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Old Jag fuel system
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7D4



Joined: 06 Sep 2017
Posts: 9

PostPosted: Wed Sep 06, 2017 11:10 pm    Post subject: Old Jag fuel system Reply with quote

I have a '74 XJ12L (US model).

It has the dual tanks, 4 Strombergs, and dual external electric fuel pumps. 2nd owner car in original stock condition.

Each pump has two motors, making them the biggest fuel pumps I've ever seen. When the tanks are empty the pumps keep running. When there's something to pump they turn off independently so they seem to be controlled by separate pressure switches. (When the car is not running.)

Do these pumps have pressure switches built inside them? I could check this by disconnecting them and running separate power to them, but I'm hoping someone can save me the trouble. If not, what/where is controlling them? I'm having a hard time finding literature for these old V12's.
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Penguin45



Joined: 28 Jul 2014
Posts: 381
Location: Padiham

PostPosted: Thu Sep 07, 2017 12:24 am    Post subject: Reply with quote

Eyup, welcome aboard.

What you are describing is perfectly normal for any SU fuel pump. It has "in" and "out" diaphragms in the pump housing. When the float valves in the carburettors close, the back pressure on the fuel line stalls the pump until the level drops and the valves can function again. An empty tank has no fuel to deliver, so the system will not pressurise - the pump(s) run.

SU fuel pumps, parts and information can be found HERE.

P45.
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Rick
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Joined: 27 Apr 2005
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PostPosted: Thu Sep 07, 2017 7:49 am    Post subject: Reply with quote

Welcome to the forum, I used to run a S1 XJ12L with the 4x Strombergs (this was 22 years ago mind).

RJ
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7D4



Joined: 06 Sep 2017
Posts: 9

PostPosted: Thu Sep 07, 2017 5:21 pm    Post subject: wait, there's more.... Reply with quote

The plot thickens:

Somebody has made a repair. One of the original pumps has been electrically disconnected and there is an aftermarket pump installed downstream of both original pumps.

Since the original pump that is still electrically connected doesn't vibrate at all, I think the aftermarket pump is trying to pull gas--petrol, sorry!--from both tanks THROUGH two dead pumps.

The aftermarket pump was underneath one of the originals and I thought it was a filter until I moved the old pump out of the way, saw the wires, and felt it vibrating. The number on it is 8016 041. (I Googled it and got nothing.)

If both original pumps are dead, at minimum I'll need to get another aftermarket pump and a new filter.

I have a couple questions:

The fuel line coming from the front on the left side appears to be a return line. True?

There are two "solonoid valves" mounted behind the pumps. What do these do? One of mine has been abandoned: no fuel line or electrical connections.
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peter scott



Joined: 18 Dec 2007
Posts: 7113
Location: Edinburgh

PostPosted: Thu Sep 07, 2017 7:46 pm    Post subject: Re: wait, there's more.... Reply with quote

7D4 wrote:

There are two "solenoid valves" mounted behind the pumps. What do these do? One of mine has been abandoned: no fuel line or electrical connections.


Switching between the two tanks perhaps?

Peter
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ukdave2002



Joined: 23 Nov 2007
Posts: 4100
Location: South Cheshire

PostPosted: Thu Sep 07, 2017 8:28 pm    Post subject: Reply with quote

7D4

SU fuel pumps are simple and cheap to overhaul, as long as the coil is in tact (and they don't often fail) at worst its 30 mins and a £35 repair kit will sort them out.

Dave
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7D4



Joined: 06 Sep 2017
Posts: 9

PostPosted: Thu Sep 07, 2017 8:49 pm    Post subject: Reply with quote

I've confirmed that the old pumps are dead but there was fuel in the lines so that little aftermarket job was pulling something through.

The solenoid valve is controlling what I believe is the return line. Perhaps the valve is there in case of rollover. It would prevent fuel from draining out of the tanks through the return line if the car is upside down.

Let me make sure we are talking about the same pumps. The originals are about as long as your forearm and have what look like two motors with the pump in the middle. Maybe they only look like regular motors and are actually linear motors? Do they jerk back and forth? I wish I could attach a picture.
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MVPeters



Joined: 28 Aug 2008
Posts: 822
Location: Northern MA, USA

PostPosted: Thu Sep 07, 2017 11:04 pm    Post subject: Reply with quote

ukdave2002 wrote:
7D4
SU fuel pumps are simple and cheap to overhaul, as long as the coil is in tact (and they don't often fail) at worst it's 30 mins and a £35 repair kit will sort them out.
Dave

"Let me make sure we are talking about the same pumps. The originals are about as long as your forearm and have what look like two motors with the pump in the middle. Maybe they only look like regular motors and are actually linear motors? Do they jerk back and forth? I wish I could attach a picture."

