Rear demister not working???? Celica GT

strips in the glass, 1.3 ohm, continuity test 001

nearby bolt, is a a good ground with just 000 continuity test reading,0.6 ohm

to ground a good stable current of 14v is detected, minor burning smell detected.

should be no current is passed through it, I just don't understand.

truly grateful.

Is the earth bolt you are testing to, on the taligate, because if so, it may not be connected to the main body. The most likely point of failure, is where the cable passes from tailgate, into the main car body. Test the earth side of the element, to a good earth on the main body.

Andy C

eating strips in the glass, 1.3 ohm, continuity test 001

to a nearby bolt, is a a good ground with just 000 continuity test reading= ,0.6 ohm

e then to ground a good stable current of 14v is detected, minor burning sm= ell detected.

led as should be no current is passed through it, I just don't understand.

'd be truly grateful.

Hi Andy,

Thanks so much for the reply.

yes the earth I'm testing to is on the tailgate but it is a good earth and = is the original spec earth for the demister and another cicuit. i will test= again though. I did run a separate wire from the demister neg terminal to = that earth bolt and still no current was passed through. It's all a bit wei= rd.

cheers bud

the original spec earth for the demister and another cicuit. i will test again though. I did run a separate wire from the demister neg terminal to that earth bolt and still no current was passed through. It's all a bit weird.

Try running a temp wire from the earth side of the element to a chassis bolt in the car - not the tailgate.

Does the other circuit used by the earth bolt on the tailgate work?

Andy C

Last time I had a problem like this, but cannot remember the detail, the demister element was somehow arranged to work ONLY when the alternator is running. The fault was the earth return on and at the fully isolated alternator. ('87 Vauxhall Astra Estate)

Remember 12/14v at a device, measured with a voltmeter is no guarantee that enough current can flow to operate the device, as one might get with a corroded terminal, better to use something with a higher current demand ie an indicator lamp with a pair of wires soldered on, one croc clip and a probe of some sort.

HTH

strips in the glass, 1.3 ohm, continuity test 001

nearby bolt, is a a good ground with just 000 continuity test reading,0.6 ohm

to ground a good stable current of 14v is detected, minor burning smell detected.

should be no current is passed through it, I just don't understand.

truly grateful.

a modern multimeter can be misleading as it can show a voltage but not whether it has any actual oomph behind it.

With everything connected and turned on, check for voltage between the earth side of the screen and a body (not tailgate) earth point, if all is good it should read zero or just a little over. In your case I expect it will read near enough battery voltage, which shows that the earth circuit is broken.

and is the original spec earth for the demister and another cicuit. i will = test again though. I did run a separate wire from the demister neg terminal= to that earth bolt and still no current was passed through. It's all a bit= weird.

Hi andy,

I ran a temp wire from the neg term to another good ground for other functi= oning circuits. started the car turned the circuit on, no voltage was passe= d through.

So I connected the Mmeter to the neg term and this good ground, which passe= d a good 13.v current, put a bowl of boiling water under the glass and obse= rved to see if the demister cleared it. Apart from a very negligible amount= of water dispersion on some of the strips on the positive term side it did= n't work. Also I tested every single element for continuity and the broader= joining strips perpendicular and they showed continuity of about 1 ohm res= istance.

The only thing I can think of is that the conductive/thermal properties of = the metal strips are degraded hence not output the heat, I'm no electrician= though, bit baffled!!!

Thanks again

Hi Daffyd,

I did this test and the reading was zero. Interestingly, as you mentioned, = I did a amperage test to. With the Meter connected to the neg terminal and = a known good ground away from the tailgate the reading was 2.8 mill amps, w= ith the circuit connected up properly with it's own ground the current was =

0.4 milli amps. Maybe the strips have just lost there thermal properties t= hrough degradation, but I'm stuck!!

, I did a amperage test to. With the Meter connected to the neg terminal an= d a known good ground away from the tailgate the reading was 2.8 mill amps,= with the circuit connected up properly with it's own ground the current wa= s 0.4 milli amps. Maybe the strips have just lost there thermal properties= through degradation, but I'm stuck!!

