This morning I noticed that my Dawes AFR was RED at WOT, in 3rd,4th and 5th, throughout the revs !!

However, I could get it to lift 2 lights (i.e. go to green) by switching the rear screen heater or lights off ....

I've repeated this on the way home, in the daylight. Sure enough, 3rd gear, WOT at 4000+ =blue. Switch on screen heater=yellow/green.

I've put a meter across the battery (my +/- take off for the AFR are directly to the battery[with 2a fuse]) and I'm reading 13.8v without lights/heater and 13.5 with.... (sorry, with engine running)

Not much difference, but I seem to remember alternators putting out 14v ish - Or is 13.5 close enough.....

I've had no flat battery issues (yet), but I'm concerned that

a) I could end up with one
b) OR the ECU/Lambda are actually reacting to the extra current !

c) Maybe someone clever can link the lights/heater circuit to my AFR Prob ?

Thanks,

Mark

 

Odd one. I don't profess to know what exactly is inside a Dawes AFR meter, but on the assumption they use an LM3914 (or something else just as good), it shouldn't be affected by fluctuations in the supply voltage.

The only thing I can think is that there's a bad earth somewhere, and that is indirectly affecting the potential difference read by the AFR meter (and probably the ECU too!).

One way to test this may be to switch the AFR meter negative from the battery to somewhere on the engine block, or maybe, temporarily, directly to the lambda sensor body and see if the problem clears. If it does, it might be worth renewing the existing earths or even fitting a star earth kit.

[Edited by greasemonkey - 9/22/2003 8:09:05 PM]

 

GM, thanks for the reply -

What exactly is the AFR monitoring ? (in electrical terms)

Pot. Diff. between (currently - no pun intended) battery earth and voltage (+) output of lambda (max 12ish). However, how does the lamdba sensor measure/scale its output - Does it use 0v at all ?

ECU is measuring between lamdba + and ITS earth.

SO, the problem here is 'is the lambda outputing the correct voltage' ?

In order to connect the afr to the body of the lambda, I'll need to cut the feed from the ECU, take it through the bulkhead and connect it to the sensor body (Any ideas here ?)

Would it be easier to measure (with a meter) the lambda output across the ECU pin and the ECU earth at WOT (with an assistant!) ?

If so, anyone know which is the earth pin (98my UK Turbo)

I've noticed no difference in the running of the car with the electrics on, so I'm hoping that the lambda and ECU are OK...

Mark

 

GM, I've just seen an earthmod thread on 'electrical'. I'll probably fit an extra couple - one to the engine block (alternator) and one to either the ECU or bulkhead (clutch cylinder)..

Can't hurt I guess.... In the meantime, I didn't know I had a problem prior to the AFR so, fingers crossed eh :O

In the interim, I'd appreciate any help...

Mark

 

Nonononono. It's measuring the potential difference between battery neg and the lambda output, yes, but the voltages concerned are a fraction of what you're thinking. The maximum voltage you're likely to see is in the 0.9 to 1.0 volt area.

Not unless the sensor is knackered. At stoich it'll read around 0.6 to 0.7 volts, while 0.9 volts corresponds to roughly 10% CO.

Correct, which may or may not be the same voltage as the meter is reporting.

In reality I think the problem is more along the lines of "Is the potential difference reported by the meter an accurate reflection of the PD generated by the sensor, and is the ECU getting an accurate representation of the sensor's output?".

No, not what I meant. The body of the sensor is its earth.

Thus if you leave the connection between the ECU and the meter, but take the meter -ve off the battery and connect it to the sensor body (polish the barrel with some wet and dry and use a jubilee clip to retain the wire), you will eliminate any earthing discontinuity from the readings, albeit with the risk of introducing some power supply discontinuity.

This would certainly give you a more accurate idea of the voltages the ECU is working with, but, major caveat, you need a a digital multimeter, and a pretty expensive one to get a really accurate idea of what's going on. Analog meters have too low an impedance, and will skew the sensor voltage, while normal low-cost DVM's don't have a fast enough update rate to keep up with the sensor's voltage fluctuations. If you do it over a few seconds sustained WOT in 5th it should settle enough for you to get a reasonably close idea at the very least.

If I had to guess at it, I'd imagine the problem here is with the meter's grounding preventing it receiving an accurate lambda output. I suspect that if you tried grounding it to the lambda sensor barrel the meter will read consistently again. If it does, a few well placed extra earth points should sort it.

 

Thanks for that GM !

I have a digital Meter, but I don't know if its fast enough.

I'll move the AFR earth from the battery to the sensor (positioned at top of Downpipe ?) and test it there.

I'll then move it inside the car and fasten it to the ECU securing bolt to try and get an ECU earth reading - Hopefully BOTH of these will give consistent, correct results, in which case, I'll leave it secured to the ECU earth and also fasten an earth from there to the battery...

If the two different earth readings (ECU and body of sensor) give different results, does this suggest that the sensor earth is floating - in which case, would an earth from battery to alternator be the solution ?

Please let me know if this sounds incorrect....

Thanks,

Mark

PS _ I just re-read what I said about taking the reading from the body of the sensor (Plonker ! - obviously the body is -ve !!)

 

GM, I've decided to try the following - though probably not until saturday, as someone else is having the same problem .!

this thread

Mark

 

So, two people called Mark having exactly the same problem. This is going to get confusing with two threads running!

Only way to find out is to try it. It might not read a truly accurate voltage but even a "slow" one should get into the ballpark enough to discover whether the ECU is also getting fluctuating readings.

Yep, at the top of the downpipe.

Good move.

Yes, it does.

It should certainly help.