Glow Plug Voltage for diesel engine?

[280z1975]

I know this isn't a diesel engine forum, but this is more a general question about glow plugs and the voltage they usually use. I am not on any other forums for cars other than ClassicZcar and this place seemed better to ask the ?.

 

Right now my 1993 Mazda 626 is getting only 1.43 volts when measured at 12v on my multimeter ... this seems quite low to me ... I suspect that they are not lighting up properly to combust the fuel. I can smell fuel and also there is a good puff of white smoke that comes out of the exhaust. At the moment I suspect the relay might have gone bad. BTW, the glow plugs are all new and the battery is solidly connected.

 

-Gregg-

[eec564]

I've owned two diesel mercedes benz and worked on a lot of diesels such as suburbans and boats. On the ones I've encountered when the glow plugs are operating you'll see perhaps 7-8 volts across them, on the engine, generally more depending on how your electrical system is. If you disconnect the glowplugs you should get 12v at the wires that power each of them, and the plugs themselves should measure around 0.8 ohms.

 

Again, this is from working on my mercedes and stuff, a mazda may be different, but if you have the service manual take a look at the engine schematic and you should be able to see if the glow plugs are powered off a heavy duty relay that's ties them straight to the battery through their own fuse.

 

Something to take special note of is it can be very difficult to get a good contact between your multimeter probes and the glowplugs or wires under the hood. The odds are everything has a very slim coating of diesel and any dirt/grease sticking to it had formed a tough layer of insulation. So clean stuff with a wire brush then take measurements again.

 

Also, if the car fires RIGHT up once the glowplugs have been allowed to stay on until the wait-to-start light goes out, then they aren't the problem. When does the car smoke and where do you smell the fuel coming from?

 

-Eric

[280z1975]

I've owned two diesel mercedes benz and worked on a lot of diesels such as suburbans and boats. On the ones I've encountered when the glow plugs are operating you'll see perhaps 7-8 volts across them, on the engine, generally more depending on how your electrical system is. If you disconnect the glowplugs you should get 12v at the wires that power each of them, and the plugs themselves should measure around 0.8 ohms.

 

Again, this is from working on my mercedes and stuff, a mazda may be different, but if you have the service manual take a look at the engine schematic and you should be able to see if the glow plugs are powered off a heavy duty relay that's ties them straight to the battery through their own fuse.

 

Something to take special note of is it can be very difficult to get a good contact between your multimeter probes and the glowplugs or wires under the hood. The odds are everything has a very slim coating of diesel and any dirt/grease sticking to it had formed a tough layer of insulation. So clean stuff with a wire brush then take measurements again.

 

Also, if the car fires RIGHT up once the glowplugs have been allowed to stay on until the wait-to-start light goes out, then they aren't the problem. When does the car smoke and where do you smell the fuel coming from?

 

-Eric

 

The won't fire right up once the glow plugs have been 'warmed' and the light is out.

 

The smell of fuel is from the exhaust I believe ... which I tend to think is the extra fuel not igniting.

 

I have a manual for the car, but it's in German (which is hard to read with all the technical names being very different). The wiring diagram has it going through a relay.

 

When I measure the voltage at the one wire which directly connects to all the glowplugs it's only 1.4V, way off even from the 6V it might be operating at ... I think today I will take one glow plug out and turn on the system to see how well it lights up and gets hot.

 

-Gregg-

[eec564]

The glowplugs operate at 12v, not 6m unless your regular car battery is 6v. Check to make certain your battery is fully charged. If the light on the dash doesn't light up that means one or more of your glow plugs do not work or something is wrong with the relay system. Pull the wires off that power the plugs and measure each ones resistance. Bad ones will read high or infinite (open, no reading) ohms, or a perfect dead short (0 ohms). You can measure them on the engine by checking between a GOOD ground and the post the power wire connects to (cleaned) with the wire off.

 

If there are any bad, replace them, then see where you strand. If they are all good, then there's a problem with a loose connection or the relay system. How long do you have to crank it before it'll start? Or will it not start at all? Generally, automotive diesel engines will start without glowplugs (in moderate to warm weather) once they've been cranked enough to build up decent oil pressure.

 

 

Attn admin: Perhaps this should be moved to misc tech or trouble shooting/general?