Coolant Temperature sensor failure - Code 15 (Page 2/2)
cartercarbaficionado FEB 20, 03:11 AM

quote
Originally posted by br1anstorm:

Thanks to those who posted helpful advice, and especially the picture....

To answer @Patrick's points - yes, usually, normal starting requires minimal gas pedal. So the current situation required more than what was usual. But it hasn't previously been an issue.

Scanning the engine? Well, if I were a full-time workshop engineer with all the relevant equipment to hand, maybe. But I'm not. I have WinALDL on a 20-year old Windows XP laptop somewhere in a storage container. I may get round to unearthing that and trying to see whether it still works and what scan-readings it might provide. But not this week.

Using a multimeter to check the CTS readings? Yeah, but first I have to get in/under/around/and at the connections. Not the easiest of locations to access. One reason why I sought guidance from those who had already been there.

Checking that mice haven't eaten the wiring? (@ olejoedad) .... Yes I know that can be a hazard. But my carport seems (touch wood) to be a critter-free zone. I regularly check on the car and over the winter run it round the block occasionally: so it hasn't exactly been hidden away in storage and neglected. As for visual checks of the wiring, see point above about access. I will indeed be taking a look when I can.

Meanwhile, it's a 35-year old car. The CTS is undoubtedly original. It has been there all those years. It's not an expensive part. They are known to fail. Some indeed recommend replacement on a preventative/contingency basis. See also @Vintage-Nut comment above. Delivery of a replacement part will take several weeks. So I am minded to order a new CTS anyway, assuming I can manage to get at the old one and remove it safely.



check your cold start injector switch and tps as well. if cts went out then usually something else is going wrong. (speaking from the basketcase fieros i own.)
br1anstorm FEB 25, 09:14 AM

quote
Originally posted by Patrick:

".... Have you not scanned the engine and/or run WinALDL to see for sure what reading the CTS is reporting to the ECU?

Or at least use a multimeter to check the sensor.




Well, one thing at a time. If necessary I will try to investigate in due course whether there is a TPS or cold start injector issue, or rodents chewing the wiring.

But first step is that I have a code 15, fault with the Coolant Temperature Sensor (CTS).

I have brought my old laptop in from the attic and fired up WinALDL. I have zero experience in reading and analysing the tables and figures it produces, so this is just a first snapshot of what it appears to show so far.

I managed to start the engine (no gas pedal....!) . It stalled the first couple of times, but then fired and continued to run.

The SES warning light came on.
The idle settled at around 1000-1050 rpm (a/c not running).

I looked at all the tabs in WinALDL. In particular I checked the error code table: it showed only the code 15 CTS (low temp).

I viewed all the figures in the Sensor Data table (but couldn't save a copy, and didn't take a screenshot/photo).

I don't know what the "normal" readings should be. But I looked closely at the Coolant Temp line in the data table. The RAW data column showed '1' initially, and after some 10 minutes that had increased to 3. The converted Centigrade and Fahrenheit columns each showed around minus 34 -38 degrees. The figure did not change over the 10 minutes engine-running (should it normally do so?), while the temp gauge reading on the car dashboard moved slowly upwards to about 160-ish.

Comments and advice welcome. My non-expert assumption is that the minus 30-something readings shown in WinALDL indicate that the CTS is defective (and telling the ECM that the engine temp is well below freezing, which is clearly a nonsense).

If this is correct, does this indeed suggest that I need to replace the CTS? Or do I have to run any other checks - and if so what - to confirm the diagnosis?

Aside from the challenge of accessing the CTS - or even its connection terminals - while the sensor is installed (because it is buried below and behind the EGR solenoid, its bracket, and various wiring cables), I don't know exactly how to test the sensor with a multimeter while it is in place. If that's necessary, do I have to disconnect it and then somehow get the engine running and stick multimeter probes into the CTS socket?! Looks like a near-impossible task...

I have a replacement CTS anyway, ready to be installed. If I simply go ahead and do so (hopefully without losing too much coolant and without removing the additional reducer as well), do people agree with @Vintage-Nut's advice not to use PTFE tape (which is what the factory service manual recommends) but to coat the threads with hi-temp RTV sealant instead?

[This message has been edited by br1anstorm (edited 02-25-2025).]

olejoedad FEB 25, 10:42 AM
I would replace the CTS.

It's ok to use PTFE tape, but ONLY 1 1/4 WRAP ON THE THREADS.
Or you can use a very minimal amount of thread sealant.

The TEMP gage receives it's signal from a different sensor.
Patrick FEB 25, 01:35 PM

ALDLView
Vintage-Nut FEB 25, 02:40 PM
If was me, the new sensor would already be installed.......

Put a big tray under the engine to catch coolant
Unbolt and Remove the EGR Solenoid and push the main wire harness out of the way
Use a socket {I believe 21mm} to remove the sensor
IF the reducer fitting comes out, no big deal / put the fitting in a vise, then remove the sensor from the fitting

Let the coolant drain at the sensor hole {or before where you want to drain the coolant}
then remove the engine cap to drain the last coolant to the sensor level, then remove the thermostat

Clean all the threads
Use whatever sealant you want
Install the fitting/new sensor and tighten down
Reinstall the EGR Solenoid

Use the caught coolant or use fresh 50/50 Antifreeze/Coolant to fill the top of the housing
Cap it enough to seal the housing without the thermostat
Run the engine 30 seconds, shut it off, open the engine cap, add more coolant, put the thermostat and the cap tightly
Run the engine until the radiator hoses are very warm and cool it down...


------------------
Original Owner of a Silver '88 GT
Under 'Production Refurbishment' @ 136k Miles

[This message has been edited by Vintage-Nut (edited 02-25-2025).]

br1anstorm FEB 26, 01:24 PM
This post might well be the final instalment in this particular story.

With helpful advice from forum members (and I have to single out @Vintage-Nut for providing useful specific, practical guidance...) the Code 15 problem is solved.

I managed to access, remove and replace the Coolant Temperature Sensor with no serious difficulty. By the way, it needs a deep 19mm socket (not 21mm) .

One tip: to avoid losing a bucket-full of coolant, when I removed the CTS I immediately bunged the hole with a wine-bottle cork; so I lost only about half a cupful of coolant. And of course this meant that eventual topping up was quick and easy.

I found that the brass reducer or bushing was pretty tightly bonded to the old CTS. The reducer unscrewed easily from the engine block and came out with the CTS. I had to separate them on the workbench using a vice, in order to re-use the reducer when installing the new sensor.

With the new CTS fitted and the fault-code cleared, I topped up the coolant and went for a 30-minute drive. The ECM evidently reprogrammed itself as it should, and the engine now starts, idles and runs absolutely fine again, with no SES light, no other issues, and no new codes.

So all is well. Thanks again to all who took the time and trouble to join in the thread-discussion.

[This message has been edited by br1anstorm (edited 02-26-2025).]

olejoedad FEB 26, 02:58 PM
Thanks for checking back in with information to the resolution of your problem.
Vintage-Nut FEB 26, 03:07 PM
Awesome!

And thank you to explain and 'close the loop'!

Many members don't inform us if our help was helpful or not....