how's it wired?

[c6Dafiy]

Since I need 25 posts before being allowed to download a manual:

How exactly are the glow plugs wired?

What I have is a 1996 TD28 with the two different types of glow plugs which is hard to start when cold. The glow plug light comes on for only a second or two and the relais also switches off when the glow plug light goes out. After that, only 6V goes to the glow plugs, and that is obviously not enough glowing for an easy start.

What I want is a manual setup so I can press a switch to power the glow plugs. I can't seem to get 12V plugs here which makes a conversion difficult.

How do I do that? I've found quite a few remarks about how the system is supposed to work, some even contradicting others, and I do not believe that the rear glow plugs don't have a ground connection through the block. Where else would they have their ground connection? But then, how can they be wired in series to get a 6V supply for afterglowing?

And why shouldn't 6V be enough for glowing to get the engine started? They are 6V glow plugs after all.

I suspect the ECU is broken, and there's no way I could buy a new one. They cost about AU$ 1520 here.

[DX grunt]

G'day and welcome to the forum.

You might find that you'll get a heap of responses after you post an introduction about yourself.

25 posts aren't that hard to accumulate. If you 'welcome' other new members to the forum, show a pic or two of your truck, and other basic stuff,
It'll be here before you know it.

Some members rack up 25 posts in under an hour or so.

Where are you from?

Take care out there.

Rossco

[c6Dafiy]

Well, I'm in Germany, so Australia is a long way from here ...


Hm only thing I found is some part numbers for glow plugs for the 4.2 which are supposed to fit into a 2.8, like Bosch GPN327. Yet when you look at their catalog, they have different part numbers for glow plugs for the 4.2. So even if I can get them here, I still won't know if they'd fit my 2.8. The parts store I checked with had plugs listed for the 4.2, and they were 24V only. If I had 12V plugs, it would be easy to wire up a manul solution ...

[Yendor]

Here's the wiring diagram.

Hope this helps.

[c6Dafiy]

Thank you very much!

So if I understand the diagram correctly, there is either ground or 12V on the short rail through relais #2 ... Then what is the point of putting 12V on the short rail when the glow plugs don't have a ground connection?

And through relais #1 and the long rail, all the plugs get 12V ... So for a manual setup, I would connect the short rail to ground permanently and rewire relais #1 so that the wire that goes to connector #5 on the control unit is connected to ground through my switch instead.

Still I don't see how they get 6V on the plugs for the afterglow. How do they do that?

[Yendor]

I would say by looking at the diagram, when the afterglow stage activates glow plug relay 1 turns off and glow plug relay 2 turns on.

When glow plug relay 2 turns on this puts 12 volts on the small rail, because these 3 glow plugs aren't earth via the body the current flows through the glow plugs to the large rail then through the other 3 glow plug to ground.

This means the first 3 glow plug are in series with the rear 3 glow plugs. So depending on the resistance of the glow plug (we will assume they are the same) you will get equal voltage drop across the front and rear glow plugs.

So with a 12 volt system you will have a 6 volt drop across each group of glow plugs.

[c6Dafiy]

Hmm I just checked again and I never get 12V on the long rail, only 6V. Would that indicate that glow plug relais #1 is broken? When #1 turns on, there should be 12V on the long rail, shouldn't there?

Maybe the starter is turning somewhat slow, too, which doesn't help. And I found it's leaking from the engine coolant thermostat which I replaced a few days ago, so I'll have to do that again. Grrrrrr :(

[Silver]

activate the starter for a while, then feel for heat at the joints and terminals in the heavy wires to and from battery, starter and earth - heat showing resistance and a need for a clean and tighten. Apologies if you know that :-)

Good luck with the thermostat. I used to have a lot of trouble with an earlier car, and those generic gaskets that are in theory made for multiple housing shapes. the idea was that you'd pick the (most) correct one, and cut it free with scissors, and then pop the paper out of the bolt holes.

They were all the local parts place stocked. they never seemed to fit sufficiently well and were a frequent cause of bad language :-)

[Woof]

Would be nice if you post up an introduction mate, seeing as some of our members have assisted you.

[c6Dafiy]

Yeah I need to check the starter wiring --- one thing after another ... I'll have to order the glow plug relais and a new screw for the venting plug in the upper coolant hose. We have a holiday today so I can't do anything atm. Perhaps I can do it on Saturday ... I used some sealant out of a tube for the thermostat because there's no gasket available. It seemed to be sealed just fine at first and now it's leaking ...

