Faulty yellow temp sender?

[nrc 0]

Apologies in advance for yet another cooling thread but I could do with some advice here.

 

So I've recently got the VR back on the road and have replaced all the senders in the cooling system (also thermostat) as the oil was running at about 118°C most of the time and it seemed apparent that fan stages 1 and 2 were not engaging while the engine was running, with only the after-run kicking in, possibly stage 3. I tested fan operation by bridging connections and everything seemed OK there, so reasoned the senders might be at fault. Coolant temp on the dash was a little lower than oil, between 90 and 110°C.

 

Nowadays the oil sits a little lower at around 112/114 but the coolant temp is always sat at 70, give or take 5 degrees. While idling I've seen the fans come on for a few seconds at a time and the after-run still operates if the temperature is high enough. Signal from the blue sender (via VAGCOM) matches the oil temp on the MFA.

 

At the moment I'm suspecting the new yellow (or black as it is now) sender might is faulty - resistance too high, so reading lower temps than actual and failing to operate fans at the correct point. Does this sound reasonable and while I drain the coolant to swap the sender, is there anything else I should consider checking?

 

Cheers for reading all that :)

[swiftkid 1]

Seems like you have a similar problem to me, stage 1 fan doesn't seem like it kicks in at all and I thought stage 3 was the only one working but now I think the more likely story is that I'm being paranoid and stage 2 just sounds powerful. Anyway, have you bridged the wires on the radiator temp sender? Stage 1 takes its information directly from this sender with stage 2 taking information from both the yellow sensor and the radiator sensor, now here is my theory which is what I'm to do next, the radiator sensor is knackered so fan stage 1 isn't kicking in and stage 2 is kicking in because of the yellow sensor. That's only my theory though!

 

Your problem may be that you had an airlock in the system and now that its been bled properly its running cooler but I'm afraid I can't say for sure as my water temp seems to fluctuate a lot.

[nrc 0]

Aye, having re-checked with the rad connector stage 2 is not engaging when bridging the connections so I think stage 3 must be keeping it in check at 114°. I know this worked last year when I checked...

 

Anyway, I've swapped back to the original yellow sender so I hope that's going to sort the gauge and now I'm going to try and get stage 2 working from the rad. Unfortunately the wiring there is in a mess from the dealer aircon installation so this isn't going to be fun :/

[nrc 0]

It turned out that all senders were working fine and the temp gauge in the dash was at fault. 2nd stage of the fan started working too after a strip down and check of the fan controller.

 

In the instrument cluster the two filaments that wind around the bimetallic strip for the temp needle had fused together near the terminals, leading to a short circuit and little to no heating of the strip. Prised them apart and now the gauge is spot on! Oil's still a bit on the warm side - going to check out the main water pump next.