Manual Glow Plug Switch

chaloux

Veteran Member
Joined
Aug 15, 2007
Location
Gravenhurst, Ontario
TDI
96 A6 Avant TDI Quattro, 04 Jetta Wagon
I gave up trying to figure my glow plug problem out. I went through EVERYTHING. I put in a button because a switch is easy to leave on, a button you have to consciously push. See my sig for the link (and yes, the SYSTEM IS FUSED, it simply ties into the factory system which is fused MULTIPLE TIMES - lol. Had some flak in the thread).
 
Joined
Oct 4, 2010
Location
Columbus, OH
TDI
2003
I don't know why I didn't think to do this test before...the VAG-COM software has output tests...on of which is to test the glow plug relay J52. I hooked my multimeter up to the GP harness. I ran that output test, and bam, 11.6V at the plugs. Repeated many time, but as soon as the program got to that particular test, 11.6V on the dot. So, now I know that the harness is good, that the plugs are good, that the relay is good, that the ECU can properly send the "on" signal to the relay, VCDS shows the proper temp coming from the CTS..so what the hell else is there?
 

Vince Waldon

Top Post Dawg
Joined
Apr 25, 2009
Location
Edmonton AB Canada
TDI
2001 ALH Jetta, 2003 ALH Wagon, 2005 BEW Wagon
In general flashing and tuning play with the fuel maps but that's about it... and the code to trigger the glow plugs is probably in non-volatile memory anyways.

Remind us again... you've done the "pull the connector on the coolant temp sender" test, right?
 
Joined
Oct 4, 2010
Location
Columbus, OH
TDI
2003
Yes, and when I pull the connector at the coolant temp sense the ECU instead uses the temp from the Fuel Temp. senor. It SHOULD default to -40 but it does not. When I pull the fuel harness off, disabling the fuel temp sensor, the ECU then thinks its -4.5c or so.
 

Vince Waldon

Top Post Dawg
Joined
Apr 25, 2009
Location
Edmonton AB Canada
TDI
2001 ALH Jetta, 2003 ALH Wagon, 2005 BEW Wagon
I can see it defaulting to the fuel sensor if it detects an open circuit on the coolant sensor... pretty clever actually.

When you remove the fuel sensor and the ECU thinks it's -4.5C do the plugs then come on properly... I'm guessing not?! :(
 
Joined
Oct 4, 2010
Location
Columbus, OH
TDI
2003
If either sensor is connected it reads the correct temp. If both are unplugged it goes to -4.5 but the plugs still do not go on. So it seems that as long as I provide the 5V to the ST pin on the relay the plugs should go on. If that is true, will doing so cause any sort of CEL?
 

Vince Waldon

Top Post Dawg
Joined
Apr 25, 2009
Location
Edmonton AB Canada
TDI
2001 ALH Jetta, 2003 ALH Wagon, 2005 BEW Wagon
I doubt the ECU glowplug relay output is monitored for correct voltage so probably no CEL.

The only thing I'd be worried about is backfeeding the ECU with a 5V signal where it is actually expecting a load. Often the outputs are transistor switches that won't mind being backfed, so it's a small risk... but a risk.

If you want to be really safe you could isolate your two connections to the relay with diodes (1N4001 etc) and then there's no possibility of causing a problem.

(hideous Paint drawing follows)



Besides being safe for the ECU the diodes should probably pretty much guarantee no CEL.
 
Last edited:
Joined
Oct 4, 2010
Location
Columbus, OH
TDI
2003
That is a great diagram. Question, what happens if it is more than 5V? Also, I did the adaptation this afternoon and found out that with both sensors unplugged and at -4.5c, the ECU calls for 9.95 seconds regardless of adaptation. So, no matter how high or low I changed (32850 high, 32468 low) it still called for 9.95. Also, the Air Intake Temp was reading at about 10F higher than the outdoor temp.
 

Vince Waldon

Top Post Dawg
Joined
Apr 25, 2009
Location
Edmonton AB Canada
TDI
2001 ALH Jetta, 2003 ALH Wagon, 2005 BEW Wagon
If the glow plug relay is solid-state and you give it more voltage than expected... POOF... you may well let the magic smoke out. :)
 

turbocharged798

Veteran Member
Joined
May 21, 2009
Location
Ellenville, NY
TDI
99.5 black ALH Jetta;09 Gasser Jetta
Yes, and when I pull the connector at the coolant temp sense the ECU instead uses the temp from the Fuel Temp. senor. It SHOULD default to -40 but it does not. When I pull the fuel harness off, disabling the fuel temp sensor, the ECU then thinks its -4.5c or so.
If the ECU is reading the wrong temp, then your injection timing and quantity will be completely off as well.

