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1990 Jaguar XJS - Air Temp Sensor Alternative/Upgrade?
Hello All.
Wondering if folks have any information or ideas about the possibility of an alternative/upgrade for the air temp sensor on the breather. The called for OEM style seems a bit pricey and with my 928, folks in that forum community came up with something that is much cheaper, sworn to be more responsive and is more today's tech. See pic. Is there an XJS equivalent?
The truth IS a LOT of this is above my head. For now. But I would guess I need a sensor. I will take some measurements, then what?
I intend, with some fiddling, to install a temp sensor more like the one below in the 928. Is there anything for the XJS?
This is something I was reading (sorry about the choppy format - I am using my phone) about the XJS temp sensor:
Did a bit of digging recently, looking for the resistance values for the CTS and the MAT (Manifold Air Temperature) sensor, as I have currently fitted a MegaSquirt II in datalogging mode. The investigation revealed the following: The V12 HE and the I6 uses the same sensor according to the spares manuals on the JHDT CD. The I6 service manual gives three temperatures at which to check the resistance, and gives a resistance range for each. If you take the mid-point of these readings, it matches up exactly with the resistance table of the CTS as given in the V12 HE service manual.
So: ergo, the are exactly the same. When I measure both with the car having stood for more than a day, at around 20 deg C, they both measured in the range of 2500 ohms.
Which brings me to a problem I picked up. I used the resistance, table inserting resistances of the values given at -10, 30 and 100 degC in place of the sensors, to calibrate the MegaSquirt II ECU.
Monitoring the MS II with a laptop, the Coolant temp and air temp is reported to be about 20deg C higher than it actually is. With brand new 82 deg C thermostats, brand new CTS, a dual fan switch that comes in at 86 deg C and 92 deg C for the main and secondary electric fans, and the fans not on, and the needle a goodly amount under the N, MS II tells me the coolant temp is 100 deg C.
So, I have my doubts about the resistances quoted for the higher temps. I will be doing a bit more experimentation, and measure the resistances of my old and new CTS and MAT sensors in boiling water and report back.
If there are some others that can do the same, we
could get a good idea if this is actually a problem.
Why, you might ask? If the ECU thinks the engine is at 100 deg C instead of say 82, it will run considerably leaner, which actually is in line with quite a few people complaining about their engines running nicely when cool, but not when hot.
Looks more like this for $100 cheaper - and more accurate and responsive... Why not?
How will you wire it up?! I would love to add modern replacement sensors, I'd also note in my experience the 30yr old temp probes are more accurate than the crap SNG barrat etc sells, a few no name replacements ended up being 30% different, and the original sensor read normal operating temp after mixed driving.
If only there was room for after market digital gauges.
How will you wire it up?! I would love to add modern replacement sensors, I'd also note in my experience the 30yr old temp probes are more accurate than the crap SNG barrat etc sells, a few no name replacements ended up being 30% different, and the original sensor read normal operating temp after mixed driving.
If only there was room for after market digital gauges.
Hey Man! How are ya?
Ummm, if resistance values match with temps it's an install and two wire plug and go... Plugged into the original wiring for the original sensor. Not this one, but many of the sensors use a similar if not the same plug style as the fuel injectors in our cars...
I've been looking around today and this one seems to match. Notice the temp to resistance chart and specs. And, the needed voltage to "run" it. From what I understand our OEM sensor needs 5v and at 20c sends about 2500 ohms of resistance to the ECU. I would like to see an OEM XJS table of temp to resistance graphs so I can get a better sense of what the ECU is looking for over a range of air temps.
Anyways, I'm looking for other sellers of THIS sensor now... This one, from this company, is still a bit pricey, lol. Yup. I'm that cheap. I guess it's about $60usd...
For the air sensor (as opposed to one submerged in liquid),,, I think it would matter how quickly the temperature of the material in the sensor reacted to change in temps. These modern sensors (air that is) react immediately. Then again, how much change does one really encounter in air temps in normal conditions? I would guess not much.
