Electrics: 164: temp sensor

[woodturner-fran]

Hi all,

Just wondering what the wisdom is here wrt to a temp sensor for a 164. The gauge barely rises in the dash, it might go as far as the bottom of the green marking. Shorting the wire on the sensor to ground doesn't make it rise either (as I believe it should). The other gauges in the dash function normally.

Has anyone else here had the same problem? What did you do to sort it out? Has anyone got a universal replacement for the temp sensor in the head?

Can I ask the same Q for the sensor at the brake distribution block/handbrake switch which I believe are linked. The previous owner had pulled the wire off the sensor at the distribution block - when I connect it up the brake failure and handbrake lights come on. (brakes and handbrake are working AOK).

Thanks,

Fran


Fran

[Audrey Old]

The brake one is easy to fix (assuming the brakes work OK!). The splitter on the front inner wing has a hex plug on the upper edge - undo this (no fluid will be spilt) then screw it up tight again. This resets the electrical failure-warning trip after work has been done to the system. However, make sure the rest of the system is fine - it may be trying to tell you something!

Water temperature: ages ago I had an air-lock in the heater/head area of coolant flow, which meant unreliable readings on the gauge. I had to bleed the heater at those awkward hoses at the back of the block, also the rad hoses, so look at all those first. I was told the ballast resister on the back of the instrument head can cause this too, but thought it would affect other instruments too, so not too sure.

Incidentally, be careful with those transmission fluid hoses to the bottom of the rad: some time ago these were unobtainable new. However, I expect a hydraulics fabrication firm could make new ones (at a cost). I had to get a couple made to suit an auxiliary trans fluid cooler I fitted when I used that car for towing -a useful extra to fit.

Will re-read your mails later to see if I can help, as a busy Monday I fear...

Paul

[woodturner-fran]

Many thanks,

I'll do as you suggest on the brake front and report back.

I repaired the hose I had so got away with it, I know the fittings are UNC so that might make getting one a bit harder but any lad that does hydraulic hose repair should be able to sort it out - especially if you have both bits!
On an aside, I wonder about the efficacy of the cooler - all it is is a pipe inside the bottom of the rad that runs opn the inside from inlet to outlet. Can't imagine that it would actually cool all that much.

Fran

[woodturner-fran]

OK, tried that with the sensor in the distribution block to no avail. I tested out the handbrake switch and it seems to be AOK. When I unscrewed the sensor from the block, the plastic shaft moved in and out as you'd expect it to. Kicking myself that I didn't try it for continuity when I had it out - I'll try this tomorrow night. Car feels to be braking fine..

If it is gone, is there a simple replacement part available somewhere?


Fran

[B20F]

You should pump the brake pedal with the brake sensor removed. This way the little piston inside the distribution block can center itself. If the piston is not in it's original centered position it shuts off one brake circuit and only about 75% of the brake capicity is on hand. This means only one rearcaliper is working, one front caliper for 100% and the other front caliper only two of the four pistons are activated.

Peter
'73ES

[woodturner-fran]

Did this tonight. Made no difference to the lights coming on. When I had the sensor out I had a look in and I can see a groove in the shaft that runs in the distribution block. That groove is not dead centre in the bottom of the hole where the sensor goes - that might be a cause.

I also tested the switch for continuity - the switch works OK, ie the circuit goes open/closed as expected. The only other thing is whether the end of the piston in the switch is worn.

I think what I need to do is bleed all the brakes out, then have another look at the distribution block and see if I can centre that shaft. Failing that its time to order a new sensor and rule that out.

Fran

[B20F]

There can also be a short to ground in the black wire going to the switch. And check the handbrake switch and wire, this is on the same circuit. The brake warning lamp is activated by grounding via those switches. Disconnect the wire from the switch and lamp and run an auxillary wire from lamp to switch. HTH!

[malb]

For the brake warning,
1. There is an amount of leeway for the piston before it starts to blanket the feeder tubes and reduce brake efficiency. Generally if you see the piston groove through the switch mounting hole, brake lines should all be open and working.
2. There is no leeway with the switch, if the groove in not centred in the switch mount hole, the switch will indicate.

Piston offset could well be a result of a brake bleed some time ago, rather than current leakage anywhere. Unless fluid is changed regularly, there is a chance that moisture will start surface corrosion in the chambers, and the piston will jam if moved over the surface corrosion. It appears that there are no seals involved with the piston, it is a firm metal to metal fit in the cylinder. As such detection system is non recoverable. I have had to replace one on a 240 and tried to reco the original as a spare.

