What have i done to my fuel gauge? My car i believe is a 300 DSL which for some reason the PO changed the instrument cluster on.

He put in one from a gasoline engine (a 280 SE i think) and apart from the tachmeter not working, the internal lights underwent the mother of all shade tree mechaine repairs. Disaster.

They stopped working so i pulled it out to see the mess. But now i have a mess.

I went to the junkyard and got a really clean instrument panel, also from a gasoline car (no glow plug window and a red line on the tacho). I connected it and found that the fuel gauge didn't even move BUT the low level fuel warning light did stay on (!).

(Previously, the low level fuel light didn't come on and the fuel guage would wonder between 1/3 and 2/3 and jump back to one third if i hit the top of the dash)

Hmm...i thought. Let's put back my old unit and see. Weirdly now my old unit is bahvin strangly. The fuel gauge now jumps right across to full and even after taking the key out stays there for about three minutes before coming back.

What have i done?

I checked the pins on the back of the plug and scrubbed them with a SS brush, as i thought perhaps this was a poor earth.

Are gasoline w126 instrument panels compatible with diesels? That might be a good question to start with, straight off the bat.

And can i improve the earth of the instrument panel itself? Where could i connect a ground wire to it?

thanks a lot for all advice

MJ in Morocco

UPDATE: I've also noticed that the clock doesn't work as well, so im certain there's a ground/earth issue.

UPDATE: According to this site https://en.vindecoder.pl/WDB12603612025321
the car started life as a 500 SE, which might explain the choke on the dash just tucked away on the left side of the steering column. HOWEVER

According to this VIN site, it's a 300 SD https://uk.vin-info.com/order-reports/WDBCB20C3FA122852

Who do you go with?

First, determine which model you have and the year of manufacture. We can't help you much without that information.

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84 300SD
85 380SE
83 528e
95 318ic

There are two VIN sites which show it to be a different car. After rethinking this today, i think it might be a 500 SE simply because of the choke and another member checked the VIN and said it had to be a 500. So let's go with that. Any help on fixing this gauge would be apreciated.

You've changed the gauge cluster so the problem isn't likely in a ground at the cluster. Pull the ground plug off the back of the cluster and check the continuity between the ground in that plug and the group of brown wired mounted just above your steering column. If you have continuity there then pull the sending wire off the sending unit at the fuel tank and check that ground.

I'm really confused that you think your car is a DSL [sic] but later, within the same post change your mind to thinking it's a SE. An SDL, as properly designated is the longer body w126 with a diesel engine. I'm not sure MB made an SDL in 1979.

Nevertheless, I'd say your problem is at the sending unit. Either a ground there or the sending unit needs to be removed and cleaned.

Have you considered which fuel to use or read the designation on the trunk of the car? I think you are confusing most of the good folks on this forum either by intent or from inexperience. Either way we need to know which engine is in this car if you expect the more seasoned veterans here to try and help you. I'm not trying to be harsh but you gotta figure out some of this stuff if you expect much help.

Does it have spark plugs or can you take a photo or draw a picture of the engine, anything?

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84 300SD
85 380SE
83 528e
95 318ic

To clarify, it's a diesel engine - the OM603 - so it doesn't have spark plugs. Im sure people here want to help but I also need them to be flexible and understand that im in morocco and the VIN number on the front of the car really doesn't mean anything. I checked with the VIN sites and got two different answers. It's a W126 which might have started life as a 500 SE but, according to one VIN website, came out of hte factory in dec 1984 as a SDL five cylinder diesel automatic.

Would it make any difference to this specific problem if it were orignally a 500 SE even though i have a diesel engine in it?

I can assure you that i'm not confusing anyone by "intent". Can't think how you would think such a thing. I have a W126 long wheelbase car which it seems has gone through many identity overhauls. Im always grateful for any advice on my frankenstein jallopy.

Your advice is excellent. Yes, i should see if there is continuity between the ground in the plug and the ground on the steering column. But any idea which number terminal in that plug is the ground? Thanks

MJ

Some pictures of the engine and the outside of the vehicle would help.

here is a picture here of the OM603 engine in my car although im struggling to see how this will help me/you with the cluster problem. But as you asked...

photos of oversized radiator with OM603 engine

(scroll down)

Update: Also, you might be curious to know that the oil pressure needle now only budges a tiny bit while the water temp goes almost right to the top when engine fires up. Jeepers.

I put a 1984 om617 in my 85 380SE body after removing the gas engine, transmission and differential yoke. I had both automobiles complete and just wanted the wonderful 617 in a black body.

So yes, I can imagine what the wiring might look like behind the gauge cluster. I tracked and diagrammed each wire on both bodies to compare them and removed anything from the 84 body that was needed to install with the diesel engine in the black car.

