Bit of technical support please

[STE VR]

Having already started a topic on my erratic dash water temp issues I’d thought I start this one as it’ll likely be a bit more in depth.

Water temp readings very erratic on dash. ECU fine and stable. Fans ok. Cooling system ok. Everything is brand new. (Major restoration).

OCD won’t let this issue lie so need help.

I’ve injected the plug that fits on the water temp sensor with a calibrated resistance. 86 ohms gives me 95 degc. When engine off dash reads very stable throughout the range. Start engine readings jump +\- 4 deg. (Thus eliminating the two new sensors I bought!!) This is consistent across most readings except when temperature is low (60 deg) the jumping is less. The higher the reading and therefore the lower the resistance the jumping is much higher. It looks like interference somewhere.

I want to eliminate the loom but can’t find where the engine loom wiring meets the dash pod. With tracing the loom it seems to me that it goes to the ecu???

I’m being a bit lazy asking here but I want to inject straight into the connection directly before the wiring hits the dash.

It’s a big job to trace so any knowledge out there would be very, very helpful.

Thanks.

Edit: 2003 T350c 3.6

Edited by STE VR on Tuesday 6th August 21:15

[STE VR]

I’m getting there slowly. Hopefully have an answer either way over the next few days. Injecting straight into the dash ecu has same effect. Must be picking ‘noise’ up from somewhere.

[STE VR]

Unfortunately not a definitive answer yet. Had added additional main earths just for test purposes but still no joy. Steady up to 91deg then starts jumping after that.

Still haven’t completely worked out how the sensor is wired. Looks to be split between main ecu and dash ecu.

Might borrow a dash ecu and see if the problem still exists.

[STE VR]

Hi, unfortunately not yet. Did try two other dash ecu units but both were faulty (given to me as suspected faulty so no surprise) Also noted that the oil pressure and battery voltage were not exactly steady either but they just seem to be ‘live data’ changes rather than the jumping of the display.

Need a properly proven dash Ecu first to rid that suspicion. That Ecu has to come from a car with a known steady temp reading above 90 degC though.

[STE VR]

s6boy said:

Could be a noisy output from the alternator. Mine stared with erratic temp and went to the whole pod flickering/weird readings.

Yep, looking very possible. It’s a brand new alternator but doesn’t mean anything when fault finding. Been there before!

nawarne said:

On the Tuscan forum - some years ago (Google search) similar problems.

One responder advised that checking and tightening the earth - Battery -VE to chassis at rear of battery box/pass' footwell was the culprit.

Nick

Thanks, yes seen that. Again all brand new and looks good. Tested out ok and even threw an additional earth to it just in case.

Basil Brush said:

Apologies in advance if this is a stupid question as I'm not sure if I am understanding your posts correctly. You know there are 2 temp senders, one feeding the dash and one for the ECU? I'm only asking as you were saying you couldn't trace back from the sender to the dash.

Thanks, yes aware of the two. The ECU one is fine, the dash one is erratic. They both seem to have their signal return going to the ECU but the dash sender seems to have its ‘positive’ wire to the dash ECU. The ECU and dash ECU must be linked somewhere for the signal return.

I’ve probably cocked up somewhere in the tracing of the wires.