Spike Resistor on Temp Sender???
Discussion
There is an in line spike resistor fitted to the back of my temperature sender on my 400 SE. Part No. RD 953066. I've never seen this before. Normally the wire is just connected to the temperature sender. I'm assuming they've fitted this to reduce the current to the gauge. Has anyone else got this and where can I buy one? only my temperature gauge has gone on the blink and before I go whipping off the sender and fitting a new one, I thought of replacing this resistor in the hope of a quick fix. I'm assuming this resistor is fitted because the wrong sender is on the car to start with. Has anyone else had this problem? Here is the picture of the part in question. The unit excluding the spade ends is about the size of a penny. Thanks in advance for any information.
Tony. TCB.
Tony. TCB.
Jack Valiant said:
Seem to remember seeing this on my old 400SE and was told at the time it was get around for the different gauge. Be interested to know what the resistance is for future use!
The resistance I believe is 6.8k but I've tried the gauge without it and it doesn't seem to make much difference if any. There is a reading but it's low. It is the original temp gauge in the car and direct earth makes the gauge go to max, so the gauge is working. I think the temp sender is faulty. The spike resistor would decrease the reading if the reading was high and needed calibrating. I now suspect the temp sender so I'll just order a new one and see if the problem is cured. If the reading is high with a new sender then I'll use the resistor or try and locate a new one. I've changed a temp sender before on another wedge and didn't have any problems without a resistor fitted.Tony. TCB.
Wedg1e said:
Spike? Surely the term is 'shunt'. I'd expect it to have a fairly low reading otherwise the current would take the lower resistance route through the meter. If it's gone high-res that would explain why it makes no difference whether in or out.
I don't understand why some wedges have this and others don't. Also, I would expect it to be fitted to a live feed but in this case it's on an earth lead. It's an original TVR gauge in both cases. I've tried to buy the same resistor but no one seems to have it, not even Rover agents and it was fitted to Rover vehicles originally I believe.Tony. TCB.
ElvisWedgely said:
I don't understand why some wedges have this and others don't. Also, I would expect it to be fitted to a live feed but in this case it's on an earth lead. It's an original TVR gauge in both cases. I've tried to buy the same resistor but no one seems to have it, not even Rover agents and it was fitted to Rover vehicles originally I believe.
Tony. TCB.
Maybe I've got it wrong; is it across the meter or in series with it? If across it ('in parallel') then a proportion of the current flowing passes through the resistor so less is available to swing the meter. If it's in series (between the meter and sender, say) then it would act to limit the total current that can flow... I was presuming the former. If it's the latter then you would expect something to change when you remove it (i.e. the meter would indicate nothing!).Tony. TCB.
It wouldn't matter where in a series circuit it was installed, positive or earthy side, it would have the same effect, which would tend to be to limit the max. swing of the meter.
The fact that some cars have it and others not is likely to be a combination of instrument and sender suppliers' tolerances.
Yes, it's in series and what you say makes sense. It is there to limit the current, only, in my case, even without the resistor the gauge reading is very low and direct earth contact sends the gauge to full. From this I've come to the conclusion that the sender itself must be faulty. When I fit the new sensor, if the reading on the gauge goes higher than expected, then I'll look into fitting another resistor in series to reduce the current flow, hence the gauge reading. I hope I've got that right???
Tony. TCB.
Tony. TCB.
So the resistor is in the wire between the gauge and sender? That's what I'd expect; the gauge gets a 12v feed which flows through the meter winding and down to the sender, which is a variable resistance (decreasing with increasing temperature). So as the sender heats up, its resistance decreases allowing more current to flow (to earth, through the engine block) which moves the needle further up the scale.
With no resistor fitted the needle can't go anywhere as the current has no return path to earth.
If the same thing happens with the resistor in, it would suggest to me that the resistor has gone open-circuit or at any rate high-resstance (so no current is flowing regardless of what the sender is doing).
Earthing the sender input to the meter is effectively max. current flow; it does at least suggest the gauge is OK. The idea of limiting the current is that where the sender's resistance falls below that needed to produce full-scale deflection, the added resistance brings the needle back down the scale. Of course passing current through the resistor heats it up and it could be that one too many heat cycles has killed it.
Or, as it's a TVR, it could be the fuffle valve
Edited for yu[pd
With no resistor fitted the needle can't go anywhere as the current has no return path to earth.
If the same thing happens with the resistor in, it would suggest to me that the resistor has gone open-circuit or at any rate high-resstance (so no current is flowing regardless of what the sender is doing).
Earthing the sender input to the meter is effectively max. current flow; it does at least suggest the gauge is OK. The idea of limiting the current is that where the sender's resistance falls below that needed to produce full-scale deflection, the added resistance brings the needle back down the scale. Of course passing current through the resistor heats it up and it could be that one too many heat cycles has killed it.
Or, as it's a TVR, it could be the fuffle valve
Edited for yu[pd
Edited by Wedg1e on Wednesday 23 March 01:32
Yeah the ones I have seen are series resistance maybe atound 1.5k though.
The main effect is (as you infer) around the high-temperature region.
Tony I would get a selection of resistors from the flea 1/4W ones, there are a selection on there e.g. http://www.ebay.co.uk/itm/131660319502 or maplin do a whole set for £8.
