John Vine Posted March 31 Share Posted March 31 (edited) My temperature gauge toggles between coolant and oil temps. Recently, during a 30+ mile journey, I noticed that the coolant struggles to reach 70C whereas oil easily reaches 80-85C. This has become more apparent over the last 6 months or so. A recent Easimap log shows coolant temperature (different sender, of course) fluctuating in the 85-95C range: So, it looks as though the thermostat is working ok but the sender for the gauge is getting tired. This is the location of the sender at the rear of the block: and in close-up: I'm assuming the sender is 71167. But would this one be a suitable replacement (apart from the terminal)? JV Edited March 31 by John Vine Link to comment Share on other sites More sharing options...
OldAndrewE Posted March 31 Share Posted March 31 I would think the sender needs to be matched with the gauge. That ebay one may well be but how would you know Link to comment Share on other sites More sharing options...
John Vine Posted March 31 Author Share Posted March 31 25 minutes ago, OldAndrewE said: I would think the sender needs to be matched with the gauge. That ebay one may well be but how would you know Indeed. Does anyone happen to know the temp/resistance spec of 71167? If I could establish that, I could ask alternative suppliers what specs their senders were. Or, I could simply buy CC's overpriced item. JV Link to comment Share on other sites More sharing options...
Member Jonathan Kay Posted March 31 Member Share Posted March 31 3 minutes ago, John Vine said: Indeed. Does anyone happen to know the temp/resistance spec of 71167? If I could establish that, I could ask alternative suppliers what specs their senders were. Or, I could simply buy CC's overpriced item. 1158 to 44 Ω, according to Caerbont. Jonathan Link to comment Share on other sites More sharing options...
John Vine Posted March 31 Author Share Posted March 31 Thanks, Jonathan. Where did you find those values? And were there temperatures alongside the resistances? JV Link to comment Share on other sites More sharing options...
Member Jonathan Kay Posted March 31 Member Share Posted March 31 Sorry, thought I'd included the link: https://www.smiths-instruments.co.uk/products/sender-units-temperature-senders/ Jonathan 1 Link to comment Share on other sites More sharing options...
John Vine Posted April 1 Author Share Posted April 1 12 hours ago, Jonathan Kay said: Sorry, thought I'd included the link: https://www.smiths-instruments.co.uk/products/sender-units-temperature-senders/ Jonathan Thanks, Jonathan. I've asked the ebay seller about the resistance range. Watch this space. JV Link to comment Share on other sites More sharing options...
John Vine Posted April 1 Author Share Posted April 1 Well, I'm puzzled now. I've just measured the resistances of my two (supposedly identical 71167) senders. The one for coolant reads 11.0 Kohms at 12C, whereas the one for oil reads 4.5 Kohms. To me, this suggests that either the senders are not identical after all, or that the coolant one will under-read at say 80C (which is what I observe). CC Parts website says "Part has recently changed. Replacement for original part" for 71167. Any thoughts, chaps? JV Link to comment Share on other sites More sharing options...
aerobod - near CYYC Posted April 1 Share Posted April 1 3 hours ago, John Vine said: Well, I'm puzzled now. I've just measured the resistances of my two (supposedly identical 71167) senders. The one for coolant reads 11.0 Kohms at 12C, whereas the one for oil reads 4.5 Kohms. To me, this suggests that either the senders are not identical after all, or that the coolant one will under-read at say 80C (which is what I observe). CC Parts website says "Part has recently changed. Replacement for original part" for 71167. Any thoughts, chaps? JV Hi John, my 2012 R400 has different coolant sensors for oil and water. The oil temp sensor is a Bosch type, the coolant temp sensor is a Ford type. The calibration voltage curve in my ECU at 10C shows 3.70/5.00V for the coolant and 3.96/5.00V for the oil sensor. The Bosch NTC sensor has a nominal 3,792 ohm resistance at 10C (which is probably within measurement and manufacturing tolerance for 4,500 ohms at 12C, my sensor would be 3,808 ohms if the calibration curve is accurate). This equates with a 1000 ohm pull-up resistor in the ECU to give 3.96V across the sensor. The Ford sensor would need a resistance of 2,846 ohms to provide 3.70V across it. Either I have a different Ford coolant sensor on my car or your sensor is somewhat high resistance. Did you measure sensor resistance directly at the sensor with the sensor wires disconnected? Link to comment Share on other sites More sharing options...
aerobod - near CYYC Posted April 1 Share Posted April 1 I’m of course forgetting the ECU temp sensor is not the same as the dash gauge temp sensor, so they will likely have a different calibration curve and resistance. Link to comment Share on other sites More sharing options...
