When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
What is this and why is it in the harness to my fuel pump on my 94 XJS convert 4.0. in the fuel pump pigtail
Having checked it's not April 1st, I reckon I can state, unequivocally, it aint off any Jaguar fuel circuit I have ever seen.
By the size of those shorn off cable ends, it looks to be from power distribution lines - eg mega fuses?
Why it was put there is even more interesting?
It's a part of the wiring harness from the fuel pump relay to the in tank pump on my 94 XJS. It had about 4 inches of harness to the pump side and a foot to the other connector, No power to the pump so I unplugged the pigtail to check it. No continuity from one end of the pigtail to the other with this in. Some kind of electric gizmo to either reduce amperage to the pump??
Bench tested the pump with out this and my pump runs. The big question is why its there and will I damage the pump without it. I accessed my intake pump 3 times now and don't really want to do it again.
If so, then it is some sort of voltage spike protection. Whether it is needed I have no idea, but Scott made one up, he said, using a different and more reliable method.
If so, then it is some sort of voltage spike protection.
Bingo, Greg . . . .
More commonly seen as a reverse biased diode across relay coil to protect the winding from change-of-state spikes.
note that tiny diode parrallel to coil of Fuel Pump Relay #2 below
Known as a radio frequency interference (RFI) filter.
To OldCoastie . . . here is a screenshot of your '94 XJS.
Note how the RFI zener diode is reverse biased (ie anode to ground)
**Edit . . . Eliminate it removes that protection . . . How much is that new fuel pump you may soon need?
PS . . . did you find the second one?
Last edited by cat_as_trophy; 07-02-2021 at 04:29 AM.
Reason: explanation
Greg and Cat, thanks for confirming what I suspected. Fortunately, I found one on E-bay that day to replace it. Just another week delay as shipping here is horribly slow these days. My knowledge of buffers/protection devices is limited but I believe Greg when he said it's probably there for a reason. This Jag has certainly been a learning experience for a former 60's to early 70's former MG/Triumph mechanic.
(Cat, my Jag has a 6 cyl and one pump so I'm guessing a second Ziener isn't needed.)
On my 94MY electrical diagram, (I was finally aware enough to look there), the device is called and RFI Protection Module. On Cat's diagram its called an RFI Filter. Aren't RFI "devices" inserted to dampen radio interference? If that's the case, I should be safe without it? I don't listen to the radio with the top down anyway.
I don’t think that’s the radio it’s protecting from. They mean electromagnetic interference, which your engine, relays and other components can, and will create, possibly contaminating a critical signal.
My 94 also had this.
The fuel pump wiring had issues with high wire temps and melted the plugs.
I don't know if these had anything to do with that but it might.
Two more pictures. This is the third time Ive replaced the fuel pump. I can now do it in 3 hrs start to finish. Vee i understand what you're saying about interference to relays and such. There are only a couple dozen relays in the trunk along with two ECUs. I can wait until the replacement shows up. In the meantime I have a Zener free jumper if I need it.
Aren't RFI "devices" inserted to dampen radio interference? If that's the case, I should be safe without it? I don't listen to the radio . . .
Not so, and sorry it took me so long to reply.
In this case, it is the switching of relays or, more commonly these days, high speed digital devices like ECMs either up or downstream, which in their operation can produce very short but high intensity voltage spikes that can take out other electrical or electronic components. The well designed placement of RFI filter/suppressor/module, such as here, not only protects that fuel pump from other switching devices, but vice versa, may be protecting even more expensive components from that fuel pump.
The wiring diagram from which I posted the screenshot in my earlier post was taken from Gus' repertoire at <jagrepair.com> for MY94, so I suggest you investigate further before overlooking second RFI protected pump. Not sayin' it's in, under or anywhere near fuel tank, and you will note it has entirely different wiring colour codes. Is it an electrical high pressure pump fed from low pressure pump at tank, or accessory injector feed or summat?
So, the RFI Filter is like the dangling strap under cars of old . . . anything that sinks away that zap as you alight, has just saved you a motza of cash for modern (read, sensitive) electronic components.
07-01-2021, 09:36 PM
