Electrical woes

My problems with melted ECUs and Idle Speed control valves are now sorted. However, the DG engine analyser is now revealing another set of probs that are bugging the life out of me....

Basically, the ECU only shows 47 degrees when the engine is hot (new sensor and old show the same) - I've tried dunking the sensor in boiling water - still only gets as high as 47 degs. The resistance of the Coolant Temp Sensor seems to be OK (checked hot and cold against autodata information) and I have checked that the ecu sees the same resistance as at the CTS itself (i.e doesnt appear to be any extra resistance between the sensor and the ECU - I think that would just make it read higher anyhow).

The other thing I did check was the voltage at the CTS and AIR temp sensor connectors - that seems to be 5.5v rather than the 5v that I would have expected - whereas the idle Pot gets 5v exactly.

As if that wasnt enough, the ECU says that battery voltage is 12.0 - 12.2v but my meter says its 13.2. I've tried checking all the connections to the ECU - it gets 13.2v from the injection relay and the earths seem sound. The only thing I can see that it doesnt get is 12v to pin 4. The wiring diagrams seem to show that as a constant live from the same feed that does the fuel pump and ignition relays but mine has the wire heading off towards the front of the car via the 13-way multiplug beside the ecu. I've tried to trace where it goes to but got lost in the spaghetti behind the radio.

Oh - and just to eliminate the ECU from the equation - I have now managed to fix my old ECU using bits rescued from a scrap R19 ECU and it is showing the same readings.

Has anyone ever seen anything like this before? I'm tempted to make a new constant feed for that pin4 and see if it makes any difference but, given my recent experiences of electrical mishaps, you can understand my reluctance. Its very annoying - the car obviously starts and runs but its never going to be right if it thinks its only running at half its proper operating temp - it wont settle down into a 750rpm idle either because it still thinks its warming up.

Any ideas greatfully accepted.

Yes, I have had this before and it was the bane of my life. I renewed most wiring, all new sensors, new cables direct to each sensor, chopped out all the old plugs, reearthed the ecu, new power feeds etc - but I could never fix the low coolant temp sensor problem. The problem is that particular sensors works on a very low and small scale of resistance, and there is something in the system which is upsetting this.

I spent over a year trying to fix this problem on my own car, and was the main reason why I wanted to develop standalone for the GTA. It is purely GTA ecu related, as when I plugged in a standard R25 V6 ecu which uses normal sensors, all of the readings were fine.

The only way I could actually get round this was to fit a C.O Pot spliced in line with the coolant temp sensor, to adjust the resistance to where it should be.

The air temp, water temp and map sensor use a shared earth. Your problem may be there.

Hmmm - nice to know its at least a problem shared - not so good that even you couldn't find out what was causing it. Do you think it could be anything to do with the sensor voltage being higher than 5v? I'm assuming that because the resistance increases with temp, the voltage returned must fall as temp increases. So a voltage that is high to start with will return a lower reading than expected.

Also - why would the battery voltage be 10% lower. Its wierd - its like a reference voltage is 10% out or something - 5.5v is 10% too high and 12v is 10% too low versus the 13.2v that it should be.

Don't know, to be honest, I can't remeber if I checked the sensor voltages.

First of all, disconnect all the other sensors such as the map and air temp, and see if it affects the coolant signal. It may be an earth loop problem.

Just tested my ECU out of the car, only +12v, earth and the Coolant sensor connected - sensor still only reaches 49 degrees in water straight from a boiled kettle but the resistances are as expected for 90-100 degree water. So - I guess I'm at the same conclusion as David - something wonky with the way the ECU is dealing with the CTS. If this is common - wonder how many other GTAs are running round with an engine management system that thinks that the engine has not quite heated up yet?

Anyhow - I believe there's a party or two to be had tonight - best wishes to all for the new year.