Say from 1k up to 2k, get it up to 90C, use an infrared thermometer to check, then put each resistor in there (you can put two in series or parallel if you like too) until your reading is correct. Check again with the engine running in case the reading changes with battery voltage.
Then once the resistance is trimmed for your car, solder some spade ends on and tape up or preferably heatshrink the resistors up so you can insert it in circuit where the old one was.
The main effect is (as you infer) around the high-temperature region.
Tony I would get a selection of resistors from the flea 1/4W ones, there are a selection on there e.g. http://www.ebay.co.uk/itm/131660319502 or maplin do a whole set for £8.
Say from 1k up to 2k, get it up to 90C, use an infrared thermometer to check, then put each resistor in there (you can put two in series or parallel if you like too) until your reading is correct. Check again with the engine running in case the reading changes with battery voltage.
Then once the resistance is trimmed for your car, solder some spade ends on and tape up or preferably heatshrink the resistors up so you can insert it in circuit where the old one was.
Thanks for all the help and suggestions gentleman. I will fit the new sender when it arrives and see what the gauge reads. Like Adam says to adjust it to the correct level, I might go to Maplins and get some of those resistors and fit them in the wire. I might connect them up parallel in the earth wire connected to the sender. That way it may look neater. Also, I could run the wire to a cooler part of the car, fit the resistors and run the wire back to the sender, and as the wire itself has resistance, I may not need too many resistors. That way, it may prolong the life of the resistors. I'll keep you informed as to what happens. I can't do much more until the temperature sender arrives. Oh, by the way, the sender cost around £35 including postage for anyone wondering how much they are. Thanks again for all your replies.
Tony. TCB.
Tony. TCB.
TVRleigh_BBWR said:
you could use a variable resister and wire it up behind the dash once trimmed so reading match using an IR temp gun, or other method.
Not a bad idea Leigh. Variable resistors cost pennies on the bay and can be adjusted. You've got me thinking now. I see no reason why it shouldn't work.Tony. TCB.
They don't handle much power so you have to be a bit more careful about how much currant runs through that circuit. I could measure mine and do the calculation.
Also they are prone to go out of adjustment, vith vibration etc, unless you use a multiturn trimpot, or lock the shaft to the body after trimming.
On the other hand 10% variation might show little change so it'll probably be OK if it can handle the power. Something like ebay item 231483375619 would do the trick - if the 1/2W rating is accurate then that's plenty. You adjust it with the little screw - around 10 turns end to end.
Also they are prone to go out of adjustment, vith vibration etc, unless you use a multiturn trimpot, or lock the shaft to the body after trimming.
On the other hand 10% variation might show little change so it'll probably be OK if it can handle the power. Something like ebay item 231483375619 would do the trick - if the 1/2W rating is accurate then that's plenty. You adjust it with the little screw - around 10 turns end to end.
Edited by adam quantrill on Thursday 24th March 08:19
adam quantrill said:
They don't handle much power so you have to be a bit more careful about how much currant runs through that circuit. I could measure mine and do the calculation.
Also they are prone to go out of adjustment, vith vibration etc, unless you use a multiturn trimpot, or lock the shaft to the body after trimming.
On the other hand 10% variation might show little change so it'll probably be OK if it can handle the power. Something like ebay item 231483375619 would do the trick - if the 1/2W rating is accurate then that's plenty. You adjust it with the little screw - around 10 turns end to end.
Hi Adam. Yes, those ones look good and fully adjustable. There is a wide choice of ratings though. I've been told that the one I have removed was rated 6.8k so should I not go for one that has a 10k rating and I can trim it down to around the 6.8k region ???Also they are prone to go out of adjustment, vith vibration etc, unless you use a multiturn trimpot, or lock the shaft to the body after trimming.
On the other hand 10% variation might show little change so it'll probably be OK if it can handle the power. Something like ebay item 231483375619 would do the trick - if the 1/2W rating is accurate then that's plenty. You adjust it with the little screw - around 10 turns end to end.
Edited by adam quantrill on Thursday 24th March 08:19
Tony. TCB.
Yeah it will be a good first approximation. IR thermometer on the flea for just under a fiver. I'll be getting one...
Thinking again maybe set it so it just reaches the red when the fan kicks in, the fan should then pull it down to near 90C. Then if the needle goes well into the red when driving, you know there's something up.
Also after you have set it up then take it out again and see if you can "pot up" the leads which have been soldered to the terminals with something like araldite or body filler. Best way to do this is to wrap some tape around the body leaving a little well around the leads/terminals and our in the glue then leave it until it sets. THis will stop the leads pulling those thin terminals off over time.
Thinking again maybe set it so it just reaches the red when the fan kicks in, the fan should then pull it down to near 90C. Then if the needle goes well into the red when driving, you know there's something up.
Also after you have set it up then take it out again and see if you can "pot up" the leads which have been soldered to the terminals with something like araldite or body filler. Best way to do this is to wrap some tape around the body leaving a little well around the leads/terminals and our in the glue then leave it until it sets. THis will stop the leads pulling those thin terminals off over time.
Edited by adam quantrill on Thursday 24th March 21:50
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