W124 300TE Fuel Pump Problem

[mark.porthouse]

Hi All,

Just thought that I would share a recent experience with you:

The car with no warning would not start. Eventually nailed it down to a problem in the petrol supply as it would start on LPG (dual fuel) once I figured how to fool it into supplying gas for startup.

After a little looking around I noted that the fuel pump wasn't running. So I removed the fuel pump relay and stuck a (decent sized) wire across pins 30 (12V supply) and 87 (to fuel pump). The wire rapidly got hot and the insulation started melting.

So I figured that I had a dodgy fuel pump and that the relay will have blown too. Got replacements for them second hand. Still no running fuel pump.

I have now just realised that the wire supplying 12V to the fuel pump from the fuel pump relay has a short to earth, as with it disconnected at both ends I'm getting a circuit through to ground. However, the wire is still complete as I can get a circuit from one end of the wire to the other.

So I reckon that a dodgy fuel pump (and it doesn't sound as sweet as it's replacement) caused too much current in the supply wire, causing the insulation to melt (or potentially I merely had some chaffing in the wire which ended up contacting the wire with ground or perhaps I melted the wire myself as perhaps it wouldn't have melted if I hadn't shorted across where the relay would normally be.). So a short to ground from the wire somewhere.

The question now is: Do I replace the whole wire. It runs from under the bonnet by the battery to emerge underneath through a hole just forward of the fuel tank (remember this is an estate with the fuel tank at the back underneath). Does anyone know if this is a single wire? Also where does it run on it's journey from front to back?

By the way, I reckon the fuel pump relay pinout is as follows (some inspiration from a poster on another forum (which wasn't entirely correct), some looking at the Haynes manual and some testing with a multimeter):
Fuel Pump Relay Pinout - W124 300TE 1988 (no catalyst, no air con)

15 -- +12V from ignition via OVP relay
30 -- +12V direct from battery (why no fuse?!!!)
87K -- transmission kickdown???
31 -- ground
87V -- to cold start valve
50 -- cold start signal
TD -- KE-Jetronic computer
TF -- KE-Jetronic computer
87 -- to fuel pump

Cheers,

Mark

 

[grober]

Think your relay designations are probably correct. the supply wire to the fuel pump could run anywhere but probably along inside the sill. [You can just lever up- the plastic trim cover/kick plate to see inside.] then probably runs under the rear seat cushion somewhere. In the absence of knowledge of where the fault lies I would be inclined to 1. disconnect the existing supply wire at both ends noting carefully the gauge/current carrying capacity of the wire. 2. Rig a temporary pump feed wire along the floor ( so you can monitor it for overheating) and try to get the pump running. If this works for a trial period then 3. run this new feed wire thro the sill-under the seat to the pump as a permanent solution. In simple single supply wire problems like this its easier to disconnect the existing faulty wiring shorting to earth and use a new substitute wire you know is OK. You may wish as a precaution to inspect the existing wiring loom anyway in case of other potential insulation damage/ shorting to earth in other wires. If you dont feel confident doing this yourself entrust the work to a competent vehicle electrician.

 

[mark.porthouse]

Hi grober,

That sounds like a sensible idea. I think that I'll do that temporary wiring - I'll also find out if I need a new fuel pump relay too that way.

I hope that I don't run into problems lifting the load bay floor as I have rear facing seats in there.

Cheers,

Mark

 

[supercharger]

i would do that for sure had a similar problem once and i did just that problem solved.
if you need a fuel pump relay will be able to help


supercharger

 

[Druk]

Not altogether sure about an '88 but later 124s had twin fuel pumps. One at the front face of the tank and one under the plastic cover just forward of the O/S rear wheel. It will still work with one pump faulty but will sound noisy on the good pump. The cable to these pumps is (iirc) a large dia white/pink cotton covered from the relay to the pumps. It runs (also iirc) through the ducting under the carpet O/S to a grommet under the rear seat on the estate.

 

[Sp!ke]

Not altogether sure about an '88 but later 124s had twin fuel pumps. One at the front face of the tank and one under the plastic cover just forward of the O/S rear wheel. It will still work with one pump faulty but will sound noisy on the good pump. The cable to these pumps is (iirc) a large dia white/pink cotton covered from the relay to the pumps. It runs (also iirc) through the ducting under the carpet O/S to a grommet under the rear seat on the estate.

I thought one of those was a fuel filter... maybe I'm wrong, just think its a bit odd to have two pumps.