Sorry that was meant for mr cheerful!!!!

ed, I did a amperage test to. With the Meter connected to the neg terminal = and a known good ground away from the tailgate the reading was 2.8 mill amp= s, with the circuit connected up properly with it's own ground the current = was 0.4 milli amps. Maybe the strips have just lost there thermal properti= es through degradation, but I'm stuck!!

Thanks daffyd tooo. I'm thinking the current is too low, there is some scor= ching degradation on the pos and neg sides where the broader connecting str= ip connects the filaments. maybe that's the problem and they need repaintin= g with that conductive paint???

cheers

functioning circuits. started the car turned the circuit on, no voltage was passed through.

good 13.v current, put a bowl of boiling water under the glass and observed to see if the demister cleared it. Apart from a very negligible amount of water dispersion on some of the strips on the positive term side it didn't work. Also I tested every single element for continuity and the broader joining strips perpendicular and they showed continuity of about 1 ohm resistance.

metal strips are degraded hence not output the heat, I'm no electrician though, bit baffled!!!

Firstly a Voltage (Volts) is a potential to *push* a Current (Amps) through a circuit, which has a certain Resistance (Ohms) so voltage doesn't flow, current flows through the circuit. The more volts the higher the current.

I = V/R

Have you proved this 14V potential you have on the + terminal of the element is coming through the fuse you think is the right one. Try pulling it and ensuring the Voltage disappears. Do that with the negative side disconnected. Out of interest, what is the value of the fuse? Also what is the highest current your MMeter can read ?

Andy C

If the HRW has degraded at any point, then it's characteristics change and it will burn itself out at that point, and you would see the broken line. ISTR you have already read the resistance or the HRW element and it's OK??

Put that meter back in the toolbox, it's no good to you. You MUST use a test lamp! Put a 21W or 55W bulb across the HRW terminals and if the lamp lights brightly, it's the HRW element, if not, work back, but first check the alternator earthing is intact (visually)- Don't forget, most HRWs only work with the engine running, because of the high current demand.

A meter is only any good to you if you know how to use it/interpret the results and with respect, what you have written before shows that you do not. Sorry to be blunt, but there it is!!

Another example - I once had a reversing light that did not work, now the bulb was intact, the earth was OK at the light cluster and there was

12volts on the other side of the bulb - why didn't it light? - there was corrosion at the terminals on the switch on the gearbox. The meter read the 12volts (which was via very low current leakage through the crap), but there was enough crap to prevent the passage of sufficient current to light the bulb! I'd have found it more quickly if I'd used a test lamp, by noting that it would have lit on one side of the reverse switch, but not the other.

HTH

scorching degradation on the pos and neg sides where the broader connecting strip connects the filaments. maybe that's the problem and they need repainting with that conductive paint???

scorching degradation on the pos and neg sides where the broader connecting= strip connects the filaments. maybe that's the problem and they need repai= nting with that conductive paint???

Thanks daffyd for the tips.

As you said with the reversing light some of these electrical issues can re= quire a bit more digging!! All tests are done with engine running to supply the required current. Circ= uit is on a 10amp fuse and 30 amp relay. I checked alternator strap, seems = fine. I managed to find a test lamp. Connecting the supply hrw anode and it= s=92 counterpart cathode the lamp lit well. On the anode circuit supply sid= e it=92s chucking out 2.5ma and around 14v pd. I have visually and continu= ity checked every part of the hrw element, all seem fine.

Conclusion?? The hrw is knackered and not conducting sufficient current to= heat the elements??

Do you agree? In other words, give up, get that paint or new tailgate lol You=92re right I=92m no auto electrician but can apply a bit of logic I thi= nk!!!

Cheers Bud Matt

nctioning circuits. started the car turned the circuit on, no voltage was p= assed through.

assed a good 13.v current, put a bowl of boiling water under the glass and = observed to see if the demister cleared it. Apart from a very negligible am= ount of water dispersion on some of the strips on the positive term side it= didn't work. Also I tested every single element for continuity and the bro= ader joining strips perpendicular and they showed continuity of about 1 ohm= resistance.

of the metal strips are degraded hence not output the heat, I'm no electri= cian though, bit baffled!!!

Thanks for getting back to me Andy,

good old ohms law eh!!!

Tested your fuse question, the voltage did disappera when pulled. It's a 10= amp btw. The anode on that circuit for the demister supply is chucking out= 14v and 2.5ma. the Mmeter goes up to 200ma btw.

