D2-40 overheat alarm but not overheating

Have you checked the 5V with the sensor disconnected? If it's correct then the circuit is not a simple Ohm's law case and it gets much more difficult.

I haven't looked at the control box. Do you think it can be opened up or is it completely potted like the alarm module on the older models?
 
I measured the open circuit voltage as 5.3V.

I haven't taken the box off the engine (yet). I don't want to disturb it before I am certain it's faulty and there is some chance of repairing it.

I am going to try some more tests using a 100 ohm variable resistor in series with the sensor as mentioned above to see if I can move the alarm trigger point to 100C plus.

Long term, if I can find a (car?) 110C temperature alarm switch that will fit I will probably wire that into the oil pressure alarm.

My natural curiosity still wants to know what has failed, though, and why!
 
That suggests the source impedance is more like 500 ohms.
How about the voltage across the sensor at 20 degrees?
I think you probably need about 50 ohms to fiddle it.
 
I have made a temporary fix using the variable resistor set to 27 ohms. After taking a series of readings of the sensor resistance up to 94C and comparing with the published curve I chose 27 ohms as the best value to move the trigger point from 85C to aound 105C (or slightly less). I will replace the variable with a 27 ohm 3W wire wound tomorrow.

I have also spoken to the local Volvo agents (Golden Arrow) and they initally got me excited when they mentioned a recall affecting the MDI box on the D2 engine series. They needed a serial number to check if mine was affected. When I gave them this they looked at the recall in more detail and reported that it only affected the D2-55 and 75 and required a wire to be moved that could short on the starter. Not much use to me!

They also confirmed that a new box costs £370 plus VAT so I will be doing some extensive sea trials over the next few months with my resistor 'bodge' before I consider replacing the box! I'll also explore my long term preferred solution which is to replace the sensor with a suitable temperature switch if I can find one that fits with the right trigger point.
 
Playtime said:
I have made a temporary fix using the variable resistor set to 27 ohms. After taking a series of readings of the sensor resistance up to 94C and comparing with the published curve I chose 27 ohms as the best value to move the trigger point from 85C to aound 105C (or slightly less). I will replace the variable with a 27 ohm 3W wire wound tomorrow.

I have also spoken to the local Volvo agents (Golden Arrow) and they initally got me excited when they mentioned a recall affecting the MDI box on the D2 engine series. They needed a serial number to check if mine was affected. When I gave them this they looked at the recall in more detail and reported that it only affected the D2-55 and 75 and required a wire to be moved that could short on the starter. Not much use to me!

They also confirmed that a new box costs £370 plus VAT so I will be doing some extensive sea trials over the next few months with my resistor 'bodge' before I consider replacing the box! I'll also explore my long term preferred solution which is to replace the sensor with a suitable temperature switch if I can find one that fits with the right trigger point.
Click to expand...

I think the position where the sensor for the oldtype eletcronic module would have been fitted is simply plugged on your version....removing that plug may offer a suitable place for a sensor. Even the old type VP one except for the fact that thats a bit pricy. (€44)

#22 in this diagram

24529.jpg
 
Thanks Vic, I'll check that out. I would feel more comfortable with a belt and braces solution. I can't fully test my current solution without cooking the engine!
 
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Have you checked the input voltage to the sensor with the 27 ohm resistor in circuit, its possible that the extra 0.3 of a volt (5.3v instead of 5v) could lower the trigger point of the alarm.
 
The open circuit voltage is 5.3V. The voltage with the sensor connected and a cold (8C) engine was almost exactly 5.0V. The 27 ohm resistor will make no difference to the cold voltage - sensor resistance alone is over 1K ohms at 8C.

I'm not sure that 5.3V is the cause of the lowered trigger point - I think it is triggered by the voltage drop across the sensor (down to 0.65V when it triggers).

The additional 27 ohms resistance raises the trigger point back to around 105C, at least in theory. I can't (or at least don't want to) test this as I would have to cook the engine. It doesn't trigger the alarm at 95C, though, which is a definite improvement.
 
Playtime said:
I'm not sure that 5.3V is the cause of the lowered trigger point - I think it is triggered by the voltage drop across the sensor (down to 0.65V when it triggers).
Click to expand...

I think it's more likely it's triggered by the voltage across the source resistor within the module, when that rises to about 4.7V. This would explain it triggering early if you've got 5.3V OCV instead of 5V. If it triggers on a low voltage limit on the sensor, then surely a high excitation voltage would make it trip late i.e. too hot not early?
I reckon you need about 50 ohms extra to offset the 0.3V at the correct trip point.
 
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