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Good day to all, I have an intermittent issue with my coolant warning light. It will come on and go off at random. I replaced the sensor in the reservoir, and fitted a new spade terminal to the connecting cable because it looked a little worse for age. I thought that was the end if the matter, but alas, the fault returned last week.
Mostly, it's off, but still flicks on and off. Has anybody had this problem I wonder?
Regards to you all, Danny.
Maybe there's a problem with the low coolant transmitter. Can't say I've ever heard of one failing but there's always a first time. Item #5 in this illustration
You might try a careful inspect of the sensor wire and/or give it a jiggle test. If there's any broken insulation to allow the strands to touch ground, the light will come on.
The low coolant sender modules can fail. Mine did. I opened up the case and found that the circuit is quite simple. I changed the IC and transistor and that got it working. To do so, you need to he reasonably proficient at soldering and use a solder-sucker to remove the old solder from the component legs.
By the way, that “sender” in the coolant tank is simply a ground rod. There is nothing to go wrong there!
Meanwhile here is a link to a good value replacement unit, if you don't fancy trying a repair:
Dear Jose, Doug and Andy, many thanks for helpful replies. I have spent some time today carrying out some basic tests. I tested the resistance between the water level in the expansion tank to ground and it was 1.8meg ohms. So then I read the resistance from the probe and it was the same, so rules out probe fault, but as you say, there is nothing can go wrong with it. Next I traced the white with red tracer back to the multi pin socket behind the battery for continuity, and that was OK. (Rhd car)
I then wiggled that plug, but it is screwed in from inside so it wouldn't move much. Then I started the car and the warning light flickered and the stayed off. So I am thinking that it might just be some corrosion on the pins, but does not seem to be easily accessible.
I would like to examine the module as Andy mentioned, and replace the IC or other components, as I am a television engineer, that job is right up my street. Can I gain access to this module if I remove the liner to the glove box? If this module checks out OK I will then check continuity from the module to the sensor via the multi pin plug / socket, but looks like a devil to access.
thank you once again chaps for your usual fantastic advice. Kindest regards Danny.
or
Last edited by DannyRobertHoward; Oct 16, 2022 at 08:01 AM.
Reason: Err
Hi Andy,
if you study the S57 Wiring Guide, it should tell you. If you don't have the S57 guide, download it free from my site in PDF format. https://jagupgrades.webstarts.com
I then wiggled that plug, but it is screwed in from inside so it wouldn't move much. Then I started the car and the warning light flickered and the stayed off. So I am thinking that it might just be some corrosion on the pins, but does not seem to be easily accessible.
Awkward, but doable, to access the pass-thru connector.
I would like to examine the module as Andy mentioned, and replace the IC or other components, as I am a television engineer, that job is right up my street. Can I gain access to this module if I remove the liner to the glove box?
Remove the 'knee panel' under the dash; only takes a few minutes. First remove the end cap at the extrame end of the dashbord and then it's just 3 or 4 scrows to remove the knee panel under the dash
Dear Jose, Doug and Andy, an update.
Removing the knee panel under the passenger side of the dashboard was as easy as Doug said it would be.
The tubular sensor unit unclipped and disconnected in a flash and on my bench.
I ran a junior hacksaw around the top and removed the pcb and connecting cables without causing any damage.
I replaced the chip and transistor as Andy suggested but the problem remained. I then removed the four IN 4148 diodes and discovered that one had a very slight leak. Four new diodes were fitted. To test I connected my bench power supply set to 12 volts DC and soldered a temporary 12v 1watt bulb across the transistor output pin and positive.
When power was applied, the bulb lit and stayed on. This is normal because I hadn't yet connected an input from the probe.
I found that a resistance of 47k ohms or lower connected between negative (chassis) and the input pin would switch the bulb off and it remained off all day.
I then removed my test lamp and glued the top back on to the tube with plumbers plastic solvent and it's back in the car working.
So fingers crossed, I am hopeful for a cure, time will tell. I am so grateful for great advice, what a fantastic forum. Kindest regards Danny.
Last edited by DannyRobertHoward; Oct 20, 2022 at 04:37 PM.
Thank you Jose, I actually earned my living as a television and video repair engineer, so this job was a pleasant change from the usual mechanical work on my series 3.I have attached a photograph of the inside of the coolant level transmitter in case it is of interest to anybody in future. Kind regards Danny. Coolant sensor pcb.
When working as designed, the warning lamp should flash.
You mentioned “the bulb stayed lit” but perhaps you meant if was flashing on and off?
The CD4011 chip is a quad 2-input NAND gate which is configured to oscillate and make the warning light flash in this application (when the probe in the coolant tank is not immersed in water). It also does one brief flash on power up which is a good bulb check function.
In my case the warning light stayed on when there was no or high resistance on the .02uf capacitor. As soon as a resistance of 47k or less was detected, the light was switched off. The warning lamp has never flashed on and off apart from sometimes flickering before staying on.
kind regards Danny.
Yes it does seem likely this is the case. I did replace the three tantalum capacitors as my first thought. I also cold checked the other caps with my capacitance meter, then read all the resistors and they were spot on. Finally I removed the four diodes and found that one had a very slight reverse leak. I am not going to investigate further unless the fault returns. I appreciate your help Andy, much appreciated. Regards Danny.
Coolant warning indicator intermittent
