1989 7.3 IDI - hard starting & hitting a brick wall

NWArcade

Registered User
Joined
Jan 20, 2020
Posts
14
Reaction score
19
Location
WA
I agree, it's concerning that things with the glow plug system seem to test out as working, with the exception of the higher-than-expected voltage when the glow plugs are under load. Even though the plugs and controller are basically new and are motorcraft, it is possible that something was bad out of the box. If that is the case, I’m leaning more toward the controller.

The controller stays locked until the 15-20 second cycle has been completed. It then clicks 2-3 times and then shuts off.

The truck has about 236k on it. I have not yet done a compression test. Thankfully the truck was bone stock when I purchased it. Zero shenanigans thankfully with the wiring or anything else, other than the typical parts on the truck failing one at a time with age:)

Individual plug grounding is an interesting theory. I’ve triple-checked the grounding at the controller too, which is solid. Oh and I've made sure the 2 fusible links are still good on the 12 feed wires from the starter solenoid to the glow plug controller.

Knowing there seems to be some anomalies with glow plug test, as a last resort, before a full compression test, I may bite the bullet and pick up another new motorcraft glow plug controller and all-new motorcraft glow plugs. Worse case it’s the same and I have a working spare on hand.

Thankfully I love this truck and know I'll solve things eventually (I'm stubborn), but the hard start issues with these rigs can drive a guy nuts.
 

Rdnck84_03

Full Access Member
Joined
Jan 15, 2019
Posts
1,038
Reaction score
1,091
Location
Kansas
Keep chipping away at it, you will eventually find the smoking gun.

James
 

Rdnck84_03

Full Access Member
Joined
Jan 15, 2019
Posts
1,038
Reaction score
1,091
Location
Kansas
I do have one question on the installed new parts. You said that the controller and glow plugs are Motorcraft, where did the new glow plug harness come from? Is it possible there is some compromised connections in the harness?

James
 

ihc1470

Registered User
Joined
Nov 19, 2023
Posts
24
Reaction score
33
Location
Idaho
Here are my results. Thoughts? Both the glow plugs and controller have less than 500 miles on them. Both are motorcraft. Glow plugs ohm out at around .2 and .3 and burn bright red with 12 volts applied. Glow plug harness is new as well.

Put your postive of your meter test lead on the single strand of wire, and your other probe to ground. Set your meter to 12 volts. On a cold engine, you should see 6 volts at the #2 cylinder glow plug. The voltage will slowly rise as the plugs heat until you get to around 11 volts at which time the controller will cut the voltage and your wait to start light goes out. IF your voltage is higher than 6 volts starting out, then some of your glow plugs are failing, or you have bad connections in the wiring. If your voltage is less than 6 volts, then you have voltage supply issues.

***VOLTAGE IS 9.66 AND DOES NOT CHANGE UNTIL THE 20-SECOND MARK AND THEN IT CYCLES AND TURNS OFF.***

Check the following. Put your positive voltmeter probe on the large lug on the GPC that is fed from the battery. Put your negative probe on the Z strip lug. You should see battery voltage at this point.

***12.50 VOLTS***

Have an assistant turn on the key, and verify that your voltage drops to near zero. If it has over half a volt, then your contactor is worn.

***.20 VOLTS***

If this test passes, put your positive probe on the positive post of your battery, and the negative on your large GPC lug coming from the battery and measure for voltage loss there.

***12.20 VOLTS***
On the very last test which is voltage drop from the battery to the controller is the 12.20 what you read? If so it would appear that you have a bad connection/wire issue from the battery to the controller. About .3 volt drop is about all I would expect to see. Normal rule of thumb is .1 volt per connection. Recheck that test and if you get same reading you might make a temp jumper wire from battery directly to controller and see if that changes anything in the way it starts.

One other test you can make if you have the equipment is to see what the amp draw is when the glow plugs are working. When I did mine recently they were drawing very close to 200 amps when first turned on and then started dropping off as the plugs warmed up. This was with new Motorcraft plugs.
 

The_Josh_Bear

Full Access Member
Joined
Jun 1, 2016
Posts
1,888
Reaction score
1,469
Location
Western WA
Thank you for such a well-written explanation of what's going on and what you've tried! That helps us a lot, and will help you later on to remember what all you've done. Nice work!

It's been a while since I had a stock 7.3 GP system. I switched over to a manual push-button and never went back. Same solenoid just minus the controller.
For testing purposes if you suspect the controller you can hook up the solenoid to a momentary switch(push-button) and run it that way. The 9.66v steady under power is really odd to me. I didn't think the controller actually affected the voltage...Maybe it does. I'd bypass it and see how she purrs. 10-12 seconds on the push button, 15 if it's in the 30's.
This would be an easy next step that seems like the most likely scenario.