Still (although I don't have any good way to test my OE sensor) I would think I need a new one. I'll do anything for this ol lady to use less fuel. But paying 120+ bucks is a bit hard for me for a few reasons.
Are you certain that you really need to replace the sensor? How is the car running and starting? These cars will use a lot of gas, especially on short stop and go around town trips. I've read road tests that stated that the HE engines could reach 20 mpg on steady speed freeway trips. That was with a new car in hopefully perfect tune. I imagine we would be lucky to see freeway mileage of 15 plus mpg. In Calif. my XJS has to be smog tested. The results have been very good, it is well within limits. The first time I smogged it the operator was amazed at how clean it ran. If it scores well on the test you can see the read out. At least the test confirms that the motor is running properly. As long as it passes! Maybe you can have the exhaust analyzed and see if your engine is running within tolerances. Good Luck.
Theres your chart. As for MPGs I get 15.759 on 80% highway 20% city drive (1 tank's worth) and thats on a mountainious 4000ft climb up the coquihalla at 130kph with 94octane. 20mpg on a new engine at flat sea level doesn't seem impossible.
Riv. Very good points... I have a guy here that would put a sticker on a car in flames,,, if I asked him to. I've never had my 1990 tested. Although, now that you say it, it makes sense and maybe I should just bring it over and have him hook it up. I think mine is running a bit rich...
I need to gather my electrical testing courage and test the probe leads from the O2 sensors in the boot, which I came across the other day... I recently finished exhaust, cat elimination, new downpipes and fitting 2 oxygen sensors and have not really tested their function, yet.
Honestly - have some reading to do. Don't know exactly what values I should be looking for from the O2 sensors. Or the fuel pot mixture dial on the ECU. Still have a LOT to learn.
Now I want to go shopping for sensors ha! Need A/F ratio gauge in digital, this fast air temp sensor you found looks like its right in range I wonder what the result would be...I guess I could spend the money to find out.
The NZEFI folks seem to NOT include a part number. I may have missed it. But I found this and will see if I can't chase down the relevant information...
California has really cracked down on fraud in smog testing and the fines for the shop are very steep, besides losing their certification. The test properly done does give you an idea on how the engine is running. All of my hobby cars except the '51, which isn't running anyway, have to be tested. I do worry every time. If I start running into a lot of problems I'll either just buy newer or much older, pre 1975, cars.
JJJ
FYI the air temp sensor in the B bank air inlet trumpet is only a trimming device for the ECU and does not do that much as far as the mixture strength is concerned. Even disconnecting it only enrichens the mixture a few percent. So using the OEM sensor is quite adequatefor the ECU/mixture system. Replacing it with a more modern and more exact/sensitive device will be lost on the ECU!
JJJ
FYI the air temp sensor in the B bank air inlet trumpet is only a trimming device for the ECU and does not do that much as far as the mixture strength is concerned. Even disconnecting it only enrichens the mixture a few percent. So using the OEM sensor is quite adequatefor the ECU/mixture system. Replacing it with a more modern and more exact/sensitive device will be lost on the ECU!
I hear ya! Thanks.
I had never changed mine, or even priced them. When I did, maybe it was just the sellers I came across, north of $100...!
The curve is steep, 4 cars now, all very different, but I had understood (watch OUT - there's that phrase again, lol) that the car understanding the density of the air (cooler air being more dense) mattered quite a bit. Maybe it's just that way with the Porsche.
What I don't like about the sensor in the the trumpet, the sensor being mostly metal,,,, the trumpet and housing being metal, is it all gets VERY hot and (unless I just don't understand it) the sensor doesn't seem 'isolated' from the conduction of heat from the trumpet into the sensor. After a while of running, I can't see how the air temp is what it's sensing anymore. Also, lots of air is underbonnet air,,, that is hot... Would hot less dense air ask the ECU for more fuel, or less?
Has anyone figured out a way to get (cooler) air MORE from outside of the bonnet? Air tubes, tunnels?