The glow plugs are just one side effect of a underlying problem Fix the problem not the effect.
 
Joined
Oct 4, 2010
Location
Columbus, OH
TDI
2003
If the ECU is reading the wrong temp, then your injection timing and quantity will be completely off as well.

The glow plugs are just one side effect of a underlying problem Fix the problem not the effect.
Thanks. To be honest, I'm not sure what that means exactly. I think it goes without saying that something is wrong that is causing the ECU to not turn on the glow plugs pre glow. As far I have determined that the plugs, harness, relay, connection to the relay, are good. IN addition the CTS and fuel temp sensors seem to report correct to the ECU. All I can think of is that some other input to the ECU is causing the ECU to not turn the plugs on. The air temp is reading mostly correct. I am not sure what else the ECU uses to turn on the plugs. Output tests with VCDS show that the ECU is capable of turning on the plugs just fine, it jsut does not want to do it during pre-glow. So yes, there is an underlying problem I am trying to get at. I would like to solve it, but if not I have no issue supplying 5V to the relay to turn the plugs on.
 

turbocharged798

Veteran Member
Joined
May 21, 2009
Location
Ellenville, NY
TDI
99.5 black ALH Jetta;09 Gasser Jetta
What it means is that the ECU uses the coolant temp to determine injection timing, quantity, and duration. If its getting a bad reading, it will not inject fuel correctly. Even gassers are that way.

Glow plugs are only part of the problem.
 

Vince Waldon

Top Post Dawg
Joined
Apr 25, 2009
Location
Edmonton AB Canada
TDI
2001 ALH Jetta, 2003 ALH Wagon, 2005 BEW Wagon
...and I"m not sure that it's wrong for it to default to fuel temp when it detects an open circuit on the coolant sensor. :)
 
Joined
Oct 4, 2010
Location
Columbus, OH
TDI
2003
I have decided to go with the diagram posted by Mr. Waldon above. I have myself some voltage regulators from radio shack that I will use to send 5V to the plugs. Now my question is, why kind of diodes will I need? I am decent with electrical work, but also unknowldgeable on parts, so I don't want to put in something that does do the correct job.
 

Vince Waldon

Top Post Dawg
Joined
Apr 25, 2009
Location
Edmonton AB Canada
TDI
2001 ALH Jetta, 2003 ALH Wagon, 2005 BEW Wagon
It's a low current low voltage situation so just about any small signal diode will work.

If you need an actual part number anything in the 1N400x series will work fine (1N4001, 1N4002, etc etc etc). Off the shelf at Radio Shack. :)

The diode will have a white band on it... that's the end that points towards the relay.
 

duwem

Veteran Member
Joined
Apr 11, 2012
Location
Wi
TDI
2002 Golf GLS TDI 5 Speed
You can raise the temp that the glow plugs come on at with VCDS.
 
Joined
Oct 4, 2010
Location
Columbus, OH
TDI
2003
Well, I am happy to report that after following the advice of Mr. Waldon, I have successfully set up a manual switch and verified that I now receive 12V on the glow plugs while the switch is turned on.
 
Joined
Oct 4, 2010
Location
Columbus, OH
TDI
2003
Well, progress was made in that I got the glow plugs to light up manually. However, as I had feared, I now get a CEL: P0684 Glow Plug Control Module Implausible Signal. Here is the diagram of what I did, what do you guys think? Is the GP relay just reporting back to teh ECU that it has voltage, and the ECU looks at it and says "I didn't send voltage, that signal is implausible!" Aside from this, it appears that I am having problems with my instrument cluster after doing this, the spedo does not work half the time, and the fuel gauge goes all over the place at times. Is the cluster grounded out at the ground under the dash by the relay panel? Is my installation causing a problem there and if so, how can I add ground locations so to not overload a ground location?
 

Vince Waldon

Top Post Dawg
Joined
Apr 25, 2009
Location
Edmonton AB Canada
TDI
2001 ALH Jetta, 2003 ALH Wagon, 2005 BEW Wagon
As discussed in this thread... by not using diodes you are back-feeding the relay circuitry.. with unpredictable results. :):)
 
Joined
Oct 4, 2010
Location
Columbus, OH
TDI
2003
No, there is a diode on that line as noted in the diagram, although I didn't draw it on there. It was one listed above from radio shack in the N4001-N4004 series. I think it was labeled 270V 1A. The white band on it is facing towards the GP relay.
 