I have visually and continuity checked every part of the hrw element, all = seem fine.

Conclusion?? The hrw is knackered and not conducting sufficient current to= heat the elements??

Do you agree? In other words, give up, get that paint or new tailgate lol

thanks fella,

matt

require a bit more digging!!

is on a 10amp fuse and 30 amp relay. I checked alternator strap, seems fine. I managed to find a test lamp. Connecting the supply hrw anode and its? counterpart cathode the lamp lit well. On the anode circuit supply side it?s chucking out 2.5ma and around 14v pd. I have visually and continuity checked every part of the hrw element, all seem fine.

heat the elements??

Are you sure that there is just one 10A fuse? Seems a bit underrated for a rear screen (BX had one 20A). My car has 2 10A fuses in parallel and a double pole relay. Never seen anything like it before but it's defo got 2.

(also has parallel fuses for heater fan motor)

I can only assume it's to reduce the feed circuit resistance by 1/2. Nothing about using a matched pair of fuses to ensure equal current share.

10a is what autodata says it should be, which can comfortably give 100 watts of heat. So the total resistance across the elements would be about 1.2 ohms or so.

If you stick a bulb in parallel with the rear element does it light up? If it does then the elements knackered, if it doesn't then there's a duff connection, probably the gnd. If you've tried a 200mA multimeter with 14V & found a good ground the odds are the current settings knackered on the meter.

A 200mA meter to check current on a car is likely to be damaged at first use! Even a 6watt interior courtesy lamp draws 500mA.

snip

heat the elements??

OK, you managed to find a test lamp. Does it use an indicator or headlamp bulb? If it only uses a piddling little sidelamp/festoon bulb, then as per my earlier comments about corroded connections, it may still light.

You need to test the 12 VOLT supply with something that draws a lot of current ie indicator 21W or headlight 55W bulb. MAKE YOUR OWN UP - don't use these silly little electrician type! I'd use a 21W (less likely to burn oneself on the bulb 8^)

It is the WIRING that conducts the current for the HRW (which does not have anode or cathode BTW, this is for batteries and semiconductors)

You have already measured the continuity/resistance of your HRW at

1.3ohms, therefore if your WIRING and it's CONNECTIONS have done their job of delivering 12VOLTS at a MINIMUM of 5 AMPS (don't try to measure this with your 200mA meter, it'll damage it when the current is correct)

I do NOT believe your HRW is toast (but using you meter on ohms, check between each of it's terminals and a close THICKER part of the element for continuity (you may have to scratch though a little orange or cream type paint covering the conductive film, do not do this at the thin points like those in the middle of the HRW)- it's just possible that the connections between the terminals and the glass are iffy) Think - if you have a wiring fault, you could change the tailgate and still have a problem. No point using silver conductive paint 'til you know exactly here to place it!

I *DO* believe the problem lies in the wiring/earthing. For this type of fault, use the meter by all means as a guide, but use a high-power requirement test lamp for definitive faulting.

I used to work for Telecom on electro-mechanical exchanges. We used test lamps for faulting. Those testers were plugged into a 50V supply and two bulbs were connected, in series across this supply, with the probe connected to the point between the bulbs. When that probe was placed on any point in equipment, it was possible, by seeing which bulb lit and how brightly, to estimate the conditions at that probe point.

LOGIC? - with your new home-made test lamp, use a good 12v supply (the battery) and look with the probe for good earths AND/OR use a good earth (battery again or RELIABLE body point) and look with the probe for good

12v. You are looking for the BRIGHT illumination that you would expect from these bulbs in their normal usage. Anything less will indicate which route to trace.

One of my sons had a 93 escort, the earthing of the some (but not all) of the tailgate electrical items relied on the (basic) tailgate hinges. Rust had reduced the ability of the hinge assembly to conduct enough current to earth to (I can't remember the problem, only the cure) I had to run a wire through the rubbers/grommets to (re)connect the tailgate to the main body of the car!

Do you know anyone else with one of these cars so you can make side-by-side comparisons? If not, then any Toyota of (ideally) similar age as electrical design practices often run through a range of models.

I was using the up to 200mA setting whereas there's a 10 amp setting where you have to move the positive probe to a different connector!! Will retry. Cheers;-)