Can anyone ID this part for me? It is from my 85 XJS V12, California emissions engine. It is located on the passenger side, back top rear of the engine compartment. It has a vacuum hose and 2 tiny male electrical nubs coming from it. Whatever kind of original electrical connector was attached to it is gone, and instead somebody used standard female crimp type connectors. These connectors are far too big, so only part of the edge of the connectors grabs the nubs. That’s no good and will never stay put. I’ve researched the part and the best I can find is that ‘maybe’ its a fuel injection full loop switch. I would like to know the correct part name so that I can know it’s function, find the correct electrical connector(s) for it, and buy a new one. I’ve attached a picture. Thanks for your help!
Looks like the full-load enrichment switch to me. When vacuum drops to xx-level the switch closes and signals the ECU to fatten the fuel mixture. You should have an additional mechanical-only full-load switch on the throttle turntable…just a small snap-action switch…which opens at about 75% throttle. The two switches are wired together providing redundancy.
Cheers
DD
Thanks for the information, Doug.
Just for fun I looked at my set-up, and found that on my 88 there is a pigtail attached to the switch. That won’t help you, though.The “tiny” connectors- there are available really small female push-on connectors that might help. Hard to tell from the photo. I think the Jag. P/no might be EAC 4371.
Thanks again for your help, Dave.
I had this Throttle Vacuum Switch (EAC4371) out of my wife’s 1990 XJ-S convertible (5.3L V12 w/ Marelli) yesterday to repair a broken wire and test. With no vacuum the switch from this car was closed. With a barely measureable amount of vacuum (1 in. Hg?) applied with my Mityvac vacuum test tester the switch opens. The switch has two male pins that two wires with female pins connect to. The pins are embedded in some kind of sealant that you can remove with difficulty. I removed the sealant, removed the wire/female pins, repaired the broken wire by soldering the broken wire and adding shrink tubing to prevent future wire problems, reassembled, tested and reinstalled it.
Regards,
Paul M. Novak
1990 Series III V12 Vanden Plas
1990 XJ-S Classic Collection convertible
1987 XJ6 Vanden Plas
1984 XJ6 Vanden Plas
1969 E-Type FHC
1957 MK VIII Saloon
Ramona, CA USA
Hi Paul, For interest I will have to run a vacuum test on mine.I’m surprised that the vacuum has to fall below 1 inch Hg before the switch closes. I would have thought it would close a bit earlier than that- kinda critical application! Hmmm. Dave. Good thing the microswitch is there, too.
Dave,
That is the only EAC4371 that I have ever tested so I don’t know for certain if that vacuum value was correct to spec since I can’t find a spec for it. It would be helpful to know what you get and see if it’s the same. I was also expecting a different value and that was one reason that I posted my reply. I thought maybe someone would know what was correct.
I have a EAC4371 in my 1990 V12 VdP that I could also test but I am focused on resolving the fuel injector firing problem in our 1990 XJ-S convertible right now and will probably test it after I get the XJ-S running again.
Regards,
Paul M. Novak
1990 Series III V12 Vanden Plas
1990 XJ-S Classic Collection convertible
1987 XJ6 Vanden Plas
1984 XJ6 Vanden Plas
1969 E-Type FHC
1957 MK VIII Saloon
Ramona, CA USA
Hi Paul, I tested my vacuum switch this morning and found the sweet spot to be 21/2 inches Hg, falling or rising vacuum. I think your switch would make me a little nervous at WOT. I did a search (thanks to Kirbert) and found a switch that looks like a perfect replacement on our cars.Standard Motorcycle MC-VOS4. Activates at 3 inchesHG. Check it out!
I wish I knew more about the Marelli system you have so I could assist, but I am only just getting comfortable with the Digital P on my own car! Hope you have success!
Best, Dave.
With no vacuum the switch from this car was closed. With
a barely measureable amount of vacuum (1 in. Hg?) applied with my
Mityvac vacuum test tester the switch opens.
Normally closed is correct; that corresponds to WOT. However, I wouldn’t
think it would open THAT easily.
The switch has two
male pins that two wires with female pins connect to. The pins are
embedded in some kind of sealant that you can remove with difficulty.
I removed the sealant, removed the wire/female pins, repaired the
broken wire by soldering the broken wire and adding shrink tubing to
prevent future wire problems, reassembled, tested and reinstalled it.
On mine, the wires were crudely soldered directly to the switch when I
bought the car.
– Kirbert
Kirby,
I will test the vacuum switch in my 1990 Jaguar V12 VdP and see when that switch opens. I was a little surprised myself about the 1” Hg needed to open the switch and thought it might be a little higher, but I couldn’t find a spec for that switch and didn’t really know when it should open. However, I could see that it was opening and closing, and it wasn’t directly related to my no-start problem, so I took it off my list of suspects and moved on. I will take another look at it once I get the car running again.
