PC Pete Posted May 27, 2013 Share Posted May 27, 2013 Morning All Finally got my R400 back from CC Midlands after PBC and IVA. I have noticed under the dash on the drivers side what looks like a resistor that gets extremely hot whenever the ignition is switched on. It is way too hot to touch and I fear it may damage surrounding cables or worse...... As I say it is way too hot to touch. Does anyone have a similar situation? Many thanks in anticipation.... Link to comment Share on other sites More sharing options...
Molecular--Bob Posted May 27, 2013 Share Posted May 27, 2013 Do you have a battery cut off switch? It sounds like a miss-wired resistor which is getting current through it when the switch is on, rather than off. Is it a largish ceramic block like this here Link to comment Share on other sites More sharing options...
sjmmarsh Posted May 27, 2013 Share Posted May 27, 2013 My thought as well, MolecularBob Steve Link to comment Share on other sites More sharing options...
Jerry Auton Posted May 27, 2013 Share Posted May 27, 2013 ..so how should these be wired, as I beleive I have the same issue. Had to replace the resistor a couple of months ago as it crumbled when i moved it to fit the captive nut retaining channels for a Brooklands set up. It appears to be hot most of the time but difficult to be sure with the knee plates in place. Link to comment Share on other sites More sharing options...
Mick Day Posted May 27, 2013 Share Posted May 27, 2013 Depending on what you have found, & if it is designed to have current flowing all the time, then it might be better to use a power resistor on a good heatsink. Just need to determine resistance & wattage. Edited by - Mick Day on 27 May 2013 15:44:25 Link to comment Share on other sites More sharing options...
sjmmarsh Posted May 27, 2013 Share Posted May 27, 2013 This link should help Steve Link to comment Share on other sites More sharing options...
MikeMolloy Posted May 28, 2013 Share Posted May 28, 2013 The power resistor should be connected to one of the "W" terminals on the FIA switch. These contacts are CLOSED when the switch is OPEN. The idea is that when you open the main switch you isolate the battery from the rest of the vehicle electrical system. If you do this while the alternator is spinning it has nothing to control the voltage and you get what is called a "load dump event". This big voltage spike can damage ECUs and the like, so the power resistor to ground is added as a load to soak up the energy from the alternator. IT SHOULD NOT GET HOT IN NORMAL OPERATION! 😳 Check your wiring! Link to comment Share on other sites More sharing options...
Jerry Auton Posted May 28, 2013 Share Posted May 28, 2013 Link to comment Share on other sites More sharing options...
PC Pete Posted May 29, 2013 Author Share Posted May 29, 2013 Many thanks for all of your replies - as speedy as ever. Apologies for not getting back earlier but I was stuck in that horrific traffic jam on the M6 yesterday for 4 hours; ironically on my way to register the car. I don't have a battery isolator switch but I agree with you Fred - it shouldn't get hot in normal operation. Strangely Caterham disagree telling me yesterday that this is normal! I can't accept this. I have pushed back on them today and await a response. On another point - has anyone experienced the engine (2 litre Duractec - brand new) hunting at tickover? Mine sits there oscillating lazily between 1000 rpm and 1500rpm. Again I await an answer from Caterham....... Thanks again for all of your responses . Link to comment Share on other sites More sharing options...
MikeMolloy Posted May 29, 2013 Share Posted May 29, 2013 You don't have a battery isolator switch? Ah, well ignore all the above then... So, what does the mystery "resistor" actually do then? Can you tell what it is wired to? Or maybe just the wire colours could tell you where to look on the wiring diagram? If it's not the FIA switch then I have no idea of the function. The only other "big boy" resistor I have come across was for the dim-dip feature on a MX5 with pop-up headlights. Link to comment Share on other sites More sharing options...
MikeMolloy Posted May 29, 2013 Share Posted May 29, 2013 Mmmm. Bit of furtling in the circuit diagrams reveals there is a 68 Ohm 4 Watt resistor between GREEN (Fused Ignition +12V) and Blue/Yellow (connection between Tacho and Grey Engine Plug Pin 11). Not exactly sure what this does but with a 4W rating it is designed to get hot. Link to comment Share on other sites More sharing options...
sjmmarsh Posted May 29, 2013 Share Posted May 29, 2013 Fred It is the same on the CSR, except that the wire is brown/yellow (NY on the diagram). Assuming the lines on the diagram go straight through the connector to the tachometer (otherwise there would be 12 connectors rather than 6) this connect to the black earth bus. Power = V ^2/R, or 144/68= 3W, so it looks like the resistor just runs hot. 3W shouldn't be that hot though - just warm? Presumably the resistor limits the power going through the gauges and protects them in the case of any spikes? Steve Link to comment Share on other sites More sharing options...
sjmmarsh Posted May 29, 2013 Share Posted May 29, 2013 This post casts some more light on the resistor - the brown/yellow wire feeds the ignition (red) light in the tachometer. As they are al LEDs now, they need a resistor to make sure they work the same as the old bulbs.... In summary - perfectly normal for it to run hot. TADTS! Steve Link to comment Share on other sites More sharing options...
MikeMolloy Posted May 29, 2013 Share Posted May 29, 2013 Ah! Quite correct - N is Brown, not Blue... Makes sense as a load for the alternator though. But if it is taking the full whack 13.5V, it will be dissipating 2.7W. Depending on the physical size of the component, (and if it isn't mounted on a heat sink) it could still get to 150 degrees surface temperature! That's a bit warm if there is any chance you could come into contact with it... 😳 If it was me I'd buy a big mutha 25W aluminium case resistor (like this here) and bolt it to a convenient bit of chassis metalwork. Link to comment Share on other sites More sharing options...
Mick Day Posted May 29, 2013 Share Posted May 29, 2013 Like wot I said then.................... Link to comment Share on other sites More sharing options...
MikeMolloy Posted May 29, 2013 Share Posted May 29, 2013 Lucky guess! 😬 Link to comment Share on other sites More sharing options...
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