identifying 35 year old relay diodes

It may not be my fault, but I haven't decoded the circuit to sufficient detail - I'm being quick, or lazy. I was assuming that those diodes were for relay-coil spike suppression, but if Ween thinks they are in series with IGN then that's an entirely different matter. If it's an IGN voltage dropper or spike protector (to protect the circuit from IGN spike damage) then it would/could be the usual low Ohmage big Wattage resistor in series with a polarity protection diode (before or after the resistor) that then powers the circuit AND has a Zenor diode to GND to limit the voltage to some maximum (eg, 16V, or 10V, etc). But in that case there would be the power take-off from the junction of the 2 diodes, and the polarity protection diode would IMO be much larger than the apparent signal diode pictured - eg, ~100mA rated; ie - a bigger 1A 1N400x or larger would be used UNLESS the relay coils are powered direct from IGN +12V (since they'd probably take in excess of 100mA each). However it could be 2 series diodes (Zenor & normal) to provide a fixed voltage drop from IGN +12V and hence no "junction" power take-off but IMO that is unlikely. Then again, I have seen many "strange" car electronics - even in Jap vehicles. And maybe one relay is a polarity protection diode (ie, has a diode in series with its coil and its contacts connects the power). But since I saw no track from the junction/common of the 2 diodes I assumed it was for coil spike protection. IE - instead of the usual reverse connected IN400x etc diode across the relay coil (line end to +ve of the coil, other end to the coil's -ve or GND) which "shorts" -ve spikes to GND, a Zenor diode is added in series with the aforementioned diode but back-to-back (ie, line end to line end, else lines away from each other). I fail to see the need for the added Zenor diode (why increase the -ve spike from ~0.7V to ~{ 0.7V plus the Zenor breakdown voltage } unless perhaps needing to limit noise or the current surge?) and hence my proposal to bypass (omit) it. Of course in the time I wrote this reply I could probably have decoded enough of the circuit, but my printer has gone on vacation and I hate reverse engineering PCB circuits on screen. (Did I mention lazy - I mean, too busy?)

Here's another shot of the trace side without a flash, perhaps a little better. I also pasted a link to a post (scroll to post #170) containing the relevant pages from the 78 Toyota fan controller service specs. I wish I were smarter about this. I started out at Ga Tech as a EE and after a particularly difficult class switched to IE. Since I went into the Navy after graduation I guess it ended up being somewhat irrelevant which engineering discipline I majored in. I'm now retired but this has really piqued my interest again. Maybe enough to take a community college class or something. http://forum.ih8mud.com/40-55-series-tech/401196-fj40-carb-cooling-fan-do-you-run-yours-9.html

It's 4:15Am here and I'm about to crash. Those pages don't help - they don't show any circuitry. And you supplied the required connection info in your 2nd last pic. But tomorrow tonight I'll see if I can su and root around with your pics assuming my girlfriend lets me off for the night (we'll be scavenging firewood for her).

oldspark wrote: It's 4:15Am here and I'm about to crash. Those pages don't help - they don't show any circuitry. And you supplied the required connection info in your 2nd last pic. But tomorrow tonight I'll see if I can su and root around with your pics assuming my girlfriend lets me off for the night (we'll be scavenging firewood for her).

Just trying to supply anything that might help. Thanks so much for your effort.

YES! Time, or delays, or procrastination. I've just been reminded of a weekend visit to a distant relative (my oops!), but I'll ensure SHE brings MY mobile router so maybe I can log in etc from there. It's now 12:30AM Zulu and I have a full day tomorrow, and still no printer. So much for my Linux Mint 15 (xfce) install...

Anyone else care to take a stab at this?

Ween wrote: the two diodes appear to be in series on the ignition lead...polarity protection as well as a slight voltage drop. guessing the resistor adjacent provides some isolation for the timer circuitry from the relay coil operation.

Ween, Can you suggest specific diodes to replace the two damaged ones with? I've been trying to sort this out for weeks and am about to throw in the towel. Thanks so much for your help.

did you think of looking on ebay perhaps for a spare for parts? i think there are a few listed for around $20 shipped? then again 1N400* series diodes are like $1.50 for a pair at radio shack, or a multi-pack of 25 diodes for under $4. did some searching on fj forums for the operation of the timer module. from the description, it seems you could wire the unit on a test bench to check operation. if the diodes are inserted backwards on the ignition lead, the unit won't operate.

Ween wrote: did you think of looking on ebay perhaps for a spare for parts? i think there are a few listed for around $20 shipped? then again 1N400* series diodes are like $1.50 for a pair at radio shack, or a multi-pack of 25 diodes for under $4. did some searching on fj forums for the operation of the timer module. from the description, it seems you could wire the unit on a test bench to check operation. if the diodes are inserted backwards on the ignition lead, the unit won't operate.

The controllers I see listed now are for 79-80 and 81 models and I hadn't considered buying a similar one to rob components from. I'll try the 1N4001 diodes first. Thanks for your help.