05 Baja odd coolant gauge issue

[mkc1962]

I took my Baja out for a drive a month or two back and on the way home the Temp gauge went straight above the H almost immediately (yes I anally monitor my gauges frequently) . Pulled over and also noticed the cooling fan was not running. Also that the AC coil had disengaged. Let it sit for a while out of concern, running heater fan at full to assist, finally got it home. When I got it home I hooked up a scan tool and with IR scanner also in hand it was not running any hotter than around 190. Now a few months later finally got it into the shop as it was time for a timing belt/water pump and  try and get everything little wrong with it back in perfect condition.  Before the work started I started it up again and noticed the temp gauge almost immediately went straight to H, checking with OBD2 scan tool and Torque app at that moment in time it was only 102 deg, IR temp scanner at multiple locations about the same..By no means hot...I figured must be a temp sensor.

I went forward with the work. All new pump,belt,tensioner etc, as well as new OE thermostat and OE temp sensor. Did notice the old temp sensor was a bit crusty looking when I pulled it out almost sort of like a dissimilar metal oxidation but never really seen a brass sensor look like that.. Even though I have the full 6k page FSM could not find a ohm value reading for the sensor so unsure if it was bad or not, all the same replaced it just due to age. All back together cranked right up, and once again almost immediately the gauge goes half then full long before the coolant system was even at 120 let alone full operating. Had the scan tool monitoring the live ecm temp and just let it run to get the air burped out of the system. At the same time constant checking with an IR temp gun, nothing ever got higher than 192. However, I figured at around 185-192 the main fan should have come on and it never did. Have tested fans and ALL relays all is ok there. Shut it down to start looking at electrical issues. From all I can see/read there is only "ONE" temp sensor on this vehicle, on the back side of the intake manifold right of and below the throttle body.

No visible critter activity to justify any of that thinking. I keep almost every connector on it dielectric greased due to southern humidity. Have checked all the Grounds I can find.

So I have a gauge going full scale almost immediately. And main cooling fan not coming on when I feel it should. However scan tool and IR testing showing it is not any hotter than it should be at normal idle condition. I am thinking some sort of common electrical issue, maybe tied to one of the ground circuits that may or may not be shared between the two. However, the FSM does not show the same GND circuits for these two. GND-03 for the cooling system, and GND-01 for the fans . FSM shows 10 GND circuits and many physical gnd ties, but I cant seem to find a page anywhere in the manual that shows the physical location of many other than the engine harness section.

Has anyone out there ever seen a similar situation? Thanks for any feedback.

 

Mike

 

[GeneralDisorder]

Radiator fan is ECU controlled and will not come on till 204. 

Temp sensor is a 3-wire and is actually two sensors in one housing. One for the ECU and one for the gauge cluster. Sounds like possibly a wiring short. A short to ground in the gauge circuit will peg the gauge all the way to hot. Resistance of the sensor drops as it is heated so a short to ground (0 ohms or close to it) will cause the gauge to read full tilt. Unplug the sensor while it's running - should drop the gauge to zero and the ECU should throw a circuit code for it's side of the sensor and will kick on the fans till the circuit is restored and the code cleared.

GD

[mkc1962]

GD, thanks for a reply, was not sure what temp the fan came on but new it was ECM controlled. its never gotten that hot so that part about the fan not being on makes sense.

I did unplug the sensor, but pre crank and it would not crank and did set a code. Never thought to try it while it was running.

with the new sensor, I did notice that when unplugged the gauge was at zero when cold, but when plugged in the gauge would never go below half. This pointed me at pulling that new sensor to compare it to the old meter wise. Both started at around 1700 ohms. When heated with a heat gun the old on (after I cleaned it with scrotbrite) would go to about 400 ohms. The new one would go to near zero ohms. But what found next is probably the cause.

The 3 pins on the new sensor vs the old were not clocked the same. 1 pin goes to ground the other 2 vary based on temp. It appears the improper clocking had the wrong two pins going to the gauge

For the moment I put the old cleaned up sensor in. I plan to send this new one back. A factory sensor should not have been such an issue. If the old one acts  up again, Will try again to get another one, but now know what to test it for before putting it in.

[GeneralDisorder]

Well those tests would seem to indicate the new sensor and possibly the old sensor are both faulty. The gauge going to full cold when disconnected would seem to validate the wiring.

All parts - across the board - have fallen in quality and quality control. It wouldn't surprise me if you got a bad "new" part. This happens to us all the time now and it's extremely frustrating from a shop owners perspective. I have to warn customers who I have sold reman alternators to that we are currently seeing about a 50% failure rate on every brand including Denso. 

GD

[mkc1962]

I agree on the new sensor. I did not heat the probes above the 190 range. At 190, the new one was at near zero ohms while the old one ( after being cleaned) hung in around 400.

Oh I fully agree the quality of parts these days has gone to crap. Obviously even when obtained from the OE dealer, no doubt this temp sensor being such. I don't turn wrench for $ anymore  just for myself. Reman starters and alternators these days are a crap shoot at best. I also put a brand new (not reman) alt on the baja as part of this service as the old one was at best putting out about 12.8 just prior to the one the road issue seen before this work. I had been hearing an occasional squealing, found to be a rear alt bearing that prompted me to meter check it. no doubt under load probably much less.. That in itself might have caused some of the pre-work gauge/ac coil issue, as I have seen low voltage supply cause similar on other vehicles. I'm now at 14.3 with the new one.

For now taking it for some test drives, leaving the scan tool connected to make sure no overheat. If I see even the slightest repeat of gauge issue, will order another OE sensor, but bench test before install. Meanwhile this new one is going back for a refund.

Thank you for your feedback.