87 300D Fuel Gauge Gremlins - PeachParts Mercedes-Benz Forum

Tpawlik · #1 08-02-2010, 12:39 PM

I just stumbled onto the great article that Jeremy 5848 wrote a couple of years ago. My needle is not jumping around. When I fill the tank the gauge reads full. After about 350 miles the gauge reads 3/4. When the needle reaches the 3/4 mark the needle plummets to empty and the reserve light comes on. I removed the sender and left the wires connected. When I tilt the sender to make the float move up and down in the tube. I can make the gauge read empty and the light comes on, but when I tip it the other way the gauge still reads 3/4 instead of full.
Has anyone had this problem? I am going to take the unit apart and see what it looks like in there.
Thanks to Jeremy for the great info.

Jeremy5848 · #2 08-02-2010, 01:02 PM

Quote:

Originally Posted by Tpawlik

I just stumbled onto the great article that Jeremy 5848 wrote a couple of years ago. My needle is not jumping around. When I fill the tank the gauge reads full. After about 350 miles the gauge reads 3/4.

Do you mean 3/4 full or 3/4 empty? 350 miles should put the tank down to about 1/4 full.

Quote:

Originally Posted by Tpawlik

When the needle reaches the 3/4 mark the needle plummets to empty and the reserve light comes on. I removed the sender and left the wires connected. When I tilt the sender to make the float move up and down in the tube. I can make the gauge read empty and the light comes on, but when I tip it the other way the gauge still reads 3/4 instead of full.
Has anyone had this problem? I am going to take the unit apart and see what it looks like in there.
Thanks to Jeremy for the great info.

Definitely open up the sender and clean it out, make sure the wires are tight and the contacts clean. Also, I wonder if your gauge (in the instrument cluster) might be hanging up, sticking somehow, or has a poor connection somewhere?

Jeremy

Tpawlik · #3 08-02-2010, 01:51 PM

3/4 full. I knew something was wrong when I had driven 150 miles and the needle barely moved off the full mark. From what I read, a poor ground could contribute to the problem. I will follow your procedure and let you know if I can fix it.
Thanks.

Jeremy5848 · #4 08-02-2010, 02:42 PM

If it comes down to it, I should be able to get you another sender from the local Pick and Pull for about $20 shipped. It will be from a gasser, but they are identical to the diesel versions and tend to be cleaner. Take yours apart first, though, no point spending money if you can fix what you've already got.

The instrument cluster is full of hidden 'gotchas.' The three main components (gauges, speedo, tach/clock) have connections that use strips of printed circuit board material that touch each other when everything is screwed together. These strips constitute the ground that runs through the system. If the plating or tinning (I'm not sure what they use) is damaged or corroded or simply dirty, or if a screw is missing or loose, a connection won't be secure. There is no guide to what will or won't work based on one or more damaged or missing connections. One of these days I really must go through my spare cluster and photograph all of the critical points so people know what to look for.

Those of us with knowledge in any specialized field relating to our cars really have a duty to make this information available to everyone else. Certainly Mercedes hasn't done that -- they expect you to be born with the knowledge or to be a factory-trained mechanic. It absolutely scares me to death, how much I know about electrical systems and knowing there's an equal or greater body of knowledge on (for example) IPs, about which I know almost nothing.

Jeremy

Tpawlik · #5 08-02-2010, 03:07 PM

I may take you up on the offer for a used sender, if I cannot save mine. I was expecting mine to look a little dirty or cracked like your was. Mine looks like it is new. Makes me think someone else tried to fix the problem by installing parts. I was thinking about tapping the ground wire on the sender harness and connecting it to a ground lug in the trunk that already has several ground wires attached. Just to rule out a bad ground, but I will check out the ground strip on the back of the instrument cluster. I am just completing a frame off restoration on a 62 Olds Starfire. It has taught me that proper grounds are essential to make electrical components behave properly. I'm only good at fixing things I can see...I cannot "see" electricity and therefore absolutely hate electrical problems.

Jeremy5848 · #6 08-02-2010, 04:02 PM

Your Starfire projects sounds like a lot of fun. I had a '65 Cutlass for many years so am familiar with the marque.

Mercedes did a good job with the grounds in the car; they are well made and properly marked in the manual. A lot of things like the instrument cluster, however, were subcontracted out to VDO and Bosch and so forth, and the subs did not always do as good a job as the Mercedes engineers in designing reliable electrics.

You don't have to "see" electricity to understand it. Think of it as water flowing through a pipe. If the pipe has a restriction in it, the flow is reduced. If there is a hole, you have a leak. As analogies go, it's not too bad.

Jeremy

sixto · #7 08-02-2010, 04:56 PM

Would you rather be burned or electrocuted?

Sixto
87 300D

Jeremy5848 · #8 08-02-2010, 06:52 PM

Or drowned?