If you had a lope at first start you could suspect one or two glow plugs or injectors. But you say it's really smooth which suggests good compression, good GPs and good injectors. It's more like the GP's are being held back by the controller, which is suggested by the steady voltage reading under load and the long glow time.

I agree with above that you should see 200+ amps when GPs first turn on, and it comes down over the 10-15 seconds. I have a clamp on meter that only registers 200 amps max and it faults out until 3-4 seconds in, then starts to read amps as they drop under 200. Pretty fun, actually.
Along those same lines you can clamp on to each wire and trigger the GPs for a bit to see the amp draw. Just un hook the trigger wire from the solenoid and tap it to the post for a second, read the amps. Move to next GP wire and see if you find one with low amperage. Beats replacing them all.

You said your starter is a good DB unit and she spins over fast-- faster than you can count in your head? If not, it's too slow. I love that @catbird7 said:
The fact it starts without issue when it's plugged in, reinforces the notion it's a glow plug or glow plug wiring issue.
This was a good catch that I missed. The one thing I'll add is that when the block/starter is warm the engine spins over faster, too. So it could be a combo of both GP's and starter since we can't hear it turn over.

Hope you get this knocked out soon,
Joshua
 

NWArcade

Registered User
Joined
Jan 20, 2020
Posts
14
Reaction score
19
Location
WA
On the very last test which is voltage drop from the battery to the controller is the 12.20 what you read? If so it would appear that you have a bad connection/wire issue from the battery to the controller. About .3 volt drop is about all I would expect to see. Normal rule of thumb is .1 volt per connection. Recheck that test and if you get same reading you might make a temp jumper wire from battery directly to controller and see if that changes anything in the way it starts.

One other test you can make if you have the equipment is to see what the amp draw is when the glow plugs are working. When I did mine recently they were drawing very close to 200 amps when first turned on and then started dropping off as the plugs warmed up. This was with new Motorcraft plugs.
Ok, good point. I'll recheck today. Running a jumper is a great idea too. I'll report back.
 

NWArcade

Registered User
Joined
Jan 20, 2020
Posts
14
Reaction score
19
Location
WA
I do have one question on the installed new parts. You said that the controller and glow plugs are Motorcraft, where did the new glow plug harness come from? Is it possible there is some compromised connections in the harness?

James
James - good question...I got it online, can't recall the seller off the top of my head, but it is of good quality and all connections check out.
 

NWArcade

Registered User
Joined
Jan 20, 2020
Posts
14
Reaction score
19
Location
WA
ihc1470 - Ok, so I just got done making up a quick 6g jumper. I hooked it up from the starter solenoid and directly to the hot bus on the glow plug controller (disconnecting the factory power). Note, that I did not have a fuse on this temp. set up but I was ready to pull the power if any issues arose. I turned the key on and the glow plug controller stayed on for about 23 seconds. Once it clicked off, it turned back on once and then off again. In the past, it would quickly click 2-3 times after the main cycle. With this temp. set up the truck fired up within about 4-5 seconds, faster than ever, especially not being plugged in and it was STONE cold (not started in 2 days + under 30 degrees outside). I did not do any other testing once the truck started right up as I did not want to keep a non-fused power supply in the truck any longer than was necessary

SO...I guess pulling the 2 factory wires out and installing a new power wire is in order. I've done a bunch of research in the past about this and there is not a lot of guidance. What I have found is using say a single 6g wire and an inline Painless type fuse rated at 200 amps. Would that be sufficient and safe or should I go with 4g and a 200 amp fuse or perhaps a 175 amp OR should I be using a fusible link setup? Let me know your thoughts about how to safely run new power (wire gauge, fuse or fusible link size, etc). In any case, thank you all so much for helping me troubleshoot this issue. I do know my main power wires are both putting out power, but somehow they must be compromised which is affecting the glow plug controller's ability to put out its full power.

Joshua - based on the above, I think your assessment was spot on. Oh, and the starter spins faster than I can count.
 
Last edited:

The_Josh_Bear

Full Access Member
Joined
Jun 1, 2016
Posts
1,888
Reaction score
1,469
Location
Western WA
That's a cool update for you, 4-5 seconds stone cold is not bad at all. Awesome!

I just went out and checked my setup, but I don't have the wire gauge labeled on the wire that I can see.
I have a 250 mega fuse off battery, then
(I think) it's 6ga from battery to White Rogers relay(18")
Then from the WR relay I spliced into the stock wires that feed the GP's (but fixed the melted section by cutting out the bad stuff and re-crimping and shrink wrapping it.)

I wouldn't use a 200amp fuse because the system should be drawing over 200 amps for a few seconds, which should be enough to pop that sucker.

The wire gauge for these GP systems always freaks me out, being so underrated for the amperage. But I guess it's all fine and dandy since it's factory and nobody is burning down their trucks or complaining of burned up fusible links. Same thing on other glow plug systems, too, very thin wire gauge for the amperage.
 

