SU Fuel Pump Electrical Issues

[Etype1967]

Hello to all (first post).

I have a Series 1 4.2 etype. Had to take out the original fuel pump to service / clean it out. Rebuilt the SU Fuel pump and tit now runs correctly when I do a bench test and directly connect to the car battery (12Volt. However, pump will not run when I cannect to the wires in correct location in the rear of the car. Checked with multimeter and get 10.6Volts at the rear. Earth connected.

I am a bit stuck and unsure why the fuel pump will not run in situ? I have ensured I have a good negative earth.

Would appreciate any help / ideas to resolve.

Is 10.6Volts not enough? could I have a burnt out wire?

And help woudl be good but can upload photos if it helps.

Thanks

[mgcjag]

You have low voltage...
It's a 12v system....so needs looking at....what's the voltage at the battery ?... Steve

[Etype1967]

12.6 Volts at the battery.

The battery does not seem to be the issue as I have had lots of cranking poweer to start the car.

Thank you for responding.

[DWW]

Check resistance between the pump ground and the car body or voltage between those points. If you see resistance or a voltage then it is a grounding issue else it must be a supply side issue and has to be traces all the way from the fuse box and routing from under the dash to the rear.

[Etype1967]

OK. Have been porgressing to try and fix this. The pump runs still with a direct connection to the battery.

I have been concerned that I am losing 2 volts (only getting 10.6Volts) at the wires / location that are supposed to run the SU Fuel pump in the rear.

It runs if I use a new cable to the postive point of the car battery and the negative earth to the body (fuel pump location).

If I connect the positive cable directly to the 50amp terminal 3 (horn fuse) it ran for once (now will not run). See photo of the location I connected for the test.

It appears athat the pump needs a lot of amps to run - more than 50amps. Is this correct?

Very confusing.

[Iansparky]

It sounds like you have a high resistance joint on the cable from the fuse box to the pump, this will cause the reduction in voltage, I suggest you start with a new fuse and clean up the contacts where the fuse locates, it's fuse no 4 for the fuel pump, if this doesn't work then you will need to take voltage readings to find where the problem is, again start at fuse no 4 and either work back towards the ignition switch or forwards to the pump, I am assuming the harness has at least one bullet connector joint in it before it gets to the pump although I am not sure if this is the case or if it is continuous. Let us know what you find. I know from experience that the fuel pump is sensitive to low voltage as I had a faulty alternator that wasn't charging the battery, the 1st thing to die when the battery drained was the fuel pump leaving me stranded an hour from home.
Hope you get it sorted, Ian

[mgcjag]

Basically by useing a new wire you have proved a fault on the feed to your fuel pump..typically dirty connections..look at your wireing diagram...you will see that the white wire pump feed comes from the input side of fuse 6....the cct is not fused....so clean all the contacts so you get 12v at the pump.....Steve

[Iansparky]

Hi Steve
Looking at the wiring diagram for series1 4.2,i7 the fuel pump comes from the output side of fuse number 4 and should be white/ purple or am I losing the plot

[mgcjag]

Are you looking at the earlier 3.8 diagram....Steve

[Iansparky]

I have the coventry auto components enlarged wiring diagram for the series 1 4.2 which shows the fuel pump fed from fuse no 4. I have looked at the green service manual and this shows it unfused as you said, I assume the coventry autos wiring booklet must be wrong, although if you look at the fuse data plate that SNG sell for the 4.2 it shows fuse 4 supplying the fuel pump. Very confusing

[caveman]

Having looked at 3 of the 4.2ltr wiring diagrams, within the technical section here, all indicate fuse no.6
viewtopic.php?f=16&t=700

[MarekH]

You can say "fuse 6", but remember you are coming off of the unfused side of fuse 6. It's the green side which is fused, not the white side. The white side is just another connection link in the chain.

kind regards
Marek

[caveman]

Already mentioned at #7 above.

[Iansparky]

It would seem that the booklet I have is incorrect,

https://www.autosparks.co.uk/jaguar-e-t ... on-booklet
But regardless the problem is still the same, a corroded joint on the cable from the fusebox to the fuel pump causing a drop in the voltage

[Etype1967]

Thank you to all who have commented. It has help me to continue to look for issues.

I have tried everything suggested. I found that the wires going to the amp meter were getting hot with the dash ignition on. Have cleaned and fixed that connection. Now ok. Then I thought that because the coil gets warm with the ignition on it could be faulty and causing a voltage drop. But it has a resistance of 2 ohms across the positive to negative and 16k ohms on the lead the distributor of the coil.

Now getting 11.36 volts to the pump wires but still not running unless directly connected to the battery.

Maybe the pump only works with a lot of power? Maybe the pump is the issue. It will not run if I make a direct connection on small gauge wires?

Does that Sound like it is pump replacement time?

Thank you to all again.

[MarekH]

Now getting 11.36 volts to the pump wires but still not running unless directly connected to the battery.

I'd expect a drop of 0.25v from the front to the back of the car with a brand new harness and all of the parts in between. I'd expect a 0.75v voltage drop on a 50 year old car, so you have eliminated only half of the wiring issues you have. Assuming your battery is at ~12.6v, you've done half of the work so far.

"11.36" doesn't mean much on its own. If the battery is at 11.37v, then this an excellent result, but if the battery is at 12.8v, this is still rather poor.

Does that Sound like it is pump replacement time?

No, the pump ran fine when you had new wiring "directly connected to the battery", so the problem is the lack good and direct connection to the battery.

Buying a new fuel pump doesn't alter the fact that the volts don't reach the back of the car. It's like buying a new fountain for the garden but running it from the same old leaking hosepipe.

I would look elsewhere on the forum for "ignition bypass relay" and then I would clean up all of the bullet connectors, fuseblocks, etc between the alternator and the fuel pump. Use the wiring diagram to trace the path from the source back to the battery again. The wiring diagram is useful because it tells you how many bullet connectors are in the path, but not their location. Connectors should not be easy to undo - they should be so tight that it is a strain to pull them apart. If they are tight, then there is a gas tight seal. That means there is no provision for corrosion and that means there is no resistance in the joint. No resistance means no lost volts.

kind regards
Marek

[rfs1957]

It’s not wholly clear in my mind under what conditions our friend is measuring his “voltage at the pump”.

Obviously the reading taken with a voltmeter on a disconnected feed-wire to the pump is of no value at all, as it is the sustained voltage under the pump’s load which we’re interested in.

Another aspect that might be being overlooked is just how good the earth to the pump is.

The potential difference actually experienced by the pump will depend on how good the earth connection is, and you could still measure 12.6v with a voltmeter, with the pump under load, but your voltmeter earth may be way better than the earth being experienced by the pump.

I would therefore pay a lot of attention to the earthing circuits too.

On the subject of bullet connections, my recent experiences with a brand new Autosparks loom, and new bullet connectors from the same source, is that the grip of these modern components is a disaster in comparison with the stuff in period.

The way the plated-steel tubing is formed, and the material used, which has virtually no spring to it,means that there is no longer any positive click or convincing retention of the bullet.

Added to this, the crimping of the bullets means that in some cases the wire just pulls out the first time you dismantle your new assemblies.

If I had known how crap this stuff was, I’d have been tempted to make my own loom from scratch and use a completely different system of connectors.