At least I seem to be getting somewhere with the glow plugs; a new relais should fix the problem.

[c6Dafiy]

An introduction? Ok, I'll post one :)

[Yendor]

I would test relay 1 before replacing it. Put a voltmeter across on the Green/White and Green/Red coloured wires at the relay then turn the glow plugs on, you should get 12 volts. Then check for 12 volts on the White coloured wire.

You should also test all the glow plugs make sure none are open circuit, you will need to remove both of the rails for testing.

The most important thing to check is the coolant temperature sensor part of the circuit.

I would also make sure your battery connections are clean and tight and the main earth lead that connects to the engine is also clean and tight.

[c6Dafiy]

Yes, I've been thinking about testing it, and the problem is that the relay may seem to work without the load of the glow plugs on it and might not work with the load on it ... I should check the supply to relay #1 to see if the relay gets 12V.

If a glow plug wasn't working, the glow plug light would flash, wouldn't it?

[Yendor]

Perform the test I said above with the relay plugged in, just back probe the wire from underneath the relay mounting base.

When carrying out electrical checks the first thing you should do it check ALL FUSES and the vehicles main earths.

The fuse that supplies 12 volts to relay 1 also supplies relay 2 so I don't think that will be your problem, but still check all fuses and fusible links first.

My understanding of how this system works is,

The glow plug control unit (ECU) senses engine temperature and if it is below a certain temperature it will activate stage 1 (quick heat) and supply 12 volts to all 6 glow plugs and bring on the warning light in the dash.

Then after a certain amount of time or a certain engine temperature is reached or a certain throttle sensor reading it will switch of stage 1 and switch on stage 2 (afterglow).

In stage 2 the ECU links the glow plugs in series (via the relays) this reduces the voltage to the glow plugs and reduces the glow plugs running temperature.

In stage 2 the ECU also switches off the glow plug warning light. When the ECU sees another set temperature it switch off stage 2.

In your case for some reason it sounds like stage 1 is being skipped and it is going straight to stage 2.

I can see two possible reason for this.

1) Glow plug relay 1 is faulty, the ECU should receive a signal along the White wire (pin 25 at the ECU) this would confirm the relay has switch on or not.

2) You may have a fault with the engine coolant temp. sensor, it possible could be showing the engine is warm/hot and the ECU thinks it doesn't need to use stage 1 so it skips it and goes straight to stage 2

These are just ideas of what the faulty could possible be based on what you have told me, tests need to be carried out to confirm the fault.

If it was me I would start by checking the main earths, power supplies and battery connections, make sure battery is in good working order, check all fuses, then test each individual glow plug.

Then depending on the glow plug test results you can proceed from there.

Cheers Rodney

[c6Dafiy]

Relay #1 is definitely broken. I checked the supply to the relay and the switching voltage, and they are fine. Only the relay doesn't switch through anymore, so the 12V don't get through to the output side and thus not to the glow plugs.

I also checked on the diagnostic connector, and I have 12V there permanently when I turn the key: no idea where that is coming from, perhaps it's a different wiring for German models.

[c6Dafiy]

So I've re-done the coolant thermostat now --- hopefully it won't leak this time. I've ordered the relay, should arrive tomorrow :)

Also I put in another ground connection that goes to the hook for lifting the engine on the front. I will have to make a better ground connection for the body as well, there is only one at the battery tray ...

[Yendor]

That's great you have confirmed relay 1 as being faulty.

i'm not sure about the diagnostic connector, did you check the colour of the wire?

[c6Dafiy]

It's working fine now, starts right away when cold :) I didn't check the color of the wire on the connector.

The thermostat is leaking again :( I'm getting a new housing because it probably has a hairline crack somewhere :(

[Yendor]

I'm glad you sorted out the problem with your glow plug circuit.

If the engine still seems to be turning over too slow during cranking it might be time for a starter motor overhaul, just make sure your main battery is in good condition first.

[c6Dafiy]

That the starter was going slow might have been due to a weak ground connection: Since I put in that extra little ground wire, it has been running faster.

However, the clutch on the starter doesn't work right anymore, and considering that other parts on the starter are probably worn out, I'm getting a new one. Rebuiling it would likely cost about the same.

And I need to find out what oil to use for the diffs, transfer case and transmission and how much I need. I think I better change all the oil.

[Yendor]

That's like most things these days, it's cheaper to buy a new one.

Sorry can't help with the oil, I guessing you would require a different grade oil to us anyway