NWArcade

Registered User
Joined
Jan 20, 2020
Posts
14
Reaction score
19
Location
WA
That's a cool update for you, 4-5 seconds stone cold is not bad at all. Awesome!

I just went out and checked my setup, but I don't have the wire gauge labeled on the wire that I can see.
I have a 250 mega fuse off battery, then
(I think) it's 6ga from battery to White Rogers relay(18")
Then from the WR relay I spliced into the stock wires that feed the GP's (but fixed the melted section by cutting out the bad stuff and re-crimping and shrink wrapping it.)

I wouldn't use a 200amp fuse because the system should be drawing over 200 amps for a few seconds, which should be enough to pop that sucker.

The wire gauge for these GP systems always freaks me out, being so underrated for the amperage. But I guess it's all fine and dandy since it's factory and nobody is burning down their trucks or complaining of burned up fusible links. Same thing on other glow plug systems, too, very thin wire gauge for the amperage.
Yeah, I'm sure you can appreciate how great it was to finally figure this out!!! So to confirm, you are NOT using the old fusible links? In other words, I can go from the starter solenoid with 6g to a 250 amp fuse, then continue to the hot terminal with 6g?
 

ihc1470

Registered User
Joined
Nov 19, 2023
Posts
24
Reaction score
33
Location
Idaho
If I was doing it I would now run a couple of voltage drop test and determine where the bad connection is. I have a 94 so wiring might be a little different but on mine it runs from the right battery over to a solenoid on the right fender and from there to the GP controller which in my case is on the right valve cover. I am guessing yours is behind the air cleaner. Anyway go from the battery just to the solenoid on the fender and see what you have for a voltage drop. If that proves good then go from the solenoid to the GP controller. You should be able to isolate which section has the poor connection. Another way is to just feel where it is getting hot when it powers up. If what you posted is the correct reading voltage drop wise it should not be that hard to isolate. Poor connection means resistance which will create heat. If I was to guess I would bet it is in the engine connector. If so I would just pull those wires out of the connector and splice together.
You must be registered for see images attach


When doing the voltage drop test go from terminal to wire end. At battery from post to cable end. Then cable end at battery to wire end at solenoid. Wire end to solenoid post and so on until you get to the GP controller. That way you will test connections and wires as you go along. You will need to have the circuit under load as you make each test. When I was testing mine I pulled the small white wire off the solenoid on the controller and hooked up a temp wire that I could ground. I left the key on and just touched the temp wire to ground which would turn on the system. If you have a helper they could turn key on and off as you are testing things.

Your question on how to rewire, I think I would just duplicate what Ford did bypassing the engine connector.

Be interesting to learn where you find your issue at.
 

The_Josh_Bear

Full Access Member
Joined
Jun 1, 2016
Posts
1,888
Reaction score
1,469
Location
Western WA
Yeah, I'm sure you can appreciate how great it was to finally figure this out!!! So to confirm, you are NOT using the old fusible links? In other words, I can go from the starter solenoid with 6g to a 250 amp fuse, then continue to the hot terminal with 6g?
For sure! You've done a lot of work and it's always great to reap the benefits!

I can't remember having any fuse link, but if I did I would have cut it out because the old fuse link in other areas of my wiring has failed me before. So I went with a mega fuse I can see and replace at any parts store instead.
Yes you can go from fender solenoid to fuse, then fuse to hot terminal. Just make the run to the fuse as short as practical so the entire circuit is protected, but also with enough room to change out the fuse when needed.

On my 89 the two 10ga yellow wires going from the controller back to the connector become black with red stripe from connector to the battery. I took all of this out of the loom when re-doing my GP system and wiring up the White Rogers relay. If there was a fusible link in there, I don't remember it. I'm sure there was, since there isn't anywhere else in the harness to provide the safety of a fuse link or fuse. (plus the above wiring diagram shows it)

Edit: And the reason I went with 6 gauge is that it's rated for more amps than two 10a wires are, so I called that good enough and don't have problems.
 

NWArcade

Registered User
Joined
Jan 20, 2020
Posts
14
Reaction score
19
Location
WA
Ok, the tests are complete. Years ago, I cut the 2 yellow wires out of the melted harness at the passenger fender well. One of those butt connections was bad, causing excess resistance. After careful consideration, I ended up pulling out the factory power run for the glow plug controller and replaced it with a 6g wire, a 250 amp fuse, with proper crimp connections that are protected by shrink tubing, wiring loom, etc. The glow plug system now works perfectly, for the 1st time ever, and the truck started this morning in less than 30 degrees within 5 seconds. It's never started this fast. Thank you for all your help!
 

Forum statistics

Threads
91,146
Posts
1,127,155
Members
23,972
Latest member
majorfa

Members online

Top