John Vine Posted April 1 Author Share Posted April 1 (edited) 1 hour ago, aerobod - near CYYC said: Hi John, my 2012 R400 has different coolant sensors for oil and water. The oil temp sensor is a Bosch type, the coolant temp sensor is a Ford type. The calibration voltage curve in my ECU at 10C shows 3.70/5.00V for the coolant and 3.96/5.00V for the oil sensor. The Bosch NTC sensor has a nominal 3,792 ohm resistance at 10C (which is probably within measurement and manufacturing tolerance for 4,500 ohms at 12C, my sensor would be 3,808 ohms if the calibration curve is accurate). This equates with a 1000 ohm pull-up resistor in the ECU to give 3.96V across the sensor. The Ford sensor would need a resistance of 2,846 ohms to provide 3.70V across it. Either I have a different Ford coolant sensor on my car or your sensor is somewhat high resistance. Did you measure sensor resistance directly at the sensor with the sensor wires disconnected? Thanks for that info, James. My oil temp sensor is CC p/n 71167, fitted in the finger-filter plug of the wet sump (about three years ago): CC Parts advised at the time that it was the same as the existing coolant sensor. My coolant temp sensor is whatever was fitted to the engine when the kit was delivered in 2008. I'm assuming from CC's advice that it is also p/n 71167, but I note that CC Parts sold a "replacement" part at some point (which could well have slightly different characteristics). Yes, I detached the coolant sender lead then measured resistance between sensor terminal and engine housing. I also measured both coolant and oil sensor resistances via the connectors on my dash toggle switch (after disconnecting them). The coolant sensor value was the same. Anyway, I've just ordered a Smith's TT6811-03 sensor from Demon Tweeks which, according to Jonathan's Caerbont link upthread, is a 71167. JV Edited April 1 by John Vine Fix typo 1 Link to comment Share on other sites More sharing options...
John Vine Posted April 2 Author Share Posted April 2 I asked Caerbont if they could provide temp/resistance data for the TT6811-03 sensor (believed to be the same as CC's 71167). This is what they kindly sent me: In chart form: The Caerbont engineer said there was no calibration data below 40C ("literally could be anything", he said), so I'm unable to make any sense of my 12C readings. JV Link to comment Share on other sites More sharing options...
aerobod - near CYYC Posted April 2 Share Posted April 2 1 hour ago, John Vine said: I asked Caerbont if they could provide temp/resistance data for the TT6811-03 sensor (believed to be the same as CC's 71167). This is what they kindly sent me: In chart form: The Caerbont engineer said there was no calibration data below 40C ("literally could be anything", he said), so I'm unable to make any sense of my 12C readings. JV Hi John, The characteristics of the thermistors used are similar across all the manufacturers, although the exact calibration will vary. The Bosch curve is similar with about a 1.5 scaling factor at higher temps compared with the Caerbont curve, but similar resistance at 40C - https://www.bosch-motorsport.com/content/downloads/Raceparts/en-GB/53687307119081483.html#/Tabs=53698955/ Link to comment Share on other sites More sharing options...
John Vine Posted April 2 Author Share Posted April 2 18 minutes ago, aerobod - near CYYC said: Hi John, The characteristics of the thermistors used are similar across all the manufacturers, although the exact calibration will vary. The Bosch curve is similar with about a 1.5 scaling factor at higher temps compared with the Caerbont curve, but similar resistance at 40C - https://www.bosch-motorsport.com/content/downloads/Raceparts/en-GB/53687307119081483.html#/Tabs=53698955/ Thanks, James -- very interesting. It looks as though my 4.5Kohms at 12C could be feasible. I'll measure my new sensor when it arrives, and report back. JV 1 Link to comment Share on other sites More sharing options...
John Vine Posted April 13 Author Share Posted April 13 An update... Demon-Tweeks let me down, I'm afraid. Although the sender was shown as in stock when I ordered it, nothing arrived. When I queried this, DT regretted that it was now out of stock. Annoying, as I wanted to fit it prior to my imminent trip to France. I approached Caerbont, who said they could supply the sender. They sent me a pro forma invoice, but specified a delivery time of a week (so, too late for me). I then asked the ever reliable Chris at Redline. Yes, he had the correct part (TT6811-03 = 71167). I ordered it yesterday (pm) and it arrived in this morning's post (Saturday). What a service! I measured the resistance to be 3290 ohms at 17C, so it looks as though that's going to fix my problem. I'll be fitting it tomorrow, along with a bleed tee. For info, the curve looks like this (estimated values below 40C, apart from 17C): JV Link to comment Share on other sites More sharing options...
Member Jonathan Kay Posted April 13 Member Share Posted April 13 Thanks. John. Well done, Redline. But I'm not sure which of those points are your measurements and which are from somewhere else... Jonathan Link to comment Share on other sites More sharing options...
John Vine Posted April 13 Author Share Posted April 13 1 hour ago, Jonathan Kay said: But I'm not sure which of those points are your measurements and which are from somewhere else... Points from 40C upwards are from Caerbont. Points below 40C (apart from the 17C one, which I measured myself at 3290 ohms) are my estimates while attempting to force a smooth graph. I couldn't find a way in Excel to differentiate them! JV Link to comment Share on other sites More sharing options...
Member Jonathan Kay Posted April 13 Member Share Posted April 13 Thanks Jonathan Link to comment Share on other sites More sharing options...
John Vine Posted April 14 Author Share Posted April 14 Well, I fitted the new sender this morning, along with a bleed tee. The new sender has definitely fixed the original problem, although the gauge still under-reads by about 3C, compared to the coolant temperature as reported by Easimap: But I can live with that. JV Link to comment Share on other sites More sharing options...
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