Joined
Oct 4, 2010
Location
Columbus, OH
TDI
2003
Alright, but where? Just to be clear I revised my diagram below. I double checked the direction of the diode today and it is correct. with the Diode installed however, I can't see how the ECU would send a code. My ideas are:

1. There is another wire between the ECU and the relay, and it is possible that the relay is communicating with the ECU through that wire. The relay reports a signal on the modified line, but the ecu knows it never sent one, hence the ECU says its "implausible".

2. The Diode may be defective

3. The Diode is not strong enough to block the 5V coming from the switch.

4. By not having a diode on the line coming from the switch, when the ECU sends a signal to keep the ECU on post-engine start, there is a voltage drop that is enough to cause a CEL.

 

Vince Waldon

Top Post Dawg
Joined
Apr 25, 2009
Location
Edmonton AB Canada
TDI
2001 ALH Jetta, 2003 ALH Wagon, 2005 BEW Wagon
Arrow marks the spot:



which corresponds to:




I'm not saying that not having both diodes is what's causing your new symptoms (because I don't have any schematics that show me what exactly is monitored by the ECU, and where) but using two diodes is the only way to know for sure that you have completely isolated the two relay driving signals from themselves, and therefore eliminated signal back-driving as the cause. :)

BTW.... a random thought occurred to me... since you clearly understand wiring and stuff it might be worth adding a LED to your system (at the ST terminal)... so that you have an indication of when power is actually being applied to the relay. This might help you figure out if you're getting weird signals to the relay that somehow get flagged by the ECU as "implausible"? This might also help you perhaps get to the bottom of your actual problem.. which is admittedly a very weird one, no doubt.
 
Last edited:

tim1

Well-known member
Joined
Dec 18, 2010
Location
ellahurst,oregon
TDI
passat 1996
On my 96, I made a whole new circuit form battery to fused relay with button on dash with indicator light. Works great. BUT, only hold in for 6 seconds or else can ruin glow plug. Ask me how I know! Tim
 

gmenounos

Vendor
Joined
Jun 26, 2003
Location
Watertown, MA, USA
TDI
'99.5 Golf GLS, '01 Jetta GLX Wagon (TDI conversion)
There is another wire between the ECU and the relay, and it is possible that the relay is communicating with the ECU through that wire. The relay reports a signal on the modified line, but the ecu knows it never sent one, hence the ECU says its "implausible".
I believe that the ST line is used by the ECU to control the glow plugs by sending a pulse-width modulated signal. The greater the duty cycle (% time that ST is 5V), the more power to the plugs. I think that the D1 line is used to send glow plug error codes back to the ECU so that if a plug fails, it knows which one.

Here's the data sheet for a glow plug controller chip, which I think is similar to what is inside the VW glow plug relay used in 2002+ cars.

http://www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/DATASHEET/CD00134658.pdf

If you look at pages 16 and 17, you'll see that there's a a failure register inside the chip that can hold the status of the individual glow plugs. As the ECU pulses the control line, not only does that cause the glow plugs to get current, but it also causes the contents of the failure register to be sent back to the ECU on the status line, in sync with the pulses on the control line.

So my guess is that by holding the control line at constant 5V for any amount of time, it's preventing the glow plug status from being returned to the ECU and that's causing the ECU to generate the error code that you're seeing.

Greg
 

Vince Waldon

Top Post Dawg
Joined
Apr 25, 2009
Location
Edmonton AB Canada
TDI
2001 ALH Jetta, 2003 ALH Wagon, 2005 BEW Wagon
Great info, thanks for taking the time to post.

Your theory sounds very plausible... and good catch on the PWM. Having said that, I dunno if the OP has the two-wire old-skool on-off drive or the later four-wire PWM harness.
 
Joined
Oct 4, 2010
Location
Columbus, OH
TDI
2003
I have the 4 wire harness. That is really interesting to be honest. So if I understand this correctly, the GP relay will send back the status to the ECU, but it will do so only when there is no voltage on the ST line, so it will send back the status between pulses or when there is no signal on the line? And by supply a constant 5V the relay does not send the status for a long enough period of time that the ECU thinks something is wrong?

So the question is, how can I fix it? I would imagine that I would need to find a way to pulsate my manual signal to the relay.

Or perhaps, the best thing to do would be to just start work on modifying the GP harness and just wire up power from the battery into the GP harness through a switch. Of course, seeing as the GP relay reports the status of the plugs back to the ECU that may also put out a code.
 
Last edited:
Top