Having suffered from Fury engine cooling problems for many years, I have been driven to discover how accurate the coolant temperature measurements provided by the Fury's temperature gauge and the ECM (observed via ALDL) are.
Clearly, the accuracy of the temperature gauge reading is a function of the degree of match between the sender and gauge calibrations, and similarly the accuracy of the ECM temperature reading is a function of how well the look-up table(s) in the ECM software match with the sender. Since this page relates to the ECM, I will confine future discussion to the ECM CTS calibration...
The diy-efi GMECM web site provides a table indicating how the electrical resistance of a genuine american GM CTS varies with temperature. No reference source is listed. The data are reproduced in the table below:
| Temperature (deg C) | Resistance (k ohm) |
| 100 | 0.177 |
| 90 | 0.241 |
| 80 | 0.332 |
| 70 | 0.467 |
| 60 | 0.667 |
| 50 | 0.973 |
| 40 | 1.459 |
| 30 | 2.238 |
| 20 | 3.52 |
| 10 | 5.67 |
| 0 | 9.42 |
| -10 | 16.18 |
| -20 | 28.68 |
| -30 | 52.7 |
| -40 | 100.7 |
Even if the above table is the correct specification for a GM CTS, clearly what matters to my engine is the calibration of my particular GM CTS. Consequently, I generated a similar table, for a GM CTS that I acquired with the ECM, by taking measurements of sender electrical resistance at different water temperatures. I did this by suspending the sender in a pan of water, which was gradually heated, and recording sender resistance (using a DVM) corresponding to water temperature (as indicated by a mercury thermometer). The result was quite a good match with the above table.
To test the other half of the ECM CTS equation, i.e. the CTS look-up table in the software, I recorded what CTS values the ECM reported (via ALDL) when I applied different resistances across its CTS pins. I did this using ANHT (mask $8D) software and a variable resistor. The result was an excellent match with the values in the above table.
Since my current engine (a Vauxhall C20XE) has no threaded boss which matches the GM CTS, and my engine comes with its own CTS, I was curious to discover if this could be used with the GM ECM instead of the genuine GM CTS so I repeated the above tests using my engine's native CTS. The result was quite a good match with the above table - so I decided to use the native XE CTS.
All the above data can be seen overlayed in this wide range plot.
Despite the similarity between the XE's CTS and the GM CTS, I decided to make the ECM's CTS reading more accurate by modifying the CTS look-up table in the software, in the region of normal running temperatures, to better match the XE CTS. Both the 1227727 and 1227749 ECM's have the ability to switch between two different CTS pull-up resistors and the ANHT (mask $8D) and Sunbird (mask $58) software switch between these at 50 C. (Having two different pull-up resistors allows a better combination of CTS measurement accuracy (achieved using an A/D converter) at low and high temperatures.) Consequently, both sets of software have a CTS look-up table for each pull-up resistor. The standard ANHT and Sunbird look-up tables for the 4 k pull-up resistor are identical, and those for the 348 ohm pull-up resistor are the same to within +/- one A/D LSB.
My new, more accurate, look-up table temperature values were obtained by computing the CTS resistances that equate to the A/D values in the look-up table, and replacing the existing temperature values with those read off this high range plot which shows 3 sets of measurements from the same XE CTS, with exponential trendlines fitted to them - to aid interpolation.
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