Anyways. I believe you when you say it doesn't have a real or great impact on the trimming of fuel/air. I've removed mine, forgot it wasn't connected more than once, and connected it listening for a change and not much could be noticed.
If I can find that sensor for 20 or 30 bucks down the line - I might just do it, just because. Cool little project.
Air density is important, but on these early systems it was not adjusted for anything like as precisely as modern ones. Atmospheric pressure likewise, which is not directly measured at all on our cars.
ignore the red arrows and look at the 6.0L air intake ducting. This gets you cold air fast, some members make their own personally I just ripped all the foam out from around the radiator...my reasoning was that hot air at low speed will give a fuel efficiency boost while all that free flow at 120kph will give the colder air the engine wants.
I've hacked out the trumpets on my filter boxes it's just a flat mesh face so it scoops up quite abit of air either way.
ignore the red arrows and look at the 6.0L air intake ducting. This gets you cold air fast, some members make their own personally I just ripped all the foam out from around the radiator...my reasoning was that hot air at low speed will give a fuel efficiency boost while all that free flow at 120kph will give the colder air the engine wants.
I've hacked out the trumpets on my filter boxes it's just a flat mesh face so it scoops up quite abit of air either way.
Interested and interesting... I like the looks of that and it wouldn't take much to do. Not really. Sitting here I cannot envision EXACTLY what I have for room at the area above the radiator on my 5.3 1990. But I am sure there is some.
So, let me see if I have this right. When an engine (not necessarily our engines but in general) is getting hot air (less 02 per volume) it adds to or reduces the amount of fuel? I guess with hot less dense air the 02 sensors are sensing less air in the exhaust and reduce fuel...? Is that right? Then the opposite for colder denser air?
Does the ECU come up with some relationship or correlation between the numbers at coolant temp and air temp at start up for fueling?
I need to say,,, I literally WANDERED into being a car guy only 3yrs ago, so I promise you these questions (how ever elementary for y'all) are very real for me and I want to know and learn more,,, so that's why I'm asking. I also find what I am learning fascinating...
I think the gist of it is correct, hot air just mixes better needing less fuel to complete the process. I get 15.75mpg with hard driving I guess for an engine designed in the late 60s? I can't complain. My friends turbo lincoln SUV is only a hair better on economy lol.
So, let me see if I have this right. When an engine (not necessarily our engines but in general) is getting hot air (less 02 per volume) it adds to or reduces the amount of fuel? I guess with hot less dense air the 02 sensors are sensing less air in the exhaust and reduce fuel...? Is that right? Then the opposite for colder denser air?
Does the ECU come up with some relationship or correlation between the numbers at coolant temp and air temp at start up for fueling?
Warm air is less dense than cold. At tickover/traffic speeds (and I am quoting Roger Bywater here) warm air gives better fuel efficiency because (I think I remember correctly his point) you get better fuel droplet evaporation into burnable fuel vapour. At higher revolutions, this is less of a consideration because the airspeed down the inlets, and the swirl and squish in the cylinders etc etc ensures good fuel vapour conditions even with cold air.
Cold air is theoretically better for power, as cold air being more dense means more oxygen molecules per cylinderful of intake air. So at higher speeds cold air is better, at lower speeds cold air is less fuel efficient. If you go out on a crisp below zero winter day, you will notice sharper bark to the exhaust, and sharper engine response than in high summer.
I seem to recall that the difference in max BHP between zero degree C cold air and (say) 35 degrees, is about 10%.
This has nothing to do with O2 sensors etc etc, whatever state of tune the engine is in it would have the same effect.
Yes the V12 ECU does alter the fueling for coolant temp, and very slightly for air temp. But this is a far less sensor-driven system than modern ones are.
Last edited by Greg in France; 04-27-2020 at 05:31 AM.
04-24-2020 | 02:45 PM
1990 Jaguar XJS - Air Temp Sensor Alternative/Upgrade?
