What is the correct voltage for Glow Plugs?

9th August 2011, 12:50

Having trouble cold starting my 1.5 Diesel. Cranking for 30+ seconds will start it. When it starts, a load of smoke comes out then it works fine. The longer I then run the engine, the easier it is to start.

One possibility is that the glow plugs aren't working right. Have disconnected the feed and tested it's voltage. When ignition is on, it stays constantly at 3volts (never seems to stop).

Haven't ruled out air getting into the system though, and thinking of changing the bleed / return piping from the injectors... But first want to rule out the glow plug system...

Thankyou in advance!

9th August 2011, 13:05

PS - the glow plug light comes on and goes off normally, while the glow plug leads stay at 3v consistantly.

9th August 2011, 13:17

PS - this was tested from when the car had been driven for 30 mins, so the engine was warm (would this prevent the glow plugs coming on fully?)

9th August 2011, 22:24

Update

Have checked the feed to the glow pug control box (relay?). It is getting 12.75 volts in, but when the glow plugs are actuated it only gives 3.00 volts out.

This was done with a cold engine.

PLEASE CAN SOMEONE CONFIRM THE VOLTS THAT THEIR DIESEL GETS TO THE GLOW PLUGS ON PREHEAT?

10th August 2011, 13:06

Update - ran a jump lead from battery +ve to glow plugs for a few seconds before starting this morning (from cold) and it started fine... definetly seems to be the preheat system. Am thinking either Control Unit, or the thermal sensor on the engine...

Please can someone still check the voltage given from the control unit? Am pretty sure that 3v is wrong but would be nice to hear from someone else...

10th August 2011, 13:21

Hi there,

It's been a while since I went rummaging around the glowplugs of a 1.5D so a lot of what I'm about to say is sketchy at best but 3v is far too low as I recall. Is this across all 4 supply cables? Sometimes the wires can get corroded and the resistance consequently shoots up so if the 3v reading is localised to one or two wires then it's likely the glow plug loom will need replacing - I take it the glowplugs have been replaced during the diagnostics? If not check the resistance across them. It's should be a pretty low reading (i.e. single digit Ohms), I realise that isn't too helpful but test all four and take the lowest resistance as being the 'correct' resistance.

If those options have already been covered chances are that there is a control unit error/failure. It's extremely basic, it's just a glorified relay if I recall correctly. Although I've never needed to investigate the relay in any great detail - so I can't help you further.

Good luck and let me know the outcome.

Adam

10th August 2011, 17:45

Seems I got the voltage supplied by the Control unit wrong, it's only sending 0.3volts! (The input is definetly 12.x volts, but when activated only gives 0.3volts). I am thinking the unit is faulty... Have tried checking all the connections / filing them to be nice and shiny but this hasn't helped.

12th August 2011, 20:16

Just to let you know, .3v looks like the voltage the circuitry in the control unit works at. Have bought a new unit for £14.50 on ebay... assume it works else I will post back.