To glow or not to glow.

[RichD]

I'm just trying to figure out the "why" of it

I can appreciate your curiosity. I've not seen the schematic for the glow plug circuit board nor am I an electrical engineer, but when I say impedance I mean that if the glow plug portion of the circuit has the wrong impedance - more or less than expected - you can end up with an unexpected result. I've seen many cases where a circuit will not behave exactly the same again until remaining power in it has been discharged. A lot of people assume that a circuit board is not live when the power source is off but there are these things called capacitors...

In troubleshooting problems (particularly on the clock), its important to ignore the why and focus on the results. Sometimes I get lab time to tinker and enjoy the luxury of chasing down the "why's" of things but most of the time the job has to get done and pragmatism prevails.

[Green1]

I have more than my share of electrical knowledge, however my understanding of this particular circuit is that it is simply relay controlled, the controller doesn't come in to the equation because the relays provide the power.
I can see why an incorrect impedance would cause the system not to work, I just don't see how it would "not work" in the way described.

[Spearo]

The dealer has kindly offered to refund my money. I should note that the other products I bought were perfect, thanks Steven.

Cheers,
Erik

[jaggedfish]

Just to add my 2 cents to the fray... I just recently had somewhat similar issues with the glow circuit and the problem was, in fact, at least one bad glow plug. This caused the glow cycles to shorten and sometimes not work at all. My suspicion would be that you had the 12 volt plugs instead of the 6 volt as mentioned earlier or one of the new plugs was a bum one. Anyway, glad to hear you have resolved your issue.

A further note... as far as I recall... relays "switch" power, not "provide" it... sorry Green1, had to throw that in there... light hearted comment.

[Spearo]

A further note... as far as I recall... relays "switch" power, not "provide" it... sorry Green1, had to throw that in there... light hearted comment.

Now you've done it!

[Green1]

A further note... as far as I recall... relays "switch" power, not "provide" it... sorry Green1, had to throw that in there... light hearted comment.

I work in an environment where power to any component is often considered to be "provided" by the component ahead of it in the circuit, and while technically you are correct that relays only switch power and do not "provide" power, it is also true that when looking at the circuit from the point of view of the glow plugs, they can't tell the difference, all they "know" is that the power comes from "that direction" on the wire (and for now we'll ignore the difference between conventional and electron current flow as it is really not related to this discussion).

The point though was that if it is relay controlled, then none of the sensitive circuitry can tell the impedance of the plugs as they are thoroughly isolated from them. The only things that incorrect impedance should affect are: how quickly the plugs heat up (or if they heat up at all) (which could cause all sorts of starting issues), and how quickly the fuse at the battery blows. For the impedance to cause the circuit to function such that there is no power provided to the plugs, WITHOUT blowing the fuse, that implies that it is NOT a simple relay providing (or for those who require more accuracy "switching") the power to that bus-bar. That becomes a very interesting and relevant point for ALL glow circuit troubleshooting in future, and hence why I would like to understand better what is actually happening.

[jaggedfish]

Grinning evilly! Lol! Figured that might inspire a good response...

Now to that response, it raises a point... In the case of my problem, the circuit actually did begin to cycle (relay was "switched" on) and thus would have provided "feedback" to the control unit. The issue seems to be that one bad plug causes incorrect "feedback" which then seems to have caused the control unit to shorten the cycle and not heat the plugs enough to allow a good start.

The obvious solution is to find the bad plug and replace it but in order to understand how the system functions, we need dialogue like this.

The temperature sensor apparently also comes into play somehow too but that is somewhat separate from this.

Cheers!

[mararmeisto]

My turn

E=IR

First: it's a DC circuit, so there is no impedance in the circuit due to a lack of frequency (other than zero hz). There is only resistance. If one were to consider the initial near-instantaneous charging time of the circut, then there is a period of time which one could say there is a 'frequency' (due to the change in voltage over time), but this disappears once the voltage reaches full potential. Generally, in a simple circuit such as this (i.e. a non-radio high-frequency circuit), this initial time is not factored into the design, therefore no impedance.

Second: I understand these circuits to be current-controlled or current-sensing, not voltage-controlled (the voltage doesn't change in the circuit, the current does - this is how it knows when the plug is hot enough). If one changes the glow plugs for the wrong voltage, generally the resistance of the plug is not correct, so the current flowing through it is either too low (glow plugs stay on, the circuit only shuts off when it times out) or the current is too high (glow plugs heat up quickly and the circuit shuts off, usually too quick to hear the second click, so you think it's not come on at all). This is a similar problem to when a turn signal bulb burns out and the signal flasher clicks faster (the current draw goes down, so the relay 'clicks' faster to tell you a bulb is out) - same thing happens when you change the bulbs to LEDs (they don't draw enough current, so the flasher relay thinks there's a bulb out and flashes faster).

If you have a bad plug, resistance of the system is off, which means the current will be wrong, which means the circuit won't cycle properly. Because the four glow plugs are ganged together with the bus bar, one bad plug messes with the whole circuit.

