voltage across glow plugs - PeachParts Mercedes-Benz Forum

funola · #1 03-05-2009, 01:38 PM

I am still chasing a no start in my 300D TURBO when engine got a cold soak overnight when temps are in the teens. All 5 glow plugs are good (each one checked with a digital ammeter- drawing over 20 amps surge, 15 amps after it gets hot.). Battery is a 3 mos old Duracrap I mean Duralast. I took some voltage readings at the glow plugs with negative lead on an injector hardline and and positive lead on a glow plug 6 mm nut. When first energized, it reads 9.4 volts, as they get hot, the voltage climbs and end at 10 volts when the glow plug relay clickes off. Do these voltages look good or are they low indicating either a weak battery or wiring?

mplafleur · #2 03-05-2009, 01:55 PM

9.4 volts seems low, but I've not measured mine.

But 20 amps at 9.4 volts is .47 ohms which is ok.

How fast does it crank?

Oracle12345 · #3 03-05-2009, 02:26 PM

who makes the glow plugs?

It could be something wrong with the fuel system might want to check there next

Number_Cruncher · #4 03-05-2009, 04:00 PM

>>Do these voltages look good

It's quite low, but as yet, it's difficult to tell if that's because the battery has gone West, or if there's a bad connection somewhere.

One way to tell is to repeat the test, but, put the positive wire onto the positive battery post (the post itself, not the terminal), and put the negative onto the glow plug 6mm nut. When the circuit is energised, you'll then measure the voltage dropped between the battery and the glow plug. If more than, say, 0.5 volts are being dropped, then, there's a bad connection somewhere.

If the voltage drop is OK, check the voltage across the battery posts while the glow plugs are energised - I would hope that the voltage would not drop to anywhere near 10 volts, which is a reasonable cranking voltage, not glow plugs as the only load.

funola · #5 03-05-2009, 07:17 PM

I did measure the voltage across the battery when the glow plugs were on and it was around 10.5 volts. However, connection was made to the battery cable teminals, not the battery posts. I will charge up the battery tonight and do the test again tomorrow making sure I connect to the battery posts and also do the voltage drop test.

Glow plugs are all Bosch. They are pretty old and may not get as hot as new ones. I did not have nearly as much trouble starting it last winter. Maybe I also have a fuel issue (a bit of air) or starter motor issue or a combination of a little of each issue?

Quote:

Originally Posted by Number_Cruncher

>>Do these voltages look good

It's quite low, but as yet, it's difficult to tell if that's because the battery has gone West, or if there's a bad connection somewhere.

One way to tell is to repeat the test, but, put the positive wire onto the positive battery post (the post itself, not the terminal), and put the negative onto the glow plug 6mm nut. When the circuit is energised, you'll then measure the voltage dropped between the battery and the glow plug. If more than, say, 0.5 volts are being dropped, then, there's a bad connection somewhere.

If the voltage drop is OK, check the voltage across the battery posts while the glow plugs are energised - I would hope that the voltage would not drop to anywhere near 10 volts, which is a reasonable cranking voltage, not glow plugs as the only load.

Oracle12345 · #6 03-05-2009, 09:32 PM

A good working system should be at least 11 -12volts at each glow plug. make sure all the wiring is good and no corrioson. Did you check the glow plugs with an ohm meter?

just because they fine in terms of voltage doesnt mean there good. Ive pull 5 bad glow plugs out of cars that failed the ohm test and passed the voltage test.

vstech · #7 03-05-2009, 09:50 PM

ok, 9.5v, sucks. that's bad supply. it looks like you do have a battery issue, or at least a connection problem. a new duralast should be doing better than that. once you verify that the battery itself is only putting out 9.5 cold on the glow, you can be assured a good battery will be putting out nearly 11v.
ok, does your car have a block heater on it? you may need it to get started in the teens...

Oracle12345 · #8 03-05-2009, 10:06 PM

Quote:

Originally Posted by vstech

ok, 9.5v, sucks. that's bad supply. it looks like you do have a battery issue, or at least a connection problem. a new duralast should be doing better than that. once you verify that the battery itself is only putting out 9.5 cold on the glow, you can be assured a good battery will be putting out nearly 11v.
ok, does your car have a block heater on it? you may need it to get started in the teens...

I have at least a 3 or 4 year old duralast in my 77 and it does the job fine, it is about $100 at autzone, one of most expensive battery they sell. on my 77 i get 11-12 volts when I first turn the car to the GP postion and it drops accordingly.

If you are getting 9.5 volts then it sounds like you get some high resistance in the plugs or the Gp wires, a bad battery, not fully charged battery @ 12.6 volts.

JonL · #9 03-05-2009, 11:19 PM

Or corroded / dirty battery terminals. Also corroded battery cables/ground straps.

Diesel911 · #10 03-05-2009, 11:44 PM

In the past I hooked a regular automotive dash type Amp Meter to 1 new Bosch Glow Plug that was not in a car and wired it to a fully charged Battery.
The Amp Meter pegged out the gauge at 60 amps (60 was the highest reading on the gauge) for 1 second or so and settled down to about a constant 16 amps after the Glow Plug was Yellow Hot.

Jeremy5848 · #11 03-06-2009, 01:15 AM

9.5 volts at the glow plug sounds a little low. The 20 amp draw is OK so maybe the problem is a weak battery. It would be interesting to see what happens if you leave the battery on trickle charge overnight and check in the morning.

funola · #12 03-06-2009, 05:27 PM

Charged battery overnight.
From start to end of glow cycle across glow plug 9.1 to 10 v

Across lead battery posts 10.4 to 11.6 V

V drop from lead battery + post to glow plug 1.6 v

Engine did not start ! Temp about 40F. starter sounded slow. Weak battery? Will bring it to Autozone for battery test tomorrow and hopefully they will refund the money like they did on the last one (Yes, I bought 2 of them!).

funola · #13 03-10-2009, 09:34 AM

I took the Duralast battery back to Autozone and the manager was sympathetic and gave me a 100% refund even though the battery tested good. Went right over to Walmart and bought the Everlast group 49 battery for $65. I made sure battery was fully charged. Took another measurement and here are the results:

Glow cycle w. meter across glow plug (ground @ injector hardline) 9.5 to 10.10 v

Glow cycle across battery posts 11.0 to 11.6

These readings are virtually the same as the Duralast battery.

Can you measure your glow plug voltage and report back? 5 glow plugs add up to around a 100 amp load which will drag the battery voltage down. I am very curious if you are really seeing 11.5 volts or higher at the glow plugs.

yellit · #14 03-10-2009, 09:59 AM

I added extra ground cables from batt negative to cylinder head/block/frame/alternator....
Also replaced GP wires with heavier cross section stranded copper cable and heavy copper crimped lugs....
That took care of the mysterious changing voltage drops....
I did the above just to rule out current path resistance in my search...
It eliminated my problem....it was the problem..... easy and cheap to do....

Number_Cruncher · #15 03-10-2009, 04:28 PM

>>Across lead battery posts 10.4 to 11.6 V

That's a bit low - the battery doesn't sound like it's very strong.

>>V drop from lead battery + post to glow plug 1.6 v

That's a bit high - you have a bad connection somewhere between the battery post and the glow plugs.

I think you've got more than 1 fault here. Replacing the battery will probably allow the engine to start - but, the bad connection will still be there.

As per Yellit's excellent post above, going through each connection between battery post and glow plug, and then the earth path back to the -ve battery post is easy and cheap to do.