92 Legacy - What is each relay in engine fusebox used for?

Many hot sides of a circuit are simply listed in the diagrams as "hot in acc/run". That means they connect to 12V when the ignition switch is in the Acc or Run position. Okay, but the ignition switch is not really the switch in their circuit. The ignition switch energizes several relays that then closes their contacts which go to 12V for the circuit under consideration.

In the engine compartment is the fuse box (the one with the headlights, hazard lights, the fuse link, and other high-amp circuits). There are 6 relays inside. The 2 at the back end are for the headlights. There are 4 more relays running in a line along the side of the engine fuse box but no mention what they are used for. I think one of them is bad but don't know which relay is used for what circuit.

I'm having weird problems with the radio. I narrowed it down to the switched 12V leads (2 of them, one to each connector on the radio, but spliced together back in the wiring harness). They go hot when the ignition switch is in the Acc or Run position. When there is no load on them (i.e., not connected to the radio) and the ignition switch is on, the voltmeter shows 12V. Yet when there is a load (by connecting the radio), they are just under 3V and why the radio won't power on.

I connected the radio directly to the battery (and not through the wiring harness at all). The 2 switched power leads (yel/red) and the constant 12V lead (blue/red) went to battery "+" and the ground (black) went to battery "-". The radio worked. When I use the wiring harness with my old radio or using a replacement that was tested good, and the ignition switch is turned on, neither radio will come on (the display remains blank). After trying several combinations of using wires in the harness or connecting direct to the battery or a ground, I found the problem is with the switch power wires (yel/red). When disconnected from the radio (i.e., under no-load), they show 12V on the voltmeter. When connected to either radio (i.e., under load), they drop to 3V. The radio probably wants 12V and not 3V to power it.

According to diagrams and the labelling on the inside fuse box cover, the switched power wires go from the radio, get spliced together in the harness, and to a fuse in the inside fuse box. It is after that point that I don't know how the fuse gets to the battery. I suspect there is a relay between the fuse and the battery that is bad. Peculiarly I have not noticed any other electrical problems and it is likely that the relay (controlled by the ignition switch) connects to several fuses to several circuits.

Of course, I'm talking of old-fashioned relays with a coil that when energized will close some contacts on a spring-loaded arm. The relays could be SCRs or other electronics that simply act like the old mechanical relays regarding closing/opening of circuit paths.

I'm wonder what each relay is used for in the engine fuse box. If one were bad, I could replace it to see if that solved the low-voltage under-load problem with the switched power leads to the radio. The radio works since wiring it directly to the battery for power and ground get it to power on. The blue/red constant 12V lead works since I can wire the switched power leads direct to the battery and the radio comes on (which also proves the ground lead is okay).

What works (i.e., radio comes on):

- Direct to battery (wiring harness not used at all). Black wire from radio to battery "-" and all 3 power wires from radio (2 yel/red and 1 blue/red) to battery "+".

- Black to wiring harness ground, blue/red to wiring harness 12V constant, and 2 yel/red wires direct to battery "+". This has the switched power leads disconnected from the wiring harness and instead go direct to battery 12V.

Voltage on switched power wires (yel/red) in wiring harness with ignition switch on:

- 12V under no load (nothing connected to the red/yel wires in the harness).

- 3V under load (radio connected).

The radio isn't dragging down the voltage because there's a short inside of it; else, it wouldv'e burned up the short in the 7 years since the radio stopped working. Remember that the radio works when connected direct to the battery, and also when all the wiring harness is used EXCEPT with the red/yel switched power leads going direct to the battery. It's like something is regulating the voltage but screws up when some actual current is drawn through the switch power circuit. The voltmeter doesn't draw enough current to provide a load on the circuit and why it shows 12V on the switched power wires in the harness.

I'm thinking that something upstream of the fuse marked "Radio" in the inside fuse box is causing the problem. That is, what is between the "radio" fuse's hot side and the battery? The constant 12V (blue/red) wire goes to a fuse in the engine fusebox but the switched power wires go to a fuse in the inside fusebox. There is something between the "radio" fuse and the battery so that it goes hot only when the ignition switch is on (i.e., in the Acc or Run position). I need to figure out just exactly what is between the "hot in Acc/Run" side of the "radio" fuse and the battery. The ignition switch is involved there but probably is not directly in the circuit but instead controls a relay whose contacts are actually in that circuit.

Reply to
Vanguard
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More than likely, one of those relays IS an accessory relay. Until someone looks it up in the service book try the following.....

Each of those relays are more than likely the SAME type and they plug in. Label them "1" "2" "3" and "4"..... Swap 1 with 2..test radio....Swap 1 and 2 back..then swap 2 with 3....test radio Swap 2 and 3 back..Swap 3 and 4...test radio each time. When you get the radio back..the relay you JUST PULLED was the bad one. These relays are common, a wrecking yard should be able to supply a handfull at a bargain price.

Reply to
Porgy Tirebiter

This is one of those times when a "test light" with an incandescent bulb is recommended. The extra current draw for said bulb will case a flaky circuit to drop in voltage as you have found without having to reconnect your radio.

Vanguard wrote:

Reply to
nobody >

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