 

[grober]

You do get twin fuel pump setups. They are connected in series one after the other. They are normally mounted close together side by side under the plastic cover along with the fuel filter Spike mentioned. Might be a different setup in estates of course On the earlier K jetronic cars there is also a small cylindrical unit T'd off the main fuel line called the accumulator under that plastic cover [ the one on the right of the three in the image --the centre one is the fuel filter-- the 2 on the left the twin fuel pumps]. It's there to maintain a residual pressure in the fuel line to assist restarts. I dont know what the cylinder mounted close to the tank Druk is referring to? picture appended

 

[Druk]

Hi Graeme. My 124TE is long gone now but the primary fuel pump was close to the front edge of the tank with a long rubber pipe to the second one in the location as in your pic. It's obvious when you get under the car whereas the second one is not which made me wonder if Mark had missed it in his first diagnosis. I've never tackled a saloon but with the tank being inboard as opposed to slung underneath, Merc obviously re-jigged the pumps.

 

[mark.porthouse]

Hi Gents,

Yes, thanks Druk, I had entirely missed the second pump. Forward of the rear axle, under the large plastic cover, to the off side (RHS) of the propshaft.

This was giving me a false earth when I tried to see if the wiring was OK. Now I've disconnected both pumps from the electricals I find that the wiring is fine.

I've just run 12V straight to the second (forwardmost) fuel pump and it appears to run fine. Will do some more tests - perhaps a flow test.

Cheers,

Mark

 

[grober]

Remember the bodies of the fuel pumps and the filter (as I recall) should be INSULATED from their mounting brackets by plastic/rubber . Sounds as if you are homing in on the problem now .

 

[mark.porthouse]

OK. Petrol is flowing nicely out of the fuel filter (toward the engine) when the union on the output of the fuel filter is undone. However, when I turn the ignition on or run the starter from the ignition key I'm getting no fuel flow. So either the fuel pump relay is bust or it is not getting a signal to switch on. Seeing as I have already replaced the relay to no effect perhaps it is the signal not working (this could be due to LPG system interaction problems). Will test shortly.

Thanks for everyone's help with the fuel pump end of things. I can see clearer now which checks I should have done in which order!

By the way if it wasn't clear. On my 1988 300TE there is one fuel pump, on it's own, by the fuel tank (between the fuel tank and the final drive) and another fuel pump (one only) mounted with the filter and accumulator (like grober's picture but with only one pump not the two side by side).

Cheers,

Mark

 

[mark.porthouse]

Hi All,

Just tested the incoming signals to the fuel pump relay from the KE-Jetronic unit:
Terminal:Voltage
TF:1V when ignition on
TD:11V when ignition on
50:circa 12V when cranking starter (note that as the car won't run on petrol I can't tell what should happen to this signal when running on petrol)

Bear in mind that I think the terminals do these things:
15 - +12V from ignition via OVP relay, hot in run and start
30 - +12V direct from battery (why no fuse?!!!)
87K - transmission kickdown???
31 - ground
87V - to cold start valve
50 - from KE-J contol unit: cold start signal
TD - to KE-J control unit from ignition control unit
TF - from KE-J control unit: fuel pump signal???
87 - to fuel pump

The terminal I suspected of driving the fuel pump relay (TF) is only showing 1V which could well not be enough to drive the relay and get the fuel pumps to do their job. As the cold start signal is good then I would say that the KE-Jetronic control unit is powered up, just not giving good output for the fuel pump.

I shall do some more checks and tests, but if anyone has anything of interest then do shout.

Cheers,

Mark

 

[mark.porthouse]

Hi, I've just taken the car to the LPG installer and we've looked at whether the LPG system is responsible for the problem. It doesn't appear to be.

I had tested the system this morning first by running the petrol pumps straight off a 12V supply line. I could hear the pumps running but there was no sign of life - the fuel distributor was obviously still not sending fuel to the injectors.

At the LPG installer we tested the voltages in the wires to the (petrol) fuel distributor. With the car set to run on LPG the voltage of line 1 was 5V and line 2 7V - this must be the LPG management system providing the reverse current through the fuel distributor to cut off the fuel supply. When on petrol I got both lines at 3.8V - I would have expected a difference, with opposite polarity to when running on LPG, in order to provide fuel pressure to cause petrol to be injected. It looks as if the KE-Jetronic ecu isn't:
1. Running the fuel pumps
2. Running the fuel distributor.
I'm going to take a look at the ecu circuit board as the sandwich guy at the LPG place (!!) said that on his 260E it had some dry joints when he had an injection problem.

