Vacuum switch EAC-4371

I searched this site trying to find the part number for a 3.0hg substitute for the Jaguar EAC-4371 vacuum switch, that has been recently mentioned here. I couldn’t find the PN, so I did the Internet research myself. A motorcycle vacuum switch made in the USA that is rated at 3.0hg is available from J&P Cycles, P/N #370-023. These switches can be adjusted for hg, and here is an explanation as to how to do it:

There are a number of these switches available for Harley Davidson motorcycles, and they range from 3.0hg to 7.0hg depending on the part number of the switch. I ordered the 3.0hg one as a spare.


Wolseley, you had more success searching for info than I did! Excellent . I read everything and it seems that these switches work opposite to what we need- ie. no vacuum and switch is OPEN. Fixed with a relay. Found the adjustable part interesting, too. Dave.

Oh oh! I didn’t know that. Lucky for my car that you brought this to my attention!! So how would you wire a relay with the switch to compensate for the substitute switch working the opposite of how the Jag switch works?

Once again I find myself thanking you for your help, Dave.

Read the thread on “Parts for sale- XJS V12” which is not too far away. Kirbert describes how to straighten things out!

Just now did a vacuum test on the original EAC-4371 switch that is on my car. The circuit is closed at 0hg., and there is continuity across the switch’s 2 terminals. 2.5hg. opens the contacts and then there is no continuity across the terminals… I used a Mighty-Vac to do the test, and the questions that now must be asked is how accurate is the vac gauge on a Mighty-Vac, and how much difference to the engine does it make if a substitute vacuum switch does not function at precisely the same readings at the EAC-4371?

Sorry, the references are losing me.

Is this to replace the vacuum full-throttle switch?

Yes, and it is located on the upper rear passenger side of the engine bay. My pictures show where it is located however the wires are disconnected from it.

The readings on my car are the same as yours. I think as long as we stay richer, rather than leaner, nothing bad will happen. So switches up to 5 inches would be ok, but below 2 inches Hg and you’re pushing your luck! (IMHO) If the mixture is a bit too rich the performance will suffer, but you won’t lose any valves! (Or pistons)

Thank you again, Dave. Your point is well taken. I agree 100%. The last thing I need is valve issues and I could care less about hi-performance.

So switches
up to 5 inches would be ok, but below 2 inchesHg and you’re pushing
your luck! (IMHO)

I concur.

If the mixture is a bit too rich the performance
will suffer, but you won’t lose any valves!

Performance won’t really suffer, as at WOT the fuel enrichment is supposed
to be engaged anyway. Fuel economy will suffer and so will emissions.

– Kirbert

I have been browsing Jaguar dealer training publications for “Digital P” injection and found some useful info about this switch:
“Full load enrichment is necessary when the engine is operating under full load conditions. A signal to the ECU is provided by full load vacuum switch when manifold depression falls bellow 4in Hg and its contacts close.”
It seems that with age spring gets weaker and switching point deviates towards 2in Hg or even less. Luckily if you remove silicone from the end of the switch, there is an adjustment bolt!

DavetheLimey found a direct replacement switch and I’ve purchased and installed one. It is rated @ 4hg. It can be purchased from Summit Racing, part # SWW-77343-04. This was a very important find, as the original EAC-4371 is no longer sold. Thanks to DavetheLimey for sharing his research.

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hicant seem to sourse this part at summit any ideas ?

Search using Stewart Warner Hobbs vacuum switch SWW-77343-04.