Logicon Problem: Fan Amp (?)
 

Cliff notes: No A/C or heat. Nothing works. FSM says fan amp is bad. Would anything cause the fan amp to go bad except age? Is the fan amp repairable?

I haven't had any A/C or heat in my car for over a year now and I'm tired of it.

Here are the symptoms:

1. A/C and Recirculation lights don't illuminate when I push them in.
2. Nothing happens when I:

1. move the blower slider to the right (turn it on...).
2. change where the air is coming from.
3. change from cold to hot, and vice versa, on the slider

3. The face does illuminate when I have my lights on.

I tested it out per the S4 FSM. All the tests in sections U-12 to U-14 turned out correct so it referred me to U-29 (attached below). I have also taken out the blower motor and hooked it directly to 12v and it worked.

My results were:

Turn blower switch ON:
f = 12v (supposed to be <2v)
h = 12v (supposed to be <2v)

Turn blower switch OFF:
f = 12v (correct)
h = 12v (correct)

Turn blower switch from low to maximum:
a = 0v (supposed to be 5v - 12v)

Turn blower switch to maximum:
i = 12v (supposed to be <2v)

Turn blower switch to medium:
i = 12v (correct)

As you can see, my results blow (sarcasm). According to the FSM, this means replace the fan amp. I just want confirmation on this.

What would cause the fan amp to go bad? Such as, something else (air mix motor?) is breaking them so I would have to replace that as well.

Also, is the fan amp repairable (kinda like the idiot lights)?

I bought this logicon/fan amp as "working when taken out of the car" after replacing my old one (same symptoms).

Thanks ahead of time.

You probably already looked at this, but the common failure point is the connector to the fan amp & the high speed relay.
It's the oxidation causing resistance causing heat causing oxidation loop.
Shinny bright metal-to-metal contact is needed for high amp loads.

In general logicons fail more often than fan amps.

The other common problem is often called "cold solder joints".
I'd resolder the the logicon PCB joints behind the connector plugs.

Is the high speed relay the "ex-hi relay" in the FSM? The round one by the blower motor? If so, I did not check it.

I'll try resoldering my logicon. Should I resolder the fan amp?

I should of tried the logicon testing sequence in the FSM. I'll do that sometime this weekend. Hell, I guess I'll just do all the tests. Can't hurt...

man you sure can put the HOMEWORK on paper,nice to know theres people who actually read the FSM
kudos

Thanks. :)

I get tired of seeing "Help me" threads where the description of the problem is "my car runs funny". :wallbash:

The ex-high relay connector carries the highest current, so if it's connector is not perfect it gets hot enough to start oxidizing the other pins in the connector.
When it fails the lower speeds will still work.

From what you describe, Id look first at the heater relay.
Then the fan amp, where the connector is soldered to the PCB.

I tested the logicon via the FSM. All the tests were fine except for "Checking power transistor output" (attached below is the testing procedure).

I received 0v regardless of where the blower switch was put to. It says that if this test fails (which it did), then check the fan amp. My fan amp has tested bad already, so...

Also, the A/C light and Recirculation light both checked out when tested via the FSM. However, neither work when the unit is actually plugged up into the dash.

I noticed an unhooked connector to the passenger side of my blower motor (it's not the blower motor connector). It was single spade with a G/B wire. I can't find it in the FSM. It's coming out the loom that splits into the Heater relay and Ex-hi relay. I'll try to get a picture this weekend.

I did the "Ex-Hi relay output" test in the FSM. The test says to hook up a test light, but I just checked voltage, which was 12v (I'm assuming that's the same thing). So it checked out fine, at least in that test.

I disconnected the heater relay and looked at the spades. Everything looked fine (no corrosion, rust, etc.).

I'll check the fan amp and see how it looks. I usually test connections/solder points via ohms. Is that the best way?


P.S. Testing that damn thing is hard. It required 3 jumper wires and a multimeter. Not exactly easy to fit 3 jumper wires in that small space without having them touch each other or multiple prongs...

P.P.S. Thanks for the help so far Sureshot.

Quick update.

1. Why would the A/C and Recirculation lights work when testing via the FSM and not when I plugged it up? To me that doesn't seem like it would indicate a fan amp problem.

2. I plugged it up and tested the output of some of the wires. The ones I tested were from the G-01 connector (14 prong).

R/G = 12v
R/B = 12v
W/G = 12v
L/B = 12v

I have no idea what the voltage is supposed to be on any of the other wires because the wiring diagram confuses me.

Did you ever figure out where the single spade with a G/B wire goes to? or what its even used for? I see mine also unconnected.

There is an older forum post on how to rebuild the amp and logicon stuff. These are fairly simple to check, and they have really fat, easy solder joints to work with. You can get ahold of pretty much every component in the amp. As far as power delivery, there could be a number of problems. The transistor in the blower box can be replaced, as a bad amp can burn it up. The ex-high relay only kicks on in the final position. The power transistor in the blower receives signal from the fan amp to control the power feed to the blower motor. I would replace this one, as transistors are annoying to test, and they cost $1.25 apiece. Aside from that, most of your gremlins are likely in the amp section of the logicon (look for the darker brown spot on the already dark brown circuit board), and at the ribbon cable connection in the main logicon. The components in the amp are mostly in rows for easy testing. Make sure to remove one side from the circuit to test. Some of the diodes and resistors can't be checked in this circuit.

https://www.rx7club.com/2nd-generati...build-1125980/
https://www.rx7club.com/2nd-generati...en2n3-1120533/