Parts Catalog Accessories Catalog How To Articles Tech Forums
Call Pelican Parts at 888-280-7799
Shopping Cart Cart | Project List | Order Status | Help




Go Back   PeachParts Mercedes-Benz Forum > Mercedes-Benz Tech Information and Support > Diesel Discussion

Reply
 
LinkBack Thread Tools Display Modes
  #1  
Old 03-05-2009, 02:38 PM
funola's Avatar
Registered User
 
Join Date: May 2005
Location: NYC
Posts: 8,112
voltage across glow plugs

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?
__________________
85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked
Reply With Quote
  #2  
Old 03-05-2009, 02:55 PM
mplafleur's Avatar
User Friendly
 
Join Date: Sep 2001
Location: Lathrup Village, Michigan
Posts: 2,939
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?
__________________
Michael LaFleur

'05 E320 CDI - 86,000 miles
'86 300SDL - 360,000 miles
'85 300SD - 150,000 miles (sold)
'89 190D - 120,000 miles (sold)
'85 300SD - 317,000 miles (sold)
'98 ML320 - 270,000 miles (sold)
'75 300D - 170,000 miles (sold)
'83 Harley Davidson FLTC (Broken again) :-(
'61 Plymouth Valiant - 60k mikes
2004 Papillon (Oliver)
2005 Tzitzu (Griffon)
2009 Welsh Corgi (Buba)

Reply With Quote
  #3  
Old 03-05-2009, 03:26 PM
Registered User
 
Join Date: Jul 2007
Location: USA
Posts: 2,156
who makes the glow plugs?

It could be something wrong with the fuel system might want to check there next
__________________
1986 300SDL, 211K,Dealership serviced its whole life
1991 190E 2.6(120k)
1983 300D(300k)
1977 300D(211k)
Reply With Quote
  #4  
Old 03-05-2009, 05:00 PM
Registered User
 
Join Date: Apr 2007
Location: UK
Posts: 357
>>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.
Reply With Quote
  #5  
Old 03-05-2009, 08:17 PM
funola's Avatar
Registered User
 
Join Date: May 2005
Location: NYC
Posts: 8,112
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 View Post
>>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.
__________________
85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked
Reply With Quote
  #6  
Old 03-05-2009, 10:32 PM
Registered User
 
Join Date: Jul 2007
Location: USA
Posts: 2,156
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.
__________________
1986 300SDL, 211K,Dealership serviced its whole life
1991 190E 2.6(120k)
1983 300D(300k)
1977 300D(211k)
Reply With Quote
  #7  
Old 03-05-2009, 10:50 PM
vstech's Avatar
DD MOD, HVAC,MCP,Mac,GMAC
 
Join Date: Mar 2006
Location: Mount Holly, NC
Posts: 26,180
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...
__________________
John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 190D 2.5Turbo
1987 190D 2.5-5SPEED!!!

1987 300TD
1987 300TD
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!
Reply With Quote
  #8  
Old 03-05-2009, 11:06 PM
Registered User
 
Join Date: Jul 2007
Location: USA
Posts: 2,156
Quote:
Originally Posted by vstech View Post
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.
__________________
1986 300SDL, 211K,Dealership serviced its whole life
1991 190E 2.6(120k)
1983 300D(300k)
1977 300D(211k)
Reply With Quote
  #9  
Old 03-06-2009, 12:19 AM
Registered User
 
Join Date: Nov 2008
Location: Long Island
Posts: 578
Or corroded / dirty battery terminals. Also corroded battery cables/ground straps.
__________________
1987 W201 190D
Reply With Quote
  #10  
Old 03-06-2009, 12:44 AM
Diesel911's Avatar
Registered User
 
Join Date: Sep 2007
Location: Long Beach,CA
Posts: 24,821
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.
__________________
84 300D, 82 Volvo 244Gl Diesel
Reply With Quote
  #11  
Old 03-06-2009, 02:15 AM
Jeremy5848's Avatar
Registered Biodiesel User
 
Join Date: Mar 2006
Location: Sonoma Wine Country
Posts: 8,393
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.
__________________

"Buster" in the '95

Our all-biodiesel family
1995 E300D (W124) . .239,000 miles My car
1996 E300D (W210) . .313,000 miles Wife's car
Santa Rosa population 170,685 (2012)
Total. . . . . . . . . . . . 722,685
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970
Reply With Quote
  #12  
Old 03-06-2009, 06:27 PM
funola's Avatar
Registered User
 
Join Date: May 2005
Location: NYC
Posts: 8,112
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!).
__________________
85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked
Reply With Quote
  #13  
Old 03-10-2009, 10:34 AM
funola's Avatar
Registered User
 
Join Date: May 2005
Location: NYC
Posts: 8,112
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.
__________________
85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked
Reply With Quote
  #14  
Old 03-10-2009, 10:59 AM
Registered User
 
Join Date: May 2004
Location: Flowood Mississippi
Posts: 438
Voltage Drop

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....
__________________
1978 Yellow 300D (The Mustard Toad)
1980 Blue 240D (The Iron Toad)
1989 Grey Mitsu.4WD Mighty Max Pickup (Needs a Diesel transplant bad)

(Open the pod bay doors HAL)
Reply With Quote
  #15  
Old 03-10-2009, 05:28 PM
Registered User
 
Join Date: Apr 2007
Location: UK
Posts: 357
>>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.
Reply With Quote
Reply

Bookmarks

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is On
Trackbacks are On
Pingbacks are On
Refbacks are On




All times are GMT -4. The time now is 05:14 PM.


Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2018, vBulletin Solutions, Inc.
Search Engine Optimization by vBSEO 3.6.0
Copyright 2018 Pelican Parts, LLC - Posts may be archived for display on the Peach Parts or Pelican Parts Website -    DMCA Registered Agent Contact Page