Does computer turn all glow plugs off if one is bad?

The other morning the glow plug light did not turn on, implying that none of them warmed up during initialisation. Cranking the engine didn't result in "not firing on all cylinders." So does the computer reprogram itself not to warm any of the glow plugs if a bad one was detected?


The morning before that, at 55F overnight low, the MIL did not come on while the glow plugs are warming up - at which time the glow plug light was on for a second. Cranking the engine was variable for a couple seconds as if "not firing on all cylinders." Afterwards, the MIL was triggered while the engine runs smoothly. Taking it to Autozone for an odp2 hookup returned a generic glow plug fault.

Depends on the model year. Sometime in '01 I think the harness changed from 2-wire to 4-wire. The ECU will give specific DTCs per plug on the 4-wire. Not so on the 2-wire.
Also, you should buy vcds, mang. OBD is literally useless for many, many issues that can crop up.
What year is your Jetta, anyway?

The answer is NO on both two and four wire systems...

The ECU activates the Relay(s) which send current to the GPs. And, yes, the 4 wire system is designed to allow the ECU to distinguish which GP is out of spec. The 2-wire system, by design, only allows the ECU to throw a single code indicating there is a GP issue. With either system, just a bad connection at the GP can cause a DTC.

Edit: The GPs don't heat up at 55f ambient. However, it probably would help in achieving a smoother start. The stumbling when the engine did fire is pretty much normal at that temp, especially if high mileage!

should be starting good at 55 deg with no GPs

check the timing, injection pump and cam

Here's a discussion of the exact same thing.
The light will come on just for a "bulb check". And the glow plugs operate independently of the dash light. A non-working dash light doesnt necessarily mean the glow plugs aren't powered.



Read this thread: http://forums.tdiclub.com/showthread.php?t=334473

OP needs to use a proper scan tool to get all the info.

The preglow lamp should come on when you turn the key on ALWAYS, regardless of ambient or coolant temps. If there is no preglow required, it will go back out almost immediately.

Despite the myth that persists here, despite my continued efforts to thwart its propagation, the glow system on your car works all year round, even in the summer. There are TWO distinct strategies at work here: preglow, which aids in cold starting, and afterglow, which aids in emissions reduction after the engine is started.

Preglow occurs starting around 50F coolant temps, but can actually vary depending on which engine/software by input from other sources like the MAP, as high altitude can create a more aggressive strategy.

Afterglow will happen at higher coolant temps. Even on a 70F "cold" start, afterglow can occur. And it works below 2500 RPMs and has a maximum time period, regardless of RPM or coolant temp, of around (going by memory here) 4 minutes. On the 2-channel ALH cars (1998 through the first month of 2002), you can HEAR the glow relay clicking on and off inside the car as the engine sweeps past 2500 RPM. The later 2002-2003 ALHs, as well as all PDs and CRs, have their relays outside either next to the ECU or in the engine compartment, and those are duty cycled... so you won't hear them. But the basic idea of pre- and after-glow still applies.

In any event, on any TDI (with the exception of some 1996 Passats with OBD-D), the glow system is monitored by the ECU all the time. If there is a fault in the system, the ECU will know it, and the MIL will come on.

It is possible to have a DTC for a glow plug circuit (either 2 or 4 (or 6 or 10 ) whichever your car has, but it is also possible to have a DTC relating to the control unit (relay) itself. LOTS of generic scan tools will not show these relay-related DTCs. While it is not very common for the ALH's relay to fail, it can happen. This could cause the preglow lamp to not come on when it should.

If the MIL does not come on when you turn the key on, and the engine cranks but will not start, you may have a failing relay 109, which powers up the ECU. This is pretty common... moreso on older cars, but even the newer ones are starting to age and mile up and having them randomly act up.

First things first, check the fuses to ensure all are good.

Screw myself every time this is not my first move when it comes to electrical. Usually it will throw a terminal 30 fault, however, I'm not sure if obd2 will read such a code.
At least get a generic $10 cable and vcds light, so you can access the codes. You won't have the ability to do much more other than maybe clear them, but, well worth it if you don't want to spend the money on Vag-Com at the moment. I don't know what I would do without my VCDS, definitely wouldn't know where to look, or how to solve an issue.

http://www.ross-tech.com/vcds/download/

Heck, I never answered the question in the title...

Even the 4 wire "box of magic" glowplug system will try to run all the glow plugs even if one is bad. The box has three big power mosfets (actual space on the board for four) parallelled together, the gates are all tied together so they switch as one.

The box itself is configurable for up to 6 glowplugs, so it makes some sort of sense that they'd only populate 3 of the four FETs as you're running the 'relay' case size at 2/3 of it's capacity.