identifying 35 year old relay diodes

I have a "Cooling Fan Control Timer" (relay) from a 1978 Toyota FJ40 Land Cruiser that I'd love to repair than try and source a replacement for. The timer is pictured below with a closeup of the likely damaged components. My intro to EE class was in 1983 and my skills now consist of being able to use a multimeter. Any help would be greatly appreciated. What I know: The module was made in Japan in 1978. It is from a 12V system and sits behind a 5A fuse. The only named component on the board is a cap with the "Nippon Chemi-Con" logo. The one burnt component appears to be a diode but the other - though appearing similar - has no marking to identify the cathode. Any idea what these two parts are and what I should purchase to replace them?

hi, a circuit board (trace side) would help. i'm guessing the diode(s) are for suppression control for the relay coil near them. you'll probably need to carefully remove them from the board for testing and replacement. with the localized heating there, the pads may remove with additional heat from the soldering iron. mark

Ween wrote: hi, a circuit board (trace side) would help. i'm guessing the diode(s) are for suppression control for the relay coil near them. you'll probably need to carefully remove them from the board for testing and replacement. with the localized heating there, the pads may remove with additional heat from the soldering iron. mark

The two components were loose when I removed the board from its housing. The solder there fell off like loose grout. The trace on the back of the board has partially delaminated there also. Here are pics of the trace side.

did some searching and found wiring diagram for '78 fj40. can you id the wires at the connector? (which wire is at which position). there are only five shown; ignition, battery, ground, fan output, and sensor input. should help in determining use of the diodes.

It's probably a Zener with series diode that some use for voltage clamping. Usually a reverse biased plain diode (IN4007 or 4007) diagonally across the 2 "end" pads will be okay especially since it's a transistor circuit. What's the blackening on the diagonally opposite end of the PCB? (Condensation arc?)

oldspark wrote: It's probably a Zener with series diode that some use for voltage clamping. Usually a reverse biased plain diode (IN4007 or 4007) diagonally across the 2 "end" pads will be okay especially since it's a transistor circuit. What's the blackening on the diagonally opposite end of the PCB? (Condensation arc?)

It's just shadowing from some brownish adhesive and my amateurish iPhone photographic skills. At first I thought a part or two had oozed its goo onto the board but all the fan controller boards have this goop on them.

Ah - now it does look like gunk/glue etc. IMO it looked like similar blackish carbon etc. I can only assume that the12volt's image rendering has improved (ha ha). Does my IN400x proposed repair make sense, and do you understand how?

Ween wrote: did some searching and found wiring diagram for '78 fj40. can you id the wires at the connector? (which wire is at which position). there are only five shown; ignition, battery, ground, fan output, and sensor input. should help in determining use of the diodes.

Here are the connections from reading the wiring diagram and matching the wire color codes.

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.

oldspark wrote: Ah - now it does look like gunk/glue etc. IMO it looked like similar blackish carbon etc. I can only assume that the12volt's image rendering has improved (ha ha). Does my IN400x proposed repair make sense, and do you understand how?

I'm afraid I don't quite follow and it's certainly not your fault. Are you saying I could replace the two diodes with a single IN400x?