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 ShopForum > Technical Information and Support > Vintage Mercedes

Reply
 
LinkBack Thread Tools Display Modes
  #1  
Old 04-03-2018, 01:31 PM
Registered User
 
Join Date: May 2008
Location: southern CA
Posts: 405
W108 Need help diagnosing bad fuel gauge reading.

The fuel gauge on my 1972 280SE 4.5 stopped working about a month ago. Now the gauge reads full whenever the key is on regardless of tank level. I have pulled the sending unit and checked the resistance, 5 OHMS when the float is at the top of the sender, 67 OHMS at the bottom, progression in between the two ends seems to be linier.

Given this, should I be looking at the gauge next? Am I checking the sender correctly by testing resistance on the workbench?

Thanks for any ideas.
__________________
1972 280SE 4.5
Reply With Quote
  #2  
Old 04-03-2018, 02:15 PM
Diseasel300's Avatar
Registered User
 
Join Date: May 2016
Location: Texas Hill Country
Posts: 3,597
The fuel sender gives the gauge a variable resistance to ground, thus at "full", there is a stronger ground reference and the gauge moves up the scale. If you have a wire to the sender that's chaffed somewhere and grounding against the chassis somewhere between the sender and the gauge, the gauge will read "Full".

Since you're getting a variable signal out of the sender, it's not likely to be the issue.
__________________
'11 Honda Accord EX - "The Daily" 60K
'83 500SL Euro - "The Money Pit" 116K
'86 300SDL - "The Diseasel" 184K
The Diseasel Thread - Everything You Didn't Know You Wanted To Know
Reply With Quote
  #3  
Old 04-03-2018, 04:54 PM
Registered User
 
Join Date: Aug 1999
Location: Florida / N.H.
Posts: 8,794
Unplug tank sensor ,

There are 3 terminals........G/W/31

With key ON, jumper W to 31....Reserve lamp should come ON.
If NO, then check fuse #5 .. [ spin it in the holder ]

Jumper G to 31..gauge should show full.

Jumper G to 31 , using a 50 ohm resistor...should show low fuel level.

If yes on all test , sender is the problem
If NO, then check that 31 is a good ground..........simply repeat the test using a jumper to a known good ground for terminal 31
__________________
A Dalton
Reply With Quote
  #4  
Old 04-03-2018, 08:14 PM
Registered User
 
Join Date: May 2008
Location: southern CA
Posts: 405
Something I should have included in my first post is that the gauge reads empty when the sending unit is disconnected. Is this relevant?

Quote:
Originally Posted by Diseasel300 View Post
The fuel sender gives the gauge a variable resistance to ground, thus at "full", there is a stronger ground reference and the gauge moves up the scale. If you have a wire to the sender that's chaffed somewhere and grounding against the chassis somewhere between the sender and the gauge, the gauge will read "Full".

Since you're getting a variable signal out of the sender, it's not likely to be the issue.
__________________
1972 280SE 4.5
Reply With Quote
  #5  
Old 04-08-2018, 11:54 AM
Registered User
 
Join Date: May 2008
Location: southern CA
Posts: 405
Quote:
Originally Posted by Arthur Dalton View Post
Unplug tank sensor ,

There are 3 terminals........G/W/31

With key ON, jumper W to 31....Reserve lamp should come ON.
If NO, then check fuse #5 .. [ spin it in the holder ]

Jumper G to 31..gauge should show full.

Jumper G to 31 , using a 50 ohm resistor...should show low fuel level.

If yes on all test , sender is the problem
If NO, then check that 31 is a good ground..........simply repeat the test using a jumper to a known good ground for terminal 31
Reserve light functions correctly.
G to 31 gauge shows full
G to 31 with 50 ohm resistor, gauge still shows full.
G to 31 with known good ground and 50 ohm resistor gauge still shows full.
G to 31 not connected gauge shows empty.

So now I am thinking it's the gauge not the sender.
__________________
1972 280SE 4.5
Reply With Quote
  #6  
Old 04-08-2018, 02:48 PM
Registered User
 
Join Date: Aug 1999
Location: Florida / N.H.
Posts: 8,794
Try a 100 ohm Resistor, just to be sure.........
__________________
A Dalton
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 02:13 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