Do a search for 'XJ12 fuel pump' to find plenty of photos.
I think you have 2 pretty standard SU fuel pumps mounted back to back to a central valve casting. Two are needed to provide volume, not pressure.
If the replacement pump does not have a pressure regulator somewhere, it should! Don't exceed 3.5psi or the carbs will flood.
I defer to the Jag experts, but I doubt if there is a return line. There were several variations for fuel delivery on XJs.
Clean the contacts with very fine emery, otherwise, as Dave said, simple to overhaul.
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ukdave2002



Joined: 23 Nov 2007
Posts: 4100
Location: South Cheshire

PostPosted: Thu Sep 07, 2017 11:36 pm    Post subject: Reply with quote

This is what I suspect you have:

Essentially you have 2x SU pumps back to back, there is nothing complicated in them, they don't have have any sort of motor, they are far simpler that!

Dave
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7D4



Joined: 06 Sep 2017
Posts: 9

PostPosted: Fri Sep 08, 2017 1:32 am    Post subject: Reply with quote

That looks like the pump. Do you just drag and drop to include a picture? On other forums I have to find an attachment button, but I can't find one on this forum. I didn't think of trying drag and drop until now. Getting the pictures is easy. I couldn't figure out how to attach/include them.

I am very curious how they look inside.

The other line is either a return line or a vent line. It is smaller tubing and has no filter

As for the valves, after thinking about it, the rollover explanation doesn't make much sense. The valves would still be open.

Since the tank filler caps are on top of the fenders it is easy to top off and easy to overfill. The solenoid valves could prevent forcing fuel through the lines to the engine while topping off. These pumps are obviously not positive displacement so you can force fuel through them when they aren't running. One solenoid valve is for the return line (or vent line) so the other valve must be for the feed line.
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peter scott



Joined: 18 Dec 2007
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Location: Edinburgh

PostPosted: Fri Sep 08, 2017 4:03 pm    Post subject: Reply with quote

If you know the address of an image on the web then just hit the Img key above the "Post a reply" box then paste the address and hit the Img key again.

See:http://sucarb.co.uk/su-fuel-pumps-spares/dual-hp-pumps-kits/fuel-pump-electronic-dual-polarity.html

The pump you are referring to is two pumps back to back. They operate by pulling and pushing a diaphragm that sucks fuel via two little valves, one on the inlet, one on the outlet of the pump. The pulling is done by a solenoid and when pulled it opens an electrical contact that disconnects power to the solenoid and a spring returns the diaphragm. The mechanism senses to stop pumping when it detects pressure on the outlet.

Peter


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7D4



Joined: 06 Sep 2017
Posts: 9

PostPosted: Fri Sep 08, 2017 6:37 pm    Post subject: Reply with quote

Now I see the "Img" button, silly me!

Thanks for the exploded view and I've already disassembled one of the pumps and found the problems.

The braided ground wire breaks where it is soldered. The contacts get caked with crud and corrosion.

If I can rebuild them without buying the kits, fine, but for the price of a rebuild kit I can find a complete aftermarket pump and it will probably be more reliable.
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ukdave2002



Joined: 23 Nov 2007
Posts: 4100
Location: South Cheshire

PostPosted: Sat Sep 09, 2017 10:44 am    Post subject: Reply with quote

The 2 faults that account for 99% of SU fuel pump problems are:

1) Points wear and if the vehicle is not used for a long period the points oxides up.

2) The diaphragm movement isn't adjusted correctly so then the points throw-over mechanism doesn't function correctly; sometimes temporally cured with a thump to the pump.

Both of these faults can be rectified in less than 30 mins, a new set of points are about £10 , a rebuild kit about £30 per pump.

Once set up correctly SU pumps are very reliable. I look after the MR spares service, we have members who have 80 year old SU pumps and have only ever changed the points.

Dave
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7D4



Joined: 06 Sep 2017
Posts: 9

PostPosted: Sat Sep 09, 2017 9:56 pm    Post subject: rebuild Reply with quote

I got one of the pumps running. I'm not going to bother with the other one.

The contacts are pretty well worn out. I was able to bend the tab to bring the contacts closer, but that didn't work for one of the drivers. I could only make that side work by applying pressure with my thumb to the contacts.

I was impressed with how it was able to siphon fuel from a tank with less than 4 gallons in it with dry lines--no priming--though it did labor at first. It smoothed out as the fuel started flowing, and this is with only one side of the pump working.

I found the site that sells SU components piece by piece, but I found the aftermarket pumps for less than $30US, some even less than $20.

I'd like to keep the original cartridge filter (ACD 60) and it is still available, but I don't have a source yet.
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MVPeters



Joined: 28 Aug 2008
Posts: 822
Location: Northern MA, USA

PostPosted: Sun Sep 10, 2017 12:36 am    Post subject: Re: rebuild Reply with quote

7D4 wrote:
........... I could only make that side work by applying pressure with my thumb to the contacts.

................... though it did labor at first. It smoothed out as the fuel started flowing, and this is with only one side of the pump working.


There's a 'rocker' assembly, #10 on the diagram Peter provided. It's on a threaded shaft to the diaphragm - rotate it 360 degrees (I can't remember which way!).

It's just priming the empty fuel line to the carbs. The ticking slows down as the pressure builds & eventually stops once the carb bowls are full. That's how SU pumps work!

What's your zip code? (mine is 01826) There's a white convertible near me I'd like to get my hands on!
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