Tpawlik · #9 08-05-2010, 08:18 AM

Well, I took the sender apart, the shaft that the float rides up and down on was a little dirty but the float traveled up and down easily, I cleaned it anyway. Lightly cleaned the three contacts on the sender with some 800 grit sandpaper. The two wires on the sender looked OK. Put it back together. Now the gauge reads full and the reserve light is on even though there is only 10 gallon of fuel in the tank. I did not start the car, only had the key in the on position. Don't know if starting the car might make the reserve light go out.
I did not check the ohm values on the sender or the ground strip at the instrument cluster. I will do that next and let you know.

bobodaclown · #10 08-05-2010, 11:54 AM

When you first turn on the vehicle the reserve light comes on for about 10 sec or so. It test the light so you know it works. After a period of time it should go out.

Jeremy5848 · #11 08-05-2010, 12:19 PM

Bobo is correct. All of the warning lights come on in preglow and (should) go out after the engine starts. So the low-fuel light is not a worry. OTOH, the gauge should not read full with only ten gallons of fuel in the tank. If you are satisfied with the way the float slides up and down, then the sender is apparently working correctly and the problem is elsewhere -- the gauge itself or a bad ground or poor connection.

BTW, if you have a DVM you can check the resistance of the sender. With the float at the top (tank full) the resistance should be near zero; with the float at the bottom (tank empty) the resistance should be about 100 Ohms. This is based on a quick measurement I just did on a spare sender for my W124.

You can also test the gauge by disconnecting the sender. The sender has three metal pins and a plastic locating pin. With the plug disconnected from the sender, and the key on (car running or not), the gauge should read zero. With the top two pins shorted together with a wire, (these two pins are farthest from the plastic pin) the gauge should read full. If it does not do these things then there is a problem with the gauge or the wiring.

Jeremy

Tpawlik · #12 08-05-2010, 08:39 PM

I took the sender apart again and tested the ohms with the float at the bottom (empty) and then slowly moved the float up the shaft. With the float touching the base where the wires connect I had 94.5 ohms. When I raised the float, as soon as it broke contact with the base, I had 0 ohms reading the entire length of the shaft. There is a little contact plate on the bottom of the float is really loose. Don't know if that's a factor?
Connected the sender to the connector and had an empty gauge reading with reserve light on, shorted the two top terminals and the gauge jumped to full and the reserve light went out. Is my resistor wire not resisting? I'm getting really tired of taking this thing apart!!! If you think it is the sender and still have a spare sender, I'd like to buy it.
Either way, I really appreciate your help and the info that everyone else provided.

Tpawlik · #13 10-14-2010, 05:19 PM

I still need help. I ohmed the sender and it performs as Jeremy said it should.
So, it seems that the guage is at fault. I checked the ground wire from the sender connector to the body and it is good. When the tank is full the gauge reads full. It only moves to the 3/4 full mark after about 400 miles then, when it get's to the 3/4 full mark it goes straight to empty and the reserve light comes on. Another function that seems correct.
I removed the speedo cable and looked behind the cluster for the ground but, did not see it. I was disuaded from completely removing the entire cluster by the number of wires connected to it. I will have to just do it to confirm the ground.
If the gauge is bad, is just the fuel gauge available or do I have to buy all the gauges in the pod?

Jeremy5848 · #14 10-14-2010, 08:58 PM

It sounds like the needle of the fuel gauge is physically dragging or hanging up on something. Have you tried gently thumping the face of the instrument cluster to see if you can get the gauge to jump down to about 1/2 after 200 miles or so?

The gauge and the "low fuel" light have separate wires from the sender but the ground is common, so you could have a problem with the ground wire or the ground connection on the instrument cluster. The instrument cluster is made of several small PC boards with components like the gauges, jumper wires, plugs, etc., soldered to them. The solder joints can fail over time; you can usually detect problems by carefully examining the sections with a bright light and a magnifying glass. The solder around a connection will appear cracked; this is sometimes called a "cold" solder joint and can be resoldered.

A picture of the parts of the instrument cluster is here: Ground and shows two ground points that interconnect the different parts of the instrument cluster. They have to be shiny clean or the cluster won't work well. I doubt that is your problem because the ground for the fuel gauge is pin 1 of the 15-pin round plug attached to the instrument cluster. If the other gauges work correctly then the ground is probably OK.

You can disconnect all of the wires and remove the cluster to your work bench. The various plugs are polarized (only fit one way) and each one is different so you can't re-install them in the wrong place. The bulbs with pigtail wires on the lower right side should be numbered 1 through 6 with number tape wrapped around the wires; if this has fallen off over the years you can always make your own with masking tape. There probably won't be a bulb in position #2.

Once you have the cluster on your bench you can remove all of the screws and take the various pieces out. The fuel, temperature, and oil pressure gauges are all on one PC board (lefthand 1/3 of the cluster). I don't know if these are available from the factory (expensive) or if you have to buy the entire cluster (very expensive). You may find that the needle of the fuel gauge is simply hanging up somehow. Careful bending may free it up.

If that doesn't work, try to find a replacement in a junkyard. Gasser and diesel Benzes use the same three gauges except the fuel gauge in the later W124 gassers is labeled "unleaded gas only." You can swap the faceplate if that bothers you. Another possibility is to get your cluster repaired. Maybe someone on the forum knows of such a service.

Jeremy

whunter · #15 10-14-2010, 09:13 PM

Fuel Gauge and Sending Unit threads
http://www.peachparts.com/shopforum/showthread.php?t=190632