I'm trying to go through the entire ignition and EFI system one item at a time to find the cause(s) of my poor idle. Some parts are on order so I decided to check a few items that I've been remiss to verify. First thing this morning was my Air temperature sensor.

I've read that the ATS is only a minor trim to the EFI and not very important so I've never even bothered to check that it was working. I should have.

My ATS was reading wildly off spec. It was 60F this morning and I was looking for somewhere between 300 and 400 Ohms. I metered it at 2.4 KOhms. With that reading I am suspicious that it's a coolant temp sensor that someone put in there. I jumped the harness connector out and instantly the RPMs dropped and she ran smoother.

Driving with the ATS jumped out she has no dead spot coming onto the throttle any more! Idle is improved, acceleration is far smoother and power is up. Partial throttle response feel right, not hesitant and meek. Huge difference! Probably as big a change as any repair/mod I've done so far.

That ATS is not just a minor EFI trim.

 

I think you hit the nail on the head. The ATS is really a minor trim, but it's range is 640 ohms at 32 deg. F, to 80 ohms at 140 deg.

The coolant temp sensor measures around 2.4k at 80 deg. F. So that is way out of the range for an ATS input.

Good work!

 

I agree, it sounds like you have a CTS in the hole that the ATS should be.

Sometimes the connectors get mixed up as well. Do yours still have the "water" and "air" labels attached?

The ATS *is* just a trim....when everything is working correctly

Cheers
DD

 

My connectors are still labeled thank goodness!

FYI, the difference between disconnected and jumped is pretty huge. 2.4KOhms is big resistance, but D/C'd is bigger. If your sensor is just off bit maybe it's not such a big ECU hit, but D/C'd or broken connection it's got to be >+5% fueling based off idle speed change and sudden milage boost. Smells better too!

 

I'm a bit surprised. On my past and present V12s disconnecting the ATS makes no perceptible difference except at idle.....a bit smoother. I believe Bywater's writes up indicate a 6% change


Cheers
DD

 

So the sensor is telling the ECU it's roughly -8000F. Ok. Surely the fuel map only covers differences in air density down to absolute zero.

Any idea what the ECU max adjustment for ATS input is?

 

Interesting. What Write-up are you referring to?

 

I think it is on the AJ6 Engineering website. The ATS is just a mixture trimming device, and disconnecting it enriches the mixture by about 10% max, I think. There may well be a difference between disconnecting it and having the wrong sensor in there, no idea.
Greg

 

The construction of inlet (air) temperature sensors usually have an exposed sensing element (negative temperature coefficient resistor)

As in these examples


Whereas coolant temperature sensors have enclosed sensor element
As in these

 

My ATS shows correct readings, I noticed a big difference when I connected it though (PO cut the wires back and left them in the loom to the CTS) she used to foul a plug now and then and the fuel consumption was terrible...

 

I guess the difference felt with/without the ATS could depend on the individual car. If you're running a bit lean then the extra 5-10% fueling would be a happy thing. If you're already running a bit fat, it wouldn't.

[shrug]

Cheers
DD

 

That's what I'm thinking as well. Many factors are involved in the simplest jaguar issue.

Turns out my air temp sensor is just failed. It isn't a water temp sensor. Got a new one on order.

 

This is a public service announcement.
With guitars...
--The Clash

According to Sean's Jaguar XJ-S pages, there are two different Air Temperature Sensor Part Numbers. Pre-HE and HE. They have wildly different output specifications. The HE model almost perfectly matches the output spec of the Coolant temperature sensor!

My sensor is fine. The problem lies elsewhere...

 

VERY interesting... That makes 2 pre-HE items on my car... When the engine was changed out to an '85 motor I think they used what ever parts they had handy good to know!

 

Now for an interesting question, you've mentioned lack of power and a poor idle. Does she behave any better when warm?

 

At the time I originally started this thread she behaved the same regardless of temp. CTS tests fine. AAV checks out too.

Currently not running. Choking out. Melted cats "fixed" but still giving me trouble. Not related to the above ATS as far as I know.

 

At the mention of cats, I have to ask about the condition of your lambda sensors? Done and tests here?