Fuel pump and lift pump problem getting the better of me

A few weeks back my 1983 mk1 Jetta started dying on the way home. I found that the lift pump had stopped working but now at some point through removing, refitting a new one and troubleshooting I seem to have created an electrical problem as the under car fuel pump has stoppoed primining when I turn the key and the new lift pump isnt getting enough power to work.

I've had the new lift pump out completely, away from the sender mechanism and connected to a supply and it works.

I've had the lift pump connected to the sender mechanism but out of the car and connected to a supply and it works.

I can see that there is 10v coming from the 2 cables in the car that power the lift pump but as soon as the car cables are connected to the sender unit and the lift pump is attached, the voltage drops to 0.5v and nothing happens.

I've also had the under car pump off and connected to a supply and that works too.

Any help would be much appreciated as it's driving me nuts now!! Why isn't the fuel pump priming anymore and coudl that be connected to why the lift pump only sees 0.5v when its all connected up?

And does anyone know which would be my fuel pump relay? Im heading to the local car parts shop this morning so I might replace the realy just to rule it out.

Thanks!!!



 

It should be the relay in position 2.
Warning there are some less than stellar non-oem relays out there.

see www.cabby-info.com for a detailed listing of fuel pump relays.

The position of them should be
Electrical System


Now with that said 10V to a fuel pump to me means that you are losing 2V to the pumps. Relays are know to be flaky, and should be replaced or jumpered out first.

Connecting one or the other up and dropping voltage isn't a good thing.

Suspects, wiring or corrosion in the connectors. Cold solder joints in the relay.
bad grounds as if there isn't a ground that is good you will see 0V on a connector to the ground pin to the voltage pin of the motor.

But you will see a voltage from the Frame to the voltage pin, means that you are having a flakey grounding issue.

Both fuel pumps are tied electrically together, that is they run a tied wire separately to both pumps.

It could be that where the wire it tied together, then branches off to the pumps is flakey and dropping voltage.

I personally would remove the fuel pump relay and either jumper the main leads, or replace it temporarily with the horn relay and re-verify all your connectivity and voltage to the two pumps.

The CIS fuel pump relays are known to develop cold solder joints.  That is as you are running down the road, the relay overheats and the soldered contacts on the relay loose connection and open, stopping the fuel pumps in their tracks…. Your car dies, and you can't re-start it.  Low and behold 20 minutes later the connections cool and your car magically comes back to life for another 20 minutes, (rinse repeat).

I have also seen the same issue with a bad lift pump where the main pump takes a dump after 20 minutes of whining….

If you haven't replaced your primary grounds, that is from the battery to the frame and the frame to the engine, then that could be an issue that needs to be addressed.  Just replacing the terminal ends of the cables is a placebo, as it your ground wire is still original  (unshielded braided/twisted) then it can drop voltage or impede current flow, where you have things acting flaky.
 

Thanks a lot for this Briano!!! I'd already been to get all new relays when you replied and after fitting them it made no difference but I only replaced the ones on the main fuse board. Then after reading your reply, I spent a couple of hours tracing the cables back from the pump and lift pump to check for dodgy connections and using a multimeter at each point. What I found was that there was zero voltage on thick the cables that go to the fuel pump. I managed to trace the cables back to the fuses and do some amateur troubleshooting which lead me to the current situation in the pic. There are 2 more smaller relays (I didn’t know that’s what they were until today) to the right of the fuse board. Relay in point A clicks on when i turn the ignition, even when I swap the 2 relays so I don’t think the relay is the problem here. Going on what I could work out from the back of the 2 relays I bridged the 2 points you can see and that got me 12v all the way to the pump but this was without the lift pump connected up at all. But even that made my day! Unfortunately, dad duties stopped play at that point and Im not entirely sure were to go now. Have I ruled out an earthing issue now do you think? Im hoping this is a connection to the back of these other relay points and when I connect it all back up to the lift pump as well I might have a running car.  Is one of these separate relays for the pump and one for the lift pump?

“Guest” does not have access to re-use this attachment.

I will assist you as I can, but pos issues aren't my real baileywick.
(Previous Owners Stupidity).

I don't know if you have an immobilizer or aftermarket alarm, but some folks did rewire the fuel pump to a different place, as well as do wierd things.  

I do know but can't remember the exact item, but I do remember that there was a TSB about rerouting the fuel pumps and a wiring change, where the wiring was re-routed to a location that I thought was above the fuse panel as an addition socket.

Fuel System has good information on testing.

http://cabby-info.com/images/Electrical/CISFuelPumpRelays.png

I can't find the TSB in question, others may know it.

Have you got a different engine fitted from standard as that is not standard wiring for a fuel pump relay?

Yes, sorry should have probably said to start with. It's a 20v t with qpeng ecu (mk1 jetta Coupe) . Just trying to go through all the info that Briano (thanks again)  has given me and work out what the wiring does but being a complete novice and on Dad duties this weekend It's been slow progress

Until I traced the cables back I had no idea these 2 smaller boxes were even relays hence not mentioning it was a different engine. It looked like a standard fuse box, relays and wiring to the pumps to me

basically what they have done is put an 18 x-relay in slot 2 which will bring on the fuel pump pin E/14, and the injector pin N constantly live with the ignition.

Then it looks like for some bizzare reason they have added another relay to power the fuel pumps, which I assume is triggered by the ECU.

If so they could have skipped the 18 relay entirely and just wired up a switched relay with the earth trigger from the ecu. it makes no sense to have the 18 relay in there as well as an extra external relay.

the other relay I would assume is for ecu/coil power. check the ignition live fuses 17 and 18, one may have been used for fuel/ecu relay trigger

Morning all, just thought I'd update this thread now that the car is back on the road. There were various linked issues which I had to get an electrician involved in to sort out. on the 1st visit he found that the fuse box had been wet at some point and all the connectors were corroded causing the 2 extra relays to randomly click on/off. I purchased a new fuse box from VW heritage (arrived in 2 days, £70 brand new and looks to be great quality) and he came back to fit that which solved the relay randomness issue.

He also found that I had the cables the wrong way around into the lift pump.  I was using the gauge cable where the live cable should have been, this is why it was only showing 10v and dropping off when it was connected.

The final issue was that after fixing the 2 issues above, the fuel pump still didn't prime with ignition and the lift pump didn't run at all. This was down to  an extra 30A fuse hidden behind the battery which was blown.

rubjonny  , I have no idea why this would have been done with the relays but I'm going to be doing a lot of work on the car so I'll probably get the electrician to sort this and tidy up the whole loom, if not replace it.

If anyone needs a great auto electrician, especially for their classic VWs then let me know and I'll pass you this guys details, he was great and only charged me £45 total. He also woudln't take payment until after I had run the car for a few days to confirm all was good. he's in Rayne, Essex.

Thanks again to you all for the help!
 

in an ideal world yopu want to find the qpeng ecu fuel relay trigger wire, run it to D/13 on the fusebox, then fit a 67, 80 or 167 relay. run fuel pump power from E/14 as per factory. You can add an extra wire to this pin with male spade on it if you like, then power some of the engine electrics off it. OEM the lambda probe heater would be fed from that spade plug.

Also the large spade N on the rear of the fusebox is live with the fuel pump, but is unfused. OEM the injectors are usually fed from this, but you could power other bits and bobs with extra inline fuses if you like