The service manual electrical diagrams were identical for the gauge and ignition but the few differences I found, glow wiring and such I robbed from the white car and routed exactly as was designed to go. I had the pleasure of completely removing the fuse box housing so I could fish the wires through the firewall in exact location as where the factory would have put them had the SE body been an SD body.

I used the Factory service manual like a bible. I suggest you use it as reference to solve your problems. It has a section on electrical trouble shooting and you're going to have to figure all this out for yourself.

I would say whoever installed that engine in your car was not prideful in his/her/its work as was acceptable in polite company.

My best to you,

https://www.*********.co/misc/Manuals/Servicev3.2/W126-1/Main.html

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84 300SD
85 380SE
83 528e
95 318ic

thanks enormously for your patience. Even this posting has helped me edge forward with fixing it.

For God's sake man. Get the right instrument cluster. If you have a diesel, then get a diesel cluster.

Im pretty sure they are the same thing, without of course the glow plug dedicated light window. From what i can gather, and from what others on the forum have said, the instruments are identical. If there were diesel ones available in the junk yard, of course i would take one. But i have only found gas models. If its not compatible, i will let the forum know for future reference.
thanks for your passionate responce.

I've seen gas clusters in cars with a diesel engine. The major differences are the gas clusters have an 'economy,' gauge which works off engine vacuum from the intake and the diesel clusters have a light for glow cycle. One of the members here crossed that line and used the vacuum gauge to track the vacuum pump on his diesel.

The biggest issue will using a gas cluster with a diesel engine is that the speedometer isn't going to be accurate. That happens from daily driving and no abuse since it's near impossible to know the quality of diesel you're getting unless you are really dedicated to purchasing your fuel from a good retailer.

Imo, if there is a ground issue within the cluster it's going to be the ground circuit on the cluster itself. Perhaps removing the gauges and cleaning all the contacts both signal and ground on both the gauges and the cluster housing might be the first step. I believe the FSM identifies the hugh mass of ground wires above the steering column as g102.

The ground on the cluster is in the 8 pin rectangular connector just below the center of the speedometer. Of course the wire is brown so testing the continuity of the brown wire to the g102 location would be a wise place to start.

For clarification, diesel with clog the sending unit in the tank a lot quicker than will gas. A sludge buildup at the bottom of the tank in a diesel car is likely to be one of the issues with gauge problems in a diesel car for that reason. Typically, a diesel car with a clogged outlet screen will 'run out of fuel,' while there is still fuel in the tank. Often the sending unit will feed false information to the gauge making it appear to have fuel when there is none due to sludge buildup on the sensor unit.

Gas cars have much fewer issues with sludge buildup but one which has not been running for long periods will develop issue from amber type buildups if it's left sitting with ethanol in the tank.

That is why I wanted to know the exact designation of your car. I suppose most people here would felt as if they were fools trying to help you without sufficient information. Fuel gauge issue.

As for the oil gauge, the most common problem is a loose wire on the oil sending unit. Figuring out if that is the issue is usually as easy as wiggling the connector on the sending unit with the engine running and having someone tell you if the needle on the oil gauge is jumping around.

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84 300SD
85 380SE
83 528e
95 318ic

thank you for your time and patience. please believe me when i say i really wish i could say with certainty what the car is/was.

Although even from your post, it seems even more proof that it was originally a gas model, as you identify the vacuum tube going to the economy gauge.

I had no luck at all with the junk yard gasoline 'new' gauge so i put back in my old one, which from looking at pictures of it for sale on ebay, is a gas model.

Now i have very low oil pressure, functioning water temp and partially accurate fuel gauge. When i put on the lights,the fuel gauge changes as does the water temp. ALSO, when i connect the clock/tacho side to the earth everything goes ape **** - which leads to me to believe that i have a probelm on that circuit board itself.

Thanks for the tip about the rectangular cluster. I will check continuity there for sure.

I know i;m a PITA. But believe me when i say i appreciate enormously the effort from yourself and members to help out. Whenever i can, i feed back info into the forum to help others.

Columbo last question. The junk yard unit i bought had an electronic speedo, which i later gathered was standard on all w126s. Mine was butchered by the previous owner who put in a mechanical one from a W124 with the cable.

Is there a neat way i can connect that little green cable coming out the back and have electronic speedo?

thanks for all your patience. I'm going to declare my car a 500 SE 1980 model from now on as im convinced now the first VIN site was accurate.

MJ in Morocco

The gears in those speedometers are designed to be accurate with a particular transmission sending unit. Unless you have one which is designed for the transmission in your car it is going to be a waste of time to try any repairs.

I seem to remember that green wire being the one which connects the speedometer and the sending unit on the transmission. I have no idea how or why you would try to get that speedometer working since any information it shows will be inaccurate, either the speed or the mileage.

I believe the FSM says if your engine oil pressure is 1.7 bars when warm at idle it's OK. The oil pressure should go to immediately to 3.0 bars when you push the accelerator.

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84 300SD
85 380SE
83 528e
95 318ic