Hello all.

I got started on this path in THIS thread... The "Full Load" blue throttle vac switch (and micro switch) came up and other members chimed in to offer some awesome stuff. Namely Grant with another, now WORLD famous, Grant write up... Thanks Grant! See below.

Rather than continue to hijack, taking that thread in my own direction,,, being a pita,,, AND to make a thread specifically about the mysterious blue vac throttle switch, I thought I would make a separate thread. I had actually removed the switch a looooong ago, wrongly thinking it was a part of the air pump system. Was living in a bag, in a box.

So, I went and dug the switch out of that box and that's when the trouble began, lol. See pic. But, I dug the epoxy out of the wire/connection set up at the back of the gizmo (arguably the most protected set of spade connections I have EVER come across) and used what amounted to two speaker wire style spade/clip connectors to clip onto the spades on the switch end. No big deal. Same size.

I took readings off the 4 (FOUR) parallel leads. I was surprised at four leads, I am obviously still missing something, to get a reading of 3.7 to 3.8 V. Is this voltage correct? In the attached ED it seems there are 2 throttle switches? Is this true on the 1990? And, if so, do I need it? I had mistakenly believed the trans kick down to be the "in parallel" with this switch but it's clearly something different.

Then I hooked up the switch (which does still hold a vacuum amazingly) and used a long vac tub creating the vacuum with my mouth, to predictably get the 3.7 to 3.8 volts when vacuum was applied - 0(zero) without vac. Then, hooked up a vacuum tube between the switch and a port at the rear of the A Bank intake manifold, and that was that. I noticed a difference in running at idle - I think. Didn't know if I would or should and I think I noticed a difference this morning in taking off from a stop in regular "normal" driving on my way into work... Does this switch ONLY engage and play a role at WOT?

Anyways, my questions:
-Is it an on/off switch or should I be getting a range of voltage values?
-Is 3.7 - 3.8 correct voltage from the leads?
-What about the second switch? Needed?
----And, can someone help me understand the open loop closed loop in this situation? Does it have to do with O2 sensors?

AND, importantly. In the boot I have this diagnostic port for the O2 sensors. If it is related to the O2 sensor open/closed loop system, how do I check at that port with a DMM and what am I looking for in numbers?

Sorry, I know it's a lot. But this leads to that and it can be like a mini can I worms everytime.

 

To start, I get 3.8 volts across my leads also. This is the sensor wire to the ECU and to ground. When the switch closes you will read 0v, so yours is working.

I was assuming you were supposed to dig the black goo out of the other end of the vacuum switch opposite the wires and there you'd find the adjusting screw.

When you pop the throw open, probably more than 30 %, manifold vacuum will drop closer to 0 and that switch will enrich which should keep the engine in open loop till the swich opens and stops enriching (my assumption). If you hold throttle steady at 30% vacuum will start to drop as engine speed increases to where it will settle to match the throttle opening. This vacuum decrease will open the switch and stop enriching. At full throttle it will keep enriching until you let up.

So I think Grant is saying you can vary the vacuum level at which it starts to enrich by the screw on the other end. You can make it enact sooner and stay on longer.

The parallel wires are so that enrichment can also be activated by a micro switch on the throttle tower. The kick down switch is mounted on the throttle cable itself.

 

Correct. Once the vac drops (either by JJJ stopping sucking, or by the throttles opening) the mixtures enriches. The two switches (microswitch on capstan and van switch) operate in parallel (which is why there are 4 wires in the diagram), so, if either one closes the fuelling gets enriched. JJJ would notice a difference at tickover if he "artificially" enriched the mixture by removing vac from the switch.
Grant was indeed tuning his vac switch to give him the extra fuelling when his driving style needed it.
FYI, UK spec, and many non-USA and Canada cars did not get the microswitch, we only have the vac switch.

 

Oh, yeah... It was a challenge digging the epoxy/resin style seal over the wire end OUT to try and get to what was underneath to make a repair. In the photo, ya can see that my wire finally gave up and broke off... Needed to dig out the toughest epoxy seal in all XJS history to get to the connection.
Ok,,,, so I think I'm picking up what you're putting down by the way... In your reply

So, when the signal going to the ECU is the 3.8 volts, the fueling is basically working in its normal operating closed loop with the O2 sensors giving orders...? When the vac and volts drop to ZERO for enriched, the fueling goes to open loop and the ECU dumps a bit of fuel enriching the mix....?

One question... Say I'm driving steadily on the highway, 70mph,,,. How low will the vac go in that situation? Enough for the opening of the butterflies and the engine breathing in enough air to produce a near ZERO or zero vac situation?

I guess what I would like to do now is make sure my O2 sensors are working properly in the first place and,,,, shame on me, I don't know how to take that reading off the diagnostic port in the boot near the ECU. What numbers am I looking for with a DMM?

I don't know if I mentioned. I am missing the micro switch at the throttle capstan and I guess I can do without. I NEVER go (never) WOT... Never = Not Yet.... Maybe, lol

 

That switch is as mentioned and I simply state it like this:

Low to NO vac - contacts closed, - more fuel, or OPEN loop.
Vac @ preset value ++ = contacts OPEN = less fuel, or CLOSED lop.

This preset value is what the factory decided, and works 90% of the time. Us "purists" can fine tune it, hence the Black Goo,.

To know what levels of vac YOUR car reads when YOU drive it YOUR way is ONLY achievable by having a vac gauge in the car for a WEEK. Drive as you normally do, and note the numbers, WATCH THE ROAD FIRST AND FOREMOST, and then take YOUR switch and using a DVM, obtain the value (vac hose and a "T" joint, where those contacts do their thing.
Now, by turning that screw IN, tightens the spring, and the contacts will close earlier, as less vac drop will be needed to do that. A lighter spring load will require the vac to drop further before reacting. Balance it out to suit what YOU want from the engine for a normal driving style.

Open highway, cruising, the vac gauge in mine was in the HIGH vac zone, so those contacts are OPEN (Closed Loop). Pulling out to overtake someone, and "squeezing" the throttle, still has it in the higher vac readings, so MY switch is still open. I am talking 100KPH, to 145KPH increase, and remember, its a V12, you dont have to MASH the throttle to overtake. IF, my throttle movements are enough to mechanically operate that micro switch, then the enrichment is supplied that way.

On our 25000km Aust Round trip, we sat at insane speeds, hour after hour, in some areas, we still had NO speed limits at that time in those areas, and we still averaged 11.2L/100KMS for that trip, so I am NOT complaining, and MORE than satisfied with the balance I did achieve with that Blue/White setting, etc etc.
It initially had me soooooo confused as to how and why it did what it did, this was all happening before computers, and after 12 beers, the haze cleared, and now you have the write up, SIMPLE cars these V12's, you know I am right.

 

beaUTiful...
got ya Grant, thank you...

Q: with this thing (blue switch) disconnected - wires dangling like mine were,,, does the ECU remain in an open loop state?

Seems like it would because when the vac drops it zero, switch turns off,,, extra gas is added - is the O2 sensor control overridden temporarily? Or, can/do they work together/simultaneously...?

Sorry. I'm just trying to understand it as a 'system'.

 

it adds fuel when the switch is closed and goes to open loop. Disconnected it is in closed loop.

 

Ok... Thank you.