The temperature sensor is used to tell the ECU if the engine is warm enough to not require a glow plug cycle. This component doesn't sound like it's a part of your problem.

NOTE: I think I've got my inversly-proportional resistance effects right-way-around, but if not I apologise - I pulled this all off the top of my head and I'm answering submarine questions at the same time so I'm a little distracted (with this).

[Green1]

impedance = resistance in DC circuits

I understand how all of this COULD affect a circuit, I just don't understand how it can affect THIS circuit.

My understanding is that we have a relay (or 2 relays) switching the power for this circuit. Is there another module or wire somewhere attached to the power side of this circuit that feeds back in to the controller that I'm missing somewhere? where is this feedback coming from?

ie: if we have the following circuit:

battery -> relay -> bus-bar -> plugs -> ground

And the relay is opened and closed by the glow controller based on input from the temperature sensor and a timer, then where is the wire that would tell it not to close the relay if the resistance of the plugs were wrong? (is there another component that I'm missing? and if so, where in the circuit does it connect?

[mararmeisto]

impedance = resistance in DC circuits

There is a common misunderstanding between the seemingly basic terms Resistance and Impedance. They both regard the same concept: the opposition to the flow of electrical current. That is in fact the complete definition for Resistance. Impedance, however, is a little more complex: it is the resistance of a component at a given frequency. That difference is an important one.

My understanding is that we have a relay (or 2 relays) switching the power for this circuit. Is there another module or wire somewhere attached to the power side of this circuit that feeds back in to the controller that I'm missing somewhere? where is this feedback coming from?

ie: if we have the following circuit:

battery -> [glow plug control] relay -> bus-bar -> plugs -> ground

And the relay is opened and closed by the glow controller based on input from the temperature sensor and a timer, then where is the wire that would tell it not to close the relay if the resistance of the plugs were wrong? (is there another component that I'm missing? and if so, where in the circuit does it connect?

The glow plug control relay is itself controlled by the ECU, which has inputs from the temperature sensor and contains the timing circuit.

The current-sensing is either a function of the control relay or a line from your indicated circuit is fed back to the ECU to adjust the timing. I haven't looked at the circuit drawings in a while, so I don't quite recall.

@Spearo: While the voltage rating is important, equally so is the resistance because it will affect the rate of electron flow (current) through the plug, which will affect how quickly or not the plug heats up. A glow plug is no difference than the coil of wire inside the slot of your toaster: electricity flows through it and the coil heats up, toasting your bread. Electricity flows through the glow plug and heats up the tip, which in the 'hot spot' in the cylinder which assists in the combustion of the fuel. Once the cylinder is warm, the residual heat from the walls is sufficient to keep the combustion cycle going, which is why the glow plugs are turned off (no longer needed).

[Profister]

impedance = resistance in DC circuits

This is absolutely correct. If X=0 then Z=R. Without ECU schematics this discussion is a 'much ado about nothing'. If the glow plugs in question do not work then get the ones that work for sure and do not waste your time. This is what I did when I bought glow plugs from Lordco.

[Green1]

impedance = resistance in DC circuits

There is a common misunderstanding between the seemingly basic terms Resistance and Impedance. They both regard the same concept: the opposition to the flow of electrical current. That is in fact the complete definition for Resistance. Impedance, however, is a little more complex: it is the resistance of a component at a given frequency. That difference is an important one.

The difference is only important in AC circuits. DC does however have a frequency as well (zero) and as such impedance = resistance in DC circuits.

The glow plug control relay is itself controlled by the ECU, which has inputs from the temperature sensor and contains the timing circuit.

apart from the term "ecu" we agree here (it isn't an ECU, the delica doesn't have an ECU, it's simply a glow plug controller)

The current-sensing is either a function of the control relay or a line from your indicated circuit is fed back to the ECU to adjust the timing. I haven't looked at the circuit drawings in a while, so I don't quite recall.

And that's the question, WHAT is doing this sensing? (and where) this is definitely relevant for future because it is another possible point of failure to watch out for.

[mararmeisto]

[T]he delica doesn't have an ECU, it's simply a glow plug controller)

It's the box in the driver's door pillar.

[Green1]

[T]he delica doesn't have an ECU, it's simply a glow plug controller)

It's the box in the driver's door pillar.

yup, it's a glow plug controller. An ECU controlls the whole engine, the glow plug controller only controls the glow plugs. (easilly proved too, remove it completely, and the glow plugs stop working, but everything else continues to work just fine)

[Erebus]

No is a good time to throw in my (minor) problem.

About a year ago I replace my glow plugs with new OEM plugs

In about Feb of this year, it started doing:
*When the engine is stone cold, glow plug circuit works completely normal
*when not fully warmed up, circuit works normal, either 2 clicks a few seconds apart, or 2 clicks very close together
*when fully warm, and it has sat for 5-10 minutes, no clicks at all. Given it is hot, it starts just fine.

Any theories?

FYI, CVI told me a year ago that when the engine is fully warm, there is no need to wait for the glow controller, I could treat it like a gas engine and just start it, which worked just fine. I've stopped doing that since the above symptoms showed up.

Think it might be related to the discussion?