Despite opening both ends, the piston could not be removed to clean up the surface of the cylinder, so the unit was scrapped. Have looked at a number in scrapyards over the years and all similar. Basically, unit will indicate fault once after the onset of surface corrosion, then can't be reset as the piston has moved and jammed in the corrosion buildup.

Re temp guage, circuit is basically 12V from ignition into 3 terminal regulator, approx 5 V out from memory, to guage then to ground via sender in head. The sender is a temperature dependant resistor, and hence changes the current flowing through the meter. Meter is normally thermally activated, rather than coil and permanent magnet, to give a slow (time averaged) response.
Regulator output also shared with fuel guage making both independant of battery voltage. Shorting the sender lead to ground for a short period would not show on the guage, as it has a long averaging period, (1 minute +), and long term shorting is not advisable as it can cook the regulator and guage. From memory, I suspect that the working range of the sender is from about 50 ohm to 150 ohms.

[woodturner-fran]

Thanks so much MalB

Temp sender: I will check it out with a DMM and watch the resistance in the sender over time as the engine heats. The fact that the gauge rises a bit and then falls back makes me think its the sender - if the clock was gone it wouldn't move at all I think. If I "short" the sender wire to ground through a 50R resistor and left it there for a while wouldn't that remove the risk of cooking the gauge/regulator?


Brakes: I can see the groove in the piston. Car brakes in a straight line no pulling to either side the way you would expect if some of the callipers were seized. The wire going to the sender in the block basically has continuity to ground- ie the continuity on the DMM beeps when I connect one lead to the wire and the other to battery neg. Is this OK (I think the continuity will beep if R is <20R on the DMM I was using)?

Also when you look at the bottom of the plastic piston on the sender it is square across there is no dome head or point that would mate into the groove. Anyone else know if this is correct?


Many thanks for all your help! I'm taking her to a show tomorrow about 120 mile round trip. Hopefully I'll be back tomorrow night to check in again!!!!


Fran

[malb]

Hi, I previously said that the voltage reg in the instrument cluster gave a 5V output to the fuel and temp guages. This was based on memory and is wrong, it is 10 V out according to Haynes, from checking wiring diags for brake warning.

For temp sender, try bridging the sender lead to ground via 50 ohms, but monitor it from the cabin. i.e. conect the resistor (1/2 watt should be adequate) temporarily in place of sender with everything switched off, then switch on ignition and monitor the guage. Switch off within 35 seconds, or when it becomes obvious that the guage is rising higher than it has been. Last thing you need is to cook the guage or regulator, as you will spend the rest of your life in scrapyards trying to replace them if you do.

For the brake sensor, the lamp test circuit also exists and is providing the earth that you are measuring. With ign on and engine not running, all dash warning lights should be on. Suspect the lamps in any that aren't. Once the engine is started, these lamps should go out unless the situation that they warn about exists. e.g. the park brake is on, or the brake balance sensor is unbalanced, or the oil pressure is low.

To achieve this, there are two earth paths for each lamp, a direct one through the switch/sensor controlling the lamp and a diode isolated one through the aux output of the alternator.

If the alt is spinning and working correctly, the aux out voltage will match the main out voltage and battery voltage and there will be no voltage drop across a warning light and it will not light. If the alt aux out is way below the battery voltage, then the warning lamp will have a voltage drop accross it and will light.

In the event of a sensor activating a warning light, the lamp will earth via sensor and light up, and the self test circuit diode for that lamp will be reverse biased (non conducting) and block the sensor from providing earths to all the other warning lights.

The path for the earth that you are measuring from the disconnected brake balance sensor is to the lamp, through the self test diode, and to ground via the aux out of the stationary alternator.

Better to check as voltages rather than resistance because there are other things in the circuit (self test system).

Hope that makes sense for you, if not tell me and I will draw up a circuit that explains it better for you. Had thought of sending you a copy from Haynes, but its that cramped that its almost impossible to read the printed copy.

Plunger in the brake balance switch is flat and flush with the nylon carrier, it should sit over the groove in the piston and not make contact unless the piston is displaced, very small movement of piston should then alow piston wall to contact and issue warning. As I said yesterday, the piston has to move a long way to mask any of the ports in the system but a small movement activates the warning light.

If you need to visualise the system more, consider temporarily replacing the sensor switch with a push switch and length of cable so you can make/break earth path while sitting in the cabin with engine running/not running while observing light.

I believe that this end of the electrics, and the brakes themselves, are working properly, and that the light is coming on because the piston is offset. Previous owner probably agreed to disconnect cable from sensor in preference to spending the cars value replacing the complete balance sensor unit and flushing brakes. Its what I did in the end too.