Frickin’ Diodes ! (Cooling Fan Circuit)

[Dangermouse]

As I mentioned in another thread, I cleaned a few connections on the engine last weekend to improve my alternator connection. Afterwards I noticed that the coolant temperature gauge had moved from the 1/4 mark to just below the 1 /2 (220) mark and as it happened immediately afterwards I put it down to getting a better signal through better grounding. Not so.

Drove the car yesterday morning without an issue, but in retrospect it must have been cool enough and I must have been moving enough that it stayed cool. Coming home in the afternoon, I hit a long tailback near my house and I noticed the temp starting to rise. It was in the red just as I pulled into the subdivision so I stopped just in time to see telltale wisps/puffs of steam in the mirror. Damn! Fingers crossed I got stopped in time.

Walked to my house and printed off the detailed schematic for the cooling fans circuit. Had a cool drink and then walked back to the car with a gallon of coolant. While the engine cooled I discovered that the cooling fans were non-functional. Troubleshooting eventually lead me to suspect the diode in the otterstat circuit was open. For noobs like me reading this later, they are found in the corner of the fusebox area:

http://dmctalk.org/attachment.php?attachmentid=20130&d=1372171337

I was able to swap it with the one in AC circuit which wasn’t being used and got the fans working again.

First Question:

Is the otterstat a simple switch or is it a variable resistor (temperature related?). What are these diodes used for? Two theories:

a) Otterstat is a simple switch. The diode in the otterstat circuit is there to prevent back feed from the compressor circuit, but why would that matter?
b) The otterstat is a variable resistor and the diode is there so that it will only allow current to pass at a certain voltage, and thereby stop the fan relay from chattering?

Second Question:

Is this diode user-repairable? I assumed it was a potted component and must be replaced entirely, but I was reading Chris’s thread (here (http://dmctalk.org/showthread.php?4319-Cooling-Fans-Not-Working&highlight=diode)) this morning where someone talks about getting one from RadioShack, without the fancy case. Can the case be slid off and a replacement diode soldered to the tabs?


Third Question:

Do these things have a useful life and should they be replaced on a routine basis as a preventative measure?

[Farrar]

For those Americans in the audience, a "tailback" is a line of traffic, not a football player. ;)

The otterstat is a bimetal thermostat. There's some (but not enough) info here (http://www.ottercontrols.co.uk/sectors/automotive/bimetal-thermostats/y1-series.html). I hope that helps.

Edit: for what it's worth, here's what my replacement diodes (for my car's courtesy light circuit) look like. I bought some 1-amp diodes from Radio Shack, spliced them into wires, applied solder and heat-shrink, and crimped/soldered on the appropriate terminals.
20132
I didn't have to make them that long, but I was concerned about heat damaging the diodes since I don't have a heat-sink to use when I solder.

[Dangermouse]

Alright then Farrar, "coming back, I hit a long queue" :hihi2: Better?

I had previously assumed that it was just a standard thermostat, rather than a thermistor, and wouldn't have thought any more about it apart from that diode.

[dmc6960]

Yes your theory #1 is correct. The reason to prevent the backfeed is because the compressor and the otterstat run on different circuits. Compressor is on Accessory and otterstat is on Ignition. That is why it matters.

[Ron]

I had previously assumed that it was just a standard thermostat, rather than a thermistor, and wouldn't have thought any more about it apart from that diode.
"a" is correct.
But it's not a thermistor (their resistance varies with temperature, significantly). It is a simple open/close switch triggered at a specific temperature.

The diodes are there to allow the Mode Switch to always turn on the Fans when it turns on the the Compressor, while preventing the Otterstat from turning on the Compressor.

You can easily open the shell and replace a diode...beer says you find a cold solder joint.


Yes your theory #1 is correct. The reason to prevent the backfeed is because the compressor and the otterstat run on different circuits. Compressor is on Accessory and otterstat is on Ignition. That is why it matters.
$.02

That part doesn't really matter...the reason the Otterstat is on (fed by) a different circuit is to prevent the fans (a high load) from running down the battery when the engine is not running. Other than that, you could move the feed from either one to the other and everything would work the same (while the engine was running), since both "feed circuits", so to speak, wind up as one feed for the fan relay (after going through the diodes).

[dmc6960]

That part doesn't really matter...the reason the Otterstat is on (fed by) a different circuit is to prevent the fans (a high load) from running down the battery when the engine is not running. Other than that, you could move the feed from either one to the other and everything would work the same (while the engine was running), since both "feed circuits", so to speak, wind up as one feed for the fan relay (after going through the diodes).

If you try running the WHOLE accessory circuit or WHOLE ignition circuit off of either of those feeds, then your in for a bad day. That is what would happen since both of those 12Vs are downstream of the primary IGN and ACC relays.

[Ron]

I didn't say anything like that, Jim...I said "move the feed from either one to the other", not move everything fed along with one or the other to a single auxiliary relay (Besides, with 70 AMP auxiliary relays (IIRC), I'd guess it would be just like any other car w/o a separate position for Accessory and Run. I seriously doubt moving either of those circuits ALONE, as I said, would harm anything....But I'm not going to figure it out since all of this is moot because the question was about the purpose of the diodes. Power supply/load is not the issue. Consider the extreme- if each circuit had a dedicated nuclear power plant and were wired with double-aught braided silver cables throughout, you would still have to use the diodes for the reasons I mentioned, correctamundo?