We'll get there...

Cheers,

Mark

 

[television]

These cars normally run with a wire linking 30 and 87 if it does not then the relay is not the problem

 

[diy_dan]

Hi, I've just taken the car to the LPG installer and we've looked at whether the LPG system is responsible for the problem. It doesn't appear to be.

I had tested the system this morning first by running the petrol pumps straight off a 12V supply line. I could hear the pumps running but there was no sign of life - the fuel distributor was obviously still not sending fuel to the injectors.

At the LPG installer we tested the voltages in the wires to the (petrol) fuel distributor. With the car set to run on LPG the voltage of line 1 was 5V and line 2 7V - this must be the LPG management system providing the reverse current through the fuel distributor to cut off the fuel supply. When on petrol I got both lines at 3.8V - I would have expected a difference, with opposite polarity to when running on LPG, in order to provide fuel pressure to cause petrol to be injected. It looks as if the KE-Jetronic ecu isn't:
1. Running the fuel pumps
2. Running the fuel distributor.
I'm going to take a look at the ecu circuit board as the sandwich guy at the LPG place (!!) said that on his 260E it had some dry joints when he had an injection problem.

We'll get there...

Cheers,

Mark

How did the lpg installer stop the car from running on petrol? On the old carbs type they put a solenoid in the fuel line. When I converted my car I cut the voltage to the fuel pumps by using a relay. I think you need to have a closer look on the lpg installation since thats where the fault is. When you switch the ignition on the fuel pump should run for about 5 sec to prime the system. You said you had pressure in the pipe so it must be priming. You could pull of the wire from the coil or rev counter that goes to the lpg controller. This should force the controller to switch back to petrol and see if this gets the pump running.

 

[mark.porthouse]

Hi diy_dan,

I can force it to start it on gas anyway by holding in the fuel changeover switch in the dashboard for a few seconds.

What they did was isolate out the LPG system altogether (putting it back to it's original petrol only state). To do this they unplugged the AEB 'injector emulator' (which isn't really an emulator at all on this particular system) and shorted across a couple of the terminals on the plug - this basically puts the wiring back to non-LPG mode (I have to make sure that the dash control is set to petrol so that I don't get gas as well as petrol!). The AEB box, when in use, actually merely sends a reverse current to the fuel distributor shutting off petrol to the injectors.

The only thing that I'm uncertain about is how does the controller in the dash switch know when the car is revved to change from petrol to gas automatically after a petrol start which leads to an auto change to gas. I think that you are imply that it receives a revs reading??? I'm not sure whether it isn't a vacuum reading though???

The (petrol) fuel pump runs all the time whether on petrol or gas - this is pretty normal - my Range Rover did that as well.

Thing is my system isn't currently running the fuel pumps and it isn't controlling the fuel distributor, hence it not working in petrol mode.

I don't know that the fault is in the LPG system.

Cheers,

Mark

 

[grober]

AEB site.

I take it you have looked at the customer section of the AEB web site? Surely the literature there would tell you what the change-over signal was? http://www.aeb.it/uk_login.asp

 

[diy_dan]

I take it you have looked at the customer section of the AEB web site? Surely the literature there would tell you what the change-over signal was? http://www.aeb.it/uk_login.asp

There isnt a changeover signal, It changes at about 1500 rpm it can be set for > 1500 or <1500. I will have to research the way that system is setup. like I said I just cut the supply to the fuel pump on mine (ce300). The system have to get a feed from the spark plug or rev counter to keep supplying the gas when the engine is running. This is a safety feature. no pulse no gas.

 

[mark.porthouse]

Hi diy_dan, grober,

I've applied for a login on AEB. I'd seen that area before, but assumed that I wouldn't get access. I'll see what happens.

Yes, it makes sense if there is the safety measure of ensuring that there is an rpm feed to the controller switch - then that same feed can determine the switch over point.

I am seeing a Bosch specialist of over 20 years tomorrow morning. Will be interesting to gain some info from him.

Cheers,

Mark

 

[diy_dan]

Hi diy_dan, grober,

I've applied for a login on AEB. I'd seen that area before, but assumed that I wouldn't get access. I'll see what happens.

Yes, it makes sense if there is the safety measure of ensuring that there is an rpm feed to the controller switch - then that same feed can determine the switch over point.

I am seeing a Bosch specialist of over 20 years tomorrow morning. Will be interesting to gain some info from him.

Cheers,

Mark

any joy from the specialist?