Water Pump

[JFP in PA]

Originally Posted by Blue-S

The Bentley manual hints that the HVAC panel receives engine coolant temp from the instrument cluster, so maybe the cluster is re-scaling the ECT data to effect a slight change...or maybe not.

Based upon the 986/987 wiring schematics, I would have suspected that would be the case, which unfortunately means that the inaccuracy and non linearity of the dash gauge is transposed onto the HVAC display. Standard diagnostics for faulty dash displays always start with measuring the same data via the OBD II port, which is an indication that the port sees the sensors directly without the influence of the dash display. This is why I tend to trust my diagnostic instrumentation over what the dash is telling me…………….

"BTW -- I noticed an anomaly: at highway speed, my cluster was showing 83 deg F ambient air temp but my Intake Air Temp (on the scan tool) was showing 66 deg F! Maybe this gives me a wee bit more spark advance than I otherwise would have..."

Be aware that these measurements come from two different places, one is in the front grill, the other is inside the MAF; so cooling effects of the relative air movement as well as exposure to ambient sunlight are different…….

[Blue-S]

Yeah, I knew that the cluster's ambient temp sensor was in the grille area and I was pretty sure that the IAT was built into the MAF sensor. I was doing my test driving at 10:30 at night, so sunlight wouldn't have been a factor. I had the top down for my test drive (why not?) and the air temp definitely felt a lot more like 83 deg F than 66 deg F. I'm not particularly worried about the low IAT reading, though. Everything else that I could see looked good -- LTFT on both banks was +1 to +3% and STFT was bouncing +/- 2 around zero. I even found that my radiator fans DO work on high speed with the A/C on, so my only fan issue appears to be an inoperative left low speed fan. It was a good test drive...

[JFP in PA]

The fan issue is probably a bad resisitor, which is replaceable, but is also expensive...........

[pk2]

Originally Posted by JFP in PA

Based upon the 986/987 wiring schematics, I would have suspected that would be the case, which unfortunately means that the inaccuracy and non linearity of the dash gauge is transposed onto the HVAC display. Standard diagnostics for faulty dash displays always start with measuring the same data via the OBD II port, which is an indication that the port sees the sensors directly without the influence of the dash display. This is why I tend to trust my diagnostic instrumentation over what the dash is telling me…………….

"BTW -- I noticed an anomaly: at highway speed, my cluster was showing 83 deg F ambient air temp but my Intake Air Temp (on the scan tool) was showing 66 deg F! Maybe this gives me a wee bit more spark advance than I otherwise would have..."

Be aware that these measurements come from two different places, one is in the front grill, the other is inside the MAF; so cooling effects of the relative air movement as well as exposure to ambient sunlight are different…….

-----------------------------------------------------------------------------
JPF and Blue_ s ("s"for "sleuth"?)

Good job. What a plot twist, What a surprise! (not).

Aren't all these sensors O to5 v? and gauges the same? If so, with schematics, pretty simple measurement; sensor to tach and from tach to hvac, although it might easier said than done, that instrument cluster is just about every but, mainly one big circuit board is more like it

Anyways, if you wanted, it would seem like an easy hack to jump your way around the gauge.

Anyways, with an obcII reader, it makes all of this moot I, but it would be nice to accurate look at the dash and get an accurate read, and worst fumble with the hvac.

[Blue-S]

I just couldn't resist gathering more ECT data over the weekend:

80 mph in the cluster, 96 deg F ambient temp in the cluster
HVAC = 93 deg C (199 deg F), Scan Tool ECT = 196 deg F

The highest ECT I saw in 40 miles of driving was 201 deg F on the scan tool, and that was at 75mph in a 98 deg F ambient temp, while climbing a shallow grade. Still, I will probably install a 160 deg F thermostat the next time I have an excuse to open the cooling system. I'm just not eager to replace a year-old stock thermostat right now...

[pk2]

Good work Blue-s, Keep it coming, Now try it at 110 (kidding of course), seriously though good work. So the upshot is despite all the hankpanky in the gauge, we can trust an OBCII scanner and the HVAC readout..more or less? You sound like you don't necessarily buy into the 160 ?

My numbers: I would say that you temps are the highest (outside of the track) actually is sitting in traffic, In my experience, ya got nothin blowing throught your rads. I could use that for an excuse for doing my testing in the drive way at 0mph, but I was just lazy and wanted to see the hvac thing work. Forget I forget If I posted this but:

Idle.....needle to the left of 180.............hvac....200......normal cruising needle position
Idle.....needle over rt end of 180...........hnac....210......fans come on

Give me a spiel, talk me in or out this, I'm in no hurry to dig into the cooling styem either. But my motor takes a beating. Was there the 5HP.increase claim (yawn)? Is your take on your scanner it's just not so significant enough?
Regards, PK