Fuel gauge indication - W123
 

On the way home last night I noticed my fuel gauge was not indicating and appeared completely dead. No low fuel indicator light either. Everything else in the cluster is fine. Tank is full.


I replaced the sending unit a few years ago, with a used unit.


Now on to determine if the issue is the gauge or the sending unit again. Does anyone have an electrical schematic of the fuel indication system? Thanks.

Does the needle flicker, even slightly when you turn on the ignition key ? .

If not, remove the fuse box cover and look for any gray colored fuses and replace them all with proper copper fuses of the correct ampere rating ~ they're cheap at VW Partshaus' .

Gray colored strip fuses corrode and stop working over time .

often you can gently roll them in situ with your fingers but this is always a temporary repair, better the replace all the gray srip fuses first, them if the gauge still is recalcitrant, move on .

No the gauge us completely dead. I didn't think the circuit being fused, however the rest of the cluster is OK. I'll check that out. Thanks.

It certainly _is_ fused, the fuse # is on the card inside the fusebox cover, it's delicate and easy to break so be _gentle_ in removing it .

Each fuse has the raised # casted in the fusebox next to it, sometimes hard to read due to dust / dirt .

From what I understand, wonky fuel gauge is one of two things:

1. The actual sending unit in the tank.

2. The gauge cluster.

Based on your symptoms, I'd guess something died in the back of the cluster. If something was wrong the sending unit, I'd think you'd have some kind of movement from the needle.

Well its not the fuse, they are all good. It looks like the fuse would handle the entire cluster. Still trying to find a schematic that would cover that circuit.

Is there a float on the sending unit? I've forgotten but on many vehicles, you can measure resistance as the float level changes.

If like my 1960's U.S. cars, the most common problem in an old sender is that the brass float develops a pinhole leak and no longer floats.

W123 Fuel Gauge
 

It's a composite float and rarely fails .

The sender is directly underneath the first aid kit on the rear parcel shelf, use a stubby #2 Phillips screwdriver to remove the single screw then gently tip the plastic tray up and out, set aside .

Peer in the open hole, there's a square black plastic plug with three wires, wiggle it with the key on and see if the gauge moves .

If not, the sender comes out easily with either a LARGE pair of Water Pump Pliers (nee: Channel Locks) or a 46MM socket .

I've forgotten the wire colors , you bridge two of them to force the gauge , someone here will explain .

The sender is a long aluminum tube with extremely delicate wires inside, if you need to open it to look, use a small pin spanner or, like everyone else including Dealer Mechanics, a pair of pliers to gently squeeze the brass nut's plastic surround *just* enough to where you can unscrew it then the bottom comes off followed by the aluminum tube, revealing the float and delicate wires .

BE GENTLE if you decide to try cleaning the wires ! .

Because the gauge doesn't move at all I doubt the sender's at fault but now you know what to do .

Since neither light nor gauge work, it’s not the sender. Nor is it the cluster. With 99% certainty it’s the ground. The three connections on the sender are ground, float, and reserve light . The float contact will have a variable resistance to ground, this drives the gauge. The reserve contact will connect directly to ground ONLY when the tank is low. My bet, the ground wire is broken or poorly terminated.

So then ground is open at the sender, at the connectors ground pin. Check the connector wires.

Open Ground Circuit
 

Not necessarily ~ the gauge cluster has a bunch of common grounds so I'd look behind the binnacle first....

The fuses need to be tested in situ using a test light and nothing else first, the comment about fuses from the O.P. sounded like he doesn't fully understand .

Here's the wiring diag you're looking for https://www.startekinfo.com/StarTek/outside/12265/disc_2/program/ETM/82_300.pdf

P 120 you can see the gauge cluster power is not fused. You can do a quick test with the key on, short pin 1 and 2 on the sender connector to pin 3 (ground). The low fuel light should illuminate and the gauge should rise. If not, short pins 1 and 2 to a chassis sheet metal ground..... if it works, the ground wire on the connector is bad. If the chassis ground does not help. Check power at pin 6 ? on C151 (circular connector) on the cluster as well as the grounds.

My dad always called channel lock pliers “water pumps” I have never heard anybody else call them that...

Air Force?

New Yorker?

Baby boomer...

Just curious if you know where the term is from.

Since the instrument cluster is un-fused, be careful when working on it. Make sure the key is off whenever connecting/ disconnecting the connector/ grounds etc.

I didn't know the cluster is unfused untill looking at this diagram. Jake2tech smoked the wiring on his W124 by not taking the above precautions.

I guess Mercedes designed it unfused because they feel loosing all gauges due to a bad fuse is not a good thing. I think they could have done better with a self resetting circuit breaker.