Heater ECU/control panel - electronic guru needed

[Ozstyle]

Hi, any eletronic whizzs out there who can offer any advice. The other day the alarm/immobiliser LED on the heater control lower dash stopped, nothing at all, it used to flash when alarm activated and remain on when alarm deactivated. Checked out the function of the alarm and immobiliser, all good every thing working as it should, except the LED not emmitting any light. (Car - 94 Griffith 500, foxgaurd alarm).
Part stripped the dash, checked the immobiliser/alarm output to the black heater ECU box, all good, 5V steady output from alarm unit to the heater ECU. Checked the heater ECU, all good, 5V at output pin. Checked the ribbon cable from heater ECU to the lower dash contol unit, all good 5V output at connector. However when I plug in the ribbon connector into the lower dash contol unit the volts drop to 0.7 at the connector pin out to the board, it's not a bad connection as I'm able to check the ribbon output when connected with a fine wire probe, definate drop in volts, unplug and back to 5V at the ribbon output.
I have pulled out the heater ECU and the lower contol panel electonic card. I can see no evidence of burnt tracks or burnt out components. All the heater controls work fine, all illumination, heated mirror and ice warning LEDs work fine, no problems with any other operation, just the alarm LED.

I'm stumped, why the drop in voltage when plugging the ribbon connector in??
Even with 0.7 V should not the LED glow a touch?
Can an LED just fail? If so would it cause this voltage drop I see?
All I can think of is when all connected the heater ECU or alarm unit is dropping the voltage ouput for some reason. Only thing I can do is replace the LED. Any advice appreciated, cheers,
Ozstyle.

[Loubaruch]

No experience of Foxguard immobiliser/alarms but easy enough to check the LED. Just connect 12 volts across its connecting wires if it fails to light just reverse the connections. Possible the LED has short circuited but not a common problem.

[330p4]

Mark typically an led will have 1.2 to 1.8 V across it if led was open cct you would read 5 V still. Suggest with power off you meter the led on diode test this will show if the led is ok or short cct and should see a faint glow from led when connected correct way around.
Ian

[330p4]

DO NOT CONNECT 12 V directly across led this will blow it LEDs are current driven devices and require a series resistor to run on 12 V
Ian

[blitzracing]

You have a short somewhere- the voltage drop will be due to the 5 volt signal being fed from a resistor somewhere, so when its open circuit you see 5 volts, but when its connected the short drags it down to .7 volts, and the remaining 4.3 volts will be across the resistance. A short circuit LED would do this.

Edited by blitzracing on Sunday 26th August 16:18

[Loubaruch]

330p4 said:

DO NOT CONNECT 12 V directly across led this will blow it LEDs are current driven devices and require a series resistor to run on 12 V
Ian

Yes quite correct but many LEDs including the ones on my car have resistors already in series.

[Ozstyle]

Thanks for all the replies,
I replaced the LED with a spare one I had and it works!!
I ruled out the LED initally as it was about 3 years old (replaced when I broke original when replacing the lower dash panel). I thought LEDs lasted for decades, this one failed without warning. Glad I don't have to fork out for heater ECU/control panel repairs or replacement.

One thing I noticed and wonder if it plays a part in the failure, the cicruit board shows a D shape outline for a 3 pronged electronic component in parallel with the alarm LED - its not there. I can see there was one in the past as there are bits of prongs in the board. Wonder if this is some kind of resistor that should be there? It is the only LED on the board that does not have a resistor in series with it.

Ozstyle

[Loubaruch]

Any external resistor for the LED would be in series not parallel with the LED so the spare slots are not for that.
Possibly it could be for a transistor driving the LED as TVR used a few different makes of immobiliser/alarms and another make of Immobiliser could require a bit more umph to light the LED.

Your replacement LED most probably has a built in resistor or the current is being limited by the Foxguard.
If the LED does not appear much brighter than the others I think you have solved it.

Edited by Loubaruch on Monday 27th August 10:06

[Hedgehopper]

Is this the three-pronged electronic component you mean? If so , it's the ice-alert LED. I'm just in the process of renewing mine after veneering the faceplate.

[Ozstyle]

Hi Hedgehopper,

no, my ice detector LED is fine. At the bottom left of the board there is a red LED in your pic, the alarm LED. Next to this is a 'D' shape outline with U1 next to it, on your pic you have a resistor soldered in it; on my board there is nothing. If you look below the word 'ENVIRONMENT' there is another 'D' shape outline with Q1 next to it, in it a 3 legged black component. I would have thought there would be another one of these components in the U1 'D' outline. Maybe my LED blew because I have no resitor wired inline with the LED. However as stated this may vary depending on the alarm used with the control board. If it blows again then maybe I will put a resistor inline as per your board.

thanks

Ozstyle.

[Sardonicus]

May be worth checking the ribbon cable as not been pulled out from the module or the lower dash panel itself these connectors are quite fragile if damaged by previous ham-fisted attempts

[David Beer]

Adrian@ said:

Sorry to drag this back up but... on a 500, I have lost the power the the lower dash and the fan, due to a bodge where the previous owner had stacked an turned alloy finisher on top of the old veneer and left all the switches floating free (because the double thickness material), which caused the fan switch to break off the PC board and I presume short in some fashion. My Q is where is the ECU powered from and is it fused away from the fuse board somewhere?
Adrian@

Surely we should be asking you! Great history in TVR land though. I have only done fan supply repairs, cheap transistor fixes.

[Ozstyle]

Hi,
when I had the ECU out I made a note of the connections and tried to work out what was what. From my notes - at the end of the ECU there is a 10 pin plug and 2 individual spade connections. One of the individual spade connections has a black/yellow wire this is 12V supply when ignition on. In the 10 pin plug there is a green wire, also a 12V supply when ignition on.

The heater control/circuit is definately an odd set up, when I pulled the fuse from the fuse panel for the heater circuit nothing would work (as expected) but the fan would still run but I could not contol its speed or turn it off once on, all very TVR.

Ozstyle

[David Beer]

Hi Adrian, sorry I did not mention that, could of saved time. The three other circuits basically have variable resistors that feed back to the three ICs, ie the temperature valve gives a position that matches the control knob position. If a wire comes off, which they do, the motor spins continuously. Edit as I have just noticed you have no power at all to the unit?

Edited by David Beer on Saturday 23 March 13:55