Regards,
Paul M. Novak
1990 Series III V12 Vanden Plas
1990 XJ-S Classic Collection convertible
1987 XJ6 Vanden Plas
1984 XJ6 Vanden Plas
1969 E-Type FHC
1957 MK VIII Saloon
Ramona, CA USA
Dave,
Thanks for this additional information. I retested the vacuum switch (EAC4371) from the 1990 XJ-S convertible yesterday and confirmed that at zero vacuum the switch was closed and that somewhere right about 1” Hg the switch opened rising and falling. I redid the test several times to be certain that my readings were correct. I plan to check the vacuum switch in my 1990 V12 VdP today and will let you know how that goes. I just checked out the part you suggest (Standard Motorcycle MC-VOS4) and found it on eBay for about $45 with shipping included. I plan to check some of the local bike and auto parts shops to see if they also carry it. Although I am focused on just getting the car started again, I will swap out that switch once the engine is running again since it has no bearing on the no-start condition, but it will be something to deal with once the car is on the road again.
Regards,
Paul M. Novak
1990 Series III V12 Vanden Plas
1990 XJ-S Classic Collection convertible
1987 XJ6 Vanden Plas
1984 XJ6 Vanden Plas
1969 E-Type FHC
1957 MK VIII Saloon
Ramona, CA USA
Dave,
I took some time yesterday to test the original light blue throttle vacuum switch in my 1990 V12 VdP. With no vacuum applied the switch was closed and at just about 1 in. Hg of vacuum the switch opened. This was identical to the results I got when I tested the switch in our 1990 XJ-S convertible.
I looked up that Standard Motorcycle MC-VOS4 switch and it seems to be readily available from a variety of sources. Do you know if that switch is closed with no vacuum and opens at 3 in. Hg or is it open with no vacuum and closes at 3 in Hg (the opposite of the OEM vacuum switch)? I couldn’t find that info in any of the write-ups that I found but a lot of the info that I saw indicated “activation at 3 in. Hg” which sounded like it was normally open and closed at 3 in Hg, but I could be wrong about that. If this Standard Motorcycle MC-VOS4 switch operates opposite of how the original switch operates then the installation becomes more complex as Kirby described in another email in this thread.
One other interesting thing to note was the Jaguar Parts Catalogue for the Series III XJ12s (my 1990 V12 VdP is essentially a Daimler Double Six or XJ12) shows the part number as EAC4371 but the Parts Catalog for my wife’s 1990 XJ-S convertible shows that part number as JLM473.
I wonder if a spec changed happened with the part number change, or if my two throttle vacuum switches are operating incorrectly? I can see that a switch that opens at 3 in Hg. Would be a more conservative design that 1 in. because the fuel enrichment would start earlier. I am still curious about what the original EAC4371 and now the JLM473 specs are. I have looked in my technical documents but haven’t found that yet.
Regards,
Paul M. Novak
1990 Series III V12 Vanden Plas
1990 XJ-S Classic Collection convertible
1987 XJ6 Vanden Plas
1984 XJ6 Vanden Plas
1969 E-Type FHC
1957 MK VIII Saloon
Ramona, CA USA
I wonder if a spec changed happened with the part number change, or if
my two throttle vacuum switches are operating incorrectly? I can see
that a switch that opens at 3 in Hg. Would be a more conservative
design that 1 in. because the fuel enrichment would start earlier.
Does anyone have any idea how much vacuum this engine pulls just cruisin’
down the highway at 70 mph? Because as long as this switch is open under
those conditions, I think we’re good. We have had a few reports of engine
damage when operated at WOT, so I think being on the safe side might be
good. Of course, there are LOTS of reasons an engine can suffer damage
when operated at WOT, starting with a partially-obstructed pickup screen in
the surge tank.
– Kirbert
Well, it gets more and more interesting. “Wolseley” the OP has found a bunch of info you will want to read. The thread is quite close. I had no luck trying to find out whether the switch is NO or NC, but it seems that it is actually NO. (No vacuum) Relay required. I, too, stumbled across JLM 473, but that was a dead end, too. If my fuel enrichment was acting up on my own car, I’d be tempted to add another microswitch- There’s plenty of room to do that.
Kirby, I’ve had vacuum gauges in some earlier cars, but not this one.
If my fuel
enrichment was acting up on my own car, I’d be tempted to add another
microswitch- There’s plenty of room to do that.
What good would another microswitch do?
– Kirbert
If the microswitch fails, the vacuum switch provides redundancy, correct? Same thing could be accomplished with two microswitches, as the Jag. designers evidently felt that the engine needed a backup system for fuel enrichment, IMHO.
“If” I understand the designs correctly, doesn’t that lead to the question as to why would Jaguar use a bulky vacuum switch assy. whose diaphragm and hoses are eventually going to dry rot and fail, to do what a secondary microswitch could have done?
More importantly and again ‘assuming’ I understand the system correctly, if the vacuum switch fails it can lead to detonation. The problem is that other than hearing the engine’s detonations, how are you going to know the vacuum switch has failed? Considering the risk of melted piston tops or ruined valves, why not have 2 vacuum switches working independently of each other as cheap insurance? It would’t be hard to do.