Help with jerry-rigged Fuel Pump Relay wiring

[sytruong]

Hi all.

Recently purchased this car 1995 E420 - I know its the E500 forum, but this is a similar setup and I'm talking to a knowledgable and active group.

The person whom I bought the car from said that his friend replaced the fuel pump... Implying there was some fuel issues prior to selling.

Anyway, the SYMPTOM is I can hear fuel circulating in the fuel lines under the hood when the key is on the "ON" position. It keeps going, it doesn't just prime and turn off (what I assume is normal).

The rear seat was loose, so I discover this. Turns out, that's where the fuel pump relay is supposed to be. Looks like they jerry-rigged something to bypass the fuel pump relay?

Can someone tell me what they did so I can fix it? Or tell me how to fix it? I don't know what it's supposed to look like normally, so if you can show me that, I can fix it. Thanks!

 

[gsxr]

Yeah... amazing that a "professional" repair shop did this.

You are right, they modified the wiring so the pumps run continuously when the key is on. To fix, you'll need to remove all their patchwork and re-install the original fused 30A relay (K27). The relay is controlled by the LH module (N3/1) located in the CAN box. Most likely, the LH module is defective and needs repair or replacement, as it wasn't triggering the relay to energize the pumps and the shop couldn't figure out why or how to fix it.

However, unless the mystery wire was just stuffed into socket #4, you'll have to open up the 6-pin connector and see what exactly the shop connected the red wire to. Based on the schematic linked below, this was a Very Bad Idea... appears the pump are being powered directly from the Basic/General module (BM/GM, N16/1), which it was never designed to do. This was pulling high current through a small wire. You should fix this ASAP.

Schematics:

https://www.500eboard.co/forums/misc/Manuals/Servicev3.2/W124/w124CD1/Program/ETM/24075002.pdf

 

[gerryvz]

Wow -- what a hack job that is! As said there should be a 30A green or blue relay plugged into that plug under the seat. There should be photos of this relay here on the forum, if you do a search. I'd start by getting this relay and seeing if it works. If not, then diagnose per Dave's instructions above. The majority of these cars have bad LH computer units underhood, which cause the relays under the rear seat to make a "clicking" noise.

Fortunately these LH units are relatively available used / good units, and existing ones can also be repaired -- usually the problem is failed capacitors in them. Lots of info here on the forum about all of this. Search for fuel pump relay click and you'll find a fair bit of info.

The part number for the fuel pump relay is: 001 542 96 19

https://www.fcpeuro.com/products/mercedes-fuel-pump-relay-0015429619

Keep us apprised! Good sleuthing work so far...

By the way -

 

[sytruong]

Thanks, you guys are great. It's so nice to have a helpful and knowledgable forum around still! Most of these forums have died off and knowledge lost!!

I ordered that green fuel pump relay and start there. From what I understand, the previous owner was in a bad money situation, so I'm crossing my fingers that he did this to save $40-80, instead of buying that little green relay.

Clearly, if that doesn't work, I'll go to that LH module.

I'll keep you guys updated.

I got the car for $2200, so I'm not too upset.

 

[sytruong]

I'm suspecting the LH module may be the issue. As you could see here it's been tampered with and hex. Bolt has not been put back all the way. I'm crossing my fingers. But we'll see when the relay gets here in the mail.

 

[gsxr]

It's unlikely the relay will fix your problem... the shop probably tried that already. '

In the meantime, open up the CAN box and see what part number LH module is currently installed.

 

[sytruong]

So I took out my LH module. It was easy, I just did like everybody else said, to use a screwdriver and gently pry the module upwards. This is the serial number I found.




To take it apart, I just took off all the Torx screws and gently lifted the circuit boards out. This is what I found.



I'm by no means experienced in this type of stuff, but these capacitors look like they're in good shape. I don't see anything burnt or crispy on either side of the circuit board.

Here is a thread where another user repairs these capacitors. I don't even know if I can tell from the outside if they're still good or not. You can send him an email, I'm waiting on my response, as I sent it last night.

A

FOR SALE Thread 'LH capacitors and Installation (LH Module Repair Service)'

Feb 26, 2024

I just received another batch of capacitors to repair our LH modules today. I have 8 sets available and so far 2 have been spoken for. Updated pricing is $12 for a set mailed in a blister pack with tracking to your address. If you want 2 or more sets add $3 for each individual set. Additionally if you want me to install capacitors in your LH module the cost is $100 which includes parts and labor and priority mailing with tracking to your door.

What I have so far identified is there are some modules that have the newer style aluminum can capacitors and some that have the old brown plastic...

• Alphasud40
• Replies: 34
• Forum: Parts for Sale - **400E / E420 ONLY**

• 
• 

 

[gsxr]

That LH module is a replacement unit, the latest part number, with date code 1999. It would be REALLY odd for a newer module to fail. And, it likely does NOT need the capacitors replaced either.

See what happens when you install the relay and remove the gerry-rigged wiring...

 

[sytruong]

That LH module is a replacement unit, the latest part number, with date code 1999. It would be REALLY odd for a newer module to fail. And, it likely does NOT need the capacitors replaced either.

See what happens when you install the relay and remove the gerry-rigged wiring...

Amazing. Thank you for your insight.

How could you tell by the serial number?

 

[R129 UK]

How could you tell by the serial number?

Date code from the label.

 

[gerryvz]

Amazing. Thank you for your insight.

How could you tell by the serial number?

there is an entry in the Wiki that has all of the potential part numbers for all of the computer units for 500Es and 400Es.

 

[sytruong]

Relay came in.

I took apart the jerryrigged relay bypass. This is what it looked like. It looks like they took a ignition-on hot wire and directly powered the fuel pump with it. At least they put a fuse on it, I had one situation in an old Honda Civic, where they didn't even fuse it.



I popped the relay in, and turned the car on, and the fuel pump did what it's supposed to. Everything works normally now!



So basically, that whole workaround was because the previous owner, whoever fixed the car previously, didn't wanna pay 40 bucks for a relay. I don't really blame them because we are enthusiasts, and we would go about it the right way, and the previous owner was probably not. I'm not even mad because I bought the car from them knowing that they sold it because they had financial hardships.

Anyway, I really hope the previous owner is doing better. They actually took good care of the car. It's been sitting for a while so it's dirty, but this is a really clean car.

Thank you everyone for your help. This is a great forum. It didn't end up being the LH module (computer). I didn't end up having to resolder capacitors. It ended up being the simplest solution. But I think this thread covered all the bases from A to Z about how to go about this problem.

I hope this thread will help somebody else out in the future!

 

[gsxr]

That's crazy! Glad to hear it was an easy fix. IIRC the invoice from the repair shop was hundreds of dollars for their Rube Goldberg patch. Just insane.

 

[gerryvz]

I'm also glad you were able to fix it.

One thing I've learned philosophically with MBs over the past 25+ years, is to always start with the simplest things first, before freaking and throwing parts at the problem. Seriously, at least 50% of the time, any given problem is going to have the simplest and most elementary solution, or it's going to be directly related to something else you just did that affects that system or causes that problem.

The key is just sitting down and reasoning it out -- what are the essential elements of the problem, what are the input variables that affect the car's system, and did you do any recent work to it that somehow borked it (unknowingly). It all comes down to cause and effect.

I would also recommend you buy a spare LH computer (they are fairly easily available, often from folks here) if you are a long-term owner, because these can and do go bad (capacitors fail).