Plug Controller). The after-glow timer cycles the Glow Plugs for up to two minutes, depending on engine temperature. The after-glow timer then opens. The Wait-To-Start Indicator Lamp will not light during the after-glow period.

If the Ignition Switch is turned OFF, it can be turned to ON immediately, and the Glow Plug heating cycle will start again.

Diesel Start/Run
The diesel engine uses two batteries to provide extra power for starting and glow plug heating. Power is applied from the batteries, through heavy gauge wires, to the Starter Solenoid (located in the Starter Motor assembly). When the Wait-To-Start Indicator goes out, the Ignition Switch can be turned to START.

With the Ignition Switch in START or RUN, voltage is supplied to the Fuel Heater, Fuel Shutoff Solenoid, and the Engine Temperature Switch through Maxi-fuses K and U.

The Fuel Heater is in the Fuel Filter/Separator. It heats the diesel fuel, melting any wax that might clog the filter. The heater has an internal thermostat to turn it on as needed.

The Fuel Shutoff Solenoid controls the flow of fuel into the injection pump. With the Ignition Switch in START or RUN, the Solenoid is energized, and fuel is allowed to flow into the injection pump. When the Ignition Switch is turned off, the solenoid is deenergized, fuel flow stops, and the engine stops running.

The Engine Temperature Switch provides voltage to the Cold Timing Advance Solenoid and the Cold Idle Solenoid. When the engine temperature is below 112 F (44 C), the Engine Temperature Switch is closed. When the Ignition Switch is turned to START or RUN, the solenoids are energized, advancing injection pump timing and engine idle, allowing the engine to run more smoothly when cold. When the engine temperature reaches 112 F (44 C), the Engine Temperature Switch opens. This deenergizes the solenoids, returning the timing and idle to normal.
this describes everything in the glow harness, let alone the glow system itself. This is from the 1992 elec and vac troubleshooting manual (EVTM)

possibly if you looked at enough glow plug manufacturers' websites you could find a chart that would describe the resistance-to-temperature relationship of a ZD9 glow plug. I have something I have no idea where it came from that has numeric data of this type for GO50 and AC60G plugs. I can tell you most other things about them, like what kinds of threads and connectors they have and their dimensions, for HKT and Denso plugs (and they have compatibles for most...)
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Old 12-02-2012, 09:22 PM   #13 (permalink)
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Quote:
Originally Posted by tanbuddy View Post
I would like to see a chart of actual glow plug time vs temp chart. It would be handy for guys running manual gp switch
I've run manual switches in 6.2l and 6.5l, and never had to glow for more than 20 seconds regardless of how cold it got. Even after 17-18 hours parked on the mountain when I was a skilift mechanic, the truck started up with 20 seconds. Wit an hour or 2 of block heater time it fired right up when I glowed for as long as the seatbelt buzzer was on.
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