The ABS sensor arrived yesterday so next fine day that I'm off it will go in, hopefully without too much of a fight!
I notice that the new sensor does not come with a mounting bolt so I hope the old one is usable and doesn't shear or round off during removal...
Before I've even had a chance to fit the ABS sensor the car decided to throw me a second curve ball - we were heading out only a mile from home queued in a traffic jam when the coolant temperature warning light and the STOP light started coming on and off intermittently. I rolled the windows down and could hear the cooling fans were running permanently on high speed even when the warning light wasn't lit. Sigh.
The temperature gauge was also reading unusually low despite us being more or less stationary - it usually reads around 70 but it was not even going that high. So I turned around and crept slowly home, being reasonably sure that it was a sensor fault and not an actual overheating condition, since the gauge sensor and fan sensor are two separate sensors - genuine overheating should cause both the warning light (from the brown fan control sensor) and the gauge (from the blue sensor) to read high, but the gauge was quite a bit below 70.
I popped it straight on the Lexia when I got home and it showed the reading (from the green sensor) was normal - around 90 degrees despite the fans still going at full speed and the warning light flickering on and off. After stopping the engine I gingerly opened the filler cap to make sure the low coolant sensor wasn't triggering the warning light but the coolant level was fine. Phew! No problems with the cooling system just a dodgy output from the brown sensor, which is a common problem.
So out with the infamous air filter box to see if the brown sensor can be reached without a major strip down - which it can, just.
The brown sensor can be seen peeking out near the middle below the hose clamp:
My intention was to give the connector contacts a good clean with LPS 1 in case they had some corrosion on it. To my surprise I discovered that this sensor isn't directly connected to the wiring loom, but rather comes via another seemingly unnecessary black connector, shown unplugged above, something I haven't seen mentioned before. So there is a short patch lead between the sensor and the main loom:
The significance of this being that anyone cleaning the connector on the sensor itself who did not notice this extra connector (and I did not notice it on my previous V6 when I did the same job) would fail to clean the connector on the black plug as well - which is also a potential source of trouble. Whoops.
Anyway both connectors were duly cleaned (although they did not look that bad to me at all) and refitted. Along the way I discovered a rather nasty problem with the connector for the knock sensor when I went to refit it!
The large protective moulded rubber block is completely ripped open, right down to the point where the cable inside is fully exposed.
Fortunately the wire's own insulation is not pierced, but the cable as a whole is not protected. It seems pretty clear that it has been damaged by the hose clamp sticking out behind it when someone has had it in and out:
I don't know whether I did this on a previous occasion (I definitely didn't do it today) or whether it was already like that. I didn't have time to try to repair it today and I'm not sure of the best way to fix it - maybe a bit of oil resistant silicone in the crack ? Or maybe try to glue it back together with super glue ? I'm not sure.
I put everything back together and tested carefully - fans not coming on when they should not and no warning light.
I tested the fan activation temperatures by comparing with the green sensor reading given by the Lexia:
Low fan speed starts at 93 degrees and goes off at 90 degrees, high fan speed starts at 98 degrees and goes back to slow at 95 degrees, which is spot on from what I remember.
We then went back out in the car and went about our day albeit an hour late...
No problems at all, and I noticed a very interesting thing. The temperature reading on the gauge, which as pointed out by xantia_v6 and others, partially measures the temperature of the water returning from the radiator, has always read quite low on this car - it never reads above 70 unless you work the car hard in which case the reading goes up to about 80 or in extreme conditions 90 tops.
After fixing the fan control signal this has changed - the temperature is now rising to an indicated 80 or maybe slightly above and is staying very steady around this point neither rising or falling much despite changing driving conditions and it is not dropping back to a reading of 70 anymore - at least not at speeds below 30mph. And remember I have not touched the blue sensor for the gauge which is on the back of the head and difficult to reach anyway.
And I'm pretty sure I know why - I believe that long before it started causing the warning light to flicker and the high fan speed to come on when it shouldn't the poor high resistance connection was also causing the low fan speed to come on too early and prevent the engine from warming up properly when the car was only being driven moderately at slower speeds.
The sensor is a PTC so higher resistance is interpreted as a higher temperature, this is why unplugging the sensor causes the fans to immediately go to full speed - open circuit is interpreted as a very high temperature. But what happens if we just add some resistance in series with the sensor ?
This will make the fan controller think the temperature is higher than it really is with the effect of causing the low fan speed, high fan speed and warning light to all come on at lower temperatures than they should, but otherwise appear to be functioning normally.
The low fan speed was quite possibly coming on before the engine was even fully warmed up when the thermostat was barely starting to open, so during slow around town driving the fan running would prevent the engine reaching operating temperature. The slow fan speed is too quiet to be heard while driving the car, even with the windows down so would go unnoticed.
Because the gauge temperature shows a blend of engine temperature and radiator return temperature, the fan running when it shouldn't would cause an unusually large drop in the return temperature indicated on the dashboard gauge that wasn't necessarily reflected by the Lexia reading. Now that this is not happening the engine is able to reach proper operating temperature on the return side as indicated by the gauge as well.
The funny thing is only a couple of weeks ago I checked for a stuck open thermostat by holding the top radiator hose from a cold start to see if it stayed cold and then went suddenly hot after about 10 minutes as it should - and it did, showing that there is no leakage through a slightly stuck open cold thermostat. I did this because it seemed like it was taking far to long to reach full indicated temperature with slow driving. Now I know why!
So in summary:
1) Even if it has not yet got bad enough to make the warning light flash or the fans to obviously go on high when they shouldn't, if there is resistance from a bad connection it will lower the threshold temperatures for the fan speeds enough to prevent the engine from warming up properly under light loads - check and clean the connectors anyway, especially if your temperature gauge reads unusually low!
2) The terminals didn't look bad to me at all - if anything there was maybe a very fine white powder appearance to them, but nothing that I would say was obvious corrosion. And yet cleaning them solved the problem, so there was definitely a bad connection there. Use a good electrical contact cleaner.
3) Don't forget to do the joiner connector as well as the one on the sensor!
4) You can check normal operation by comparing fan speed activation temperatures with the coolant temperature reading of the Lexia. On mine after repair slow started at 93 and fast started at 98. In the other direction it went from fast to slow at 95 and stopped at 90. If yours come on at a significantly lower temperature there is probably an issue with the brown sensor or its connections!
Hope this is useful to others, especially if your V6 seems to take too long to warm up...
which quickly confirmed the front right sensor (pins 1 and 6) being completely open circuit. So I guess that's what it is...
