glow plug rellay

[merk]

I wonder how ECU knows a glow plug is bad? ECU only controls timing by sending a signal to the glow plug rellay that switches higher current to the glow plugs. Rellay circuit as i see it purely mechanical rellay, so I guess ECU has no back info about current not being drawn by the plug.

For example if engine is cold and one plug is detached there could be a missfire at the begining and ECU could detect that by monitoring spikes in the cranksfaht speed. But this will turn on missfire light etc.

In my case (one plug being broken ) ECU turns on glow plug problem light imidietly, you don't even need to start the engine. The light turns on even if I connect a new glow plug to the connector and plug glows (just plug itsels isnt mounted into the head).

Any ideas?
Relay is tested with light bulbs and is fine.

 

[narkeleptk]

Usually the logic in ecu's for relay controls are monitored to see if its low when it should be high (shorted) or if its high when it should be low (open) and throw a code based on that.

I have seen some logics dislike certain relays as well so maybe the system is monitoring more then just high/low signals and sensing current based on the relays coil resistance.

Hard to know much with out knowing the year/model. The ecu systems are quite different. It may not even be a control signal issue.

 

[shokjok]

Is there any internal method of the ECU monitoring the output current to the glow plugs?
If so, there shouldn't be engine turnover.

 

[Nigel Goodwin]

Is there any internal method of the ECU monitoring the output current to the glow plugs?
If so, there shouldn't be engine turnover.

Presumably you've little experience with diesel vehicles?, poor starting in cold weather due to failed glowplugs has always been a very common problem. It'd certainly be a bit of a disaster (and incredibly poor design) if the engine wouldn't even turn over if a glow plug failed - I've seen plenty of diesel cars that still started (with difficulty) in cold weather with only one functional glow plug.