Sears GMP075-3 Wiring Photo

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Old 11-11-14, 05:32 PM
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Sears GMP075-3 Wiring Photo

I just picked up this furnace to use in my Detroit rehab project. Everything looks mostly complete, but I'm having trouble comparing the wiring diagram in the service manual (GMPN?) to what I see in the cabinet.

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I see the R W and G connections at the top left of the main board, but the jumpers connected to them seem to get mixed up in a mess of wire nuts which also mix in connections to some other things in there, like the big magneto-looking thing at the bottom. Then, they combine into 3 bundle cables exiting the cabinet. One, I suppose, hopefully contains just the RWG wires, but what could these other ones be for? Maybe for setting the fan speed or something? If I plan to just plug this into a standard tstat, what do I do with these other 2 bundles? Each has a red and white wire.
 
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Old 11-11-14, 08:36 PM
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The "big magneto-looking thing" is the transformer. It steps down line voltage to 24 volts for the control circuit. Do you have any connections to Y & C (or Com.) on the board?
 
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Old 11-11-14, 11:19 PM
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I see a transformer at the top left of the unit. The one laying on the ground could have been for a dehumidifier. One of those cables could have been for an A/C condensor and one for a humidifier.

We need a closer picture with the wiring separated better to verify.
 
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Old 11-12-14, 04:27 AM
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Yeah I thought the part on the top left was the transformer, so wasn't sure why there would be 2. There isn't a C connection on the board, but I've read a few threads explaining where to tap one on this board. I think the humidifier and condenser theory is a good bet. I'll separate these wires more and take a closer look at what's going on and post back later today.
 
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Old 11-12-14, 08:53 AM
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So, the big transformer on the bottom is spliced into the white and red wires coming from the blower.
 
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Old 11-15-14, 02:24 PM
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I removed the extra pieces, hooked it up, and Everything works except the main blower win't start. I jumped the main 120v input on the board to the HEAT terminal and the blower turned on, but if I apply 24v from the transformer to the G wire I get nothing. Can I rule it a bad board?
 
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Old 11-15-14, 02:54 PM
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Measure G to ground see what voltage you get. Which will be the blue wire from transformer.
 
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Old 11-15-14, 05:15 PM
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The blue wire coming off the transformer, which is also connected to a purple wire going to ground, reads zero volts when the r,g,and w is all twisted together. The other red wire off the top of the transformer reads 24v. I'll post a pic once I get back to my desk
 
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Old 11-15-14, 05:30 PM
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That's a goodman model number from back when goodman was junk - you'll be much better off with a new furnace that has a full warranty.

New furnaces at cost aren't too expensive - it's the install + markup that's expensive.

Gmp heat exchanger failures -> please watch: Heat exchanger failure diagnosis, Goodman GMP model - YouTube
 
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Old 11-15-14, 06:33 PM
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Yeah I read that info this morning. Once I get it running, I'll see if it has that problem. For now, I'm going to try to make this one work. Here is the transformer.

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Old 11-15-14, 07:56 PM
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Glancing at this photo... Am I crazy, or does it look like the black and white wires on the bottom of the transformer are switched from what the sticker above them shows? It looks like the white wire is on the 120v and the black on the common. This is definitely how is was wired when I got it.

And if they are switched, why would it be able to go through the entire heating cycle except for the blower?

Or, I guess a better question is: If both of the top spades are supposed to be delivering 24V, can I just splice all 3 wires onto the one good red 24V output?
 
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Old 11-15-14, 08:06 PM
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If that transformer is a 4000-01E07AE79 here is a photo without the wires attached.

Glancing at this photo... Am I crazy, or does it look like the black and white wires on the bottom of the transformer are switched from what the sticker above them shows? It looks like the white wire is on the 120v and the black on the common.
Yes,it appears so. I would definitely make sure the wires are connected to the correct terminals on the control board as shown on the wiring diagram as modern furnaces require the correct polarity to operate.

Or, I guess a better question is: If both of the top spades are supposed to be delivering 24V, can I just splice all 3 wires onto the one good red 24V output?
No, one side is the "hot" side and provides 24V to the thermostat. The other side is the common side. If they touch together it will fry the transformer. It appears the purple wire is connected using what is known as a "piggyback" terminal that allows two wires to be attached to one terminal.
 
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Old 11-23-14, 07:04 AM
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Corrected the transformer wiring and still no fan. My new board has arrived so I'll install it next time I'm out there.

One more curiosity as I'm about to install the new board on this. In the photos I posted, you can see the white jumper between pins 1 and 4 on the 9-pin connector. What does this do? The instructions with the new board say to cut it (which is the same model number as the old board). Could it be possible that the reason the board doesn't send power to the fan is that this furnace already has a replacement board and this jumper was supposed to be cut? Maybe I'll try cutting it on the old board and see what happens.
 
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Old 11-23-14, 09:09 AM
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Can you post the part number of the board currently installed and the board you want to install? On some of the boards the 1 to 4 jumper wire is for 90+ efficiency furnaces.
 
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Old 11-23-14, 09:32 AM
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They are both B18099-13

This furnace is only 80%, but I think you might be right about the jumper being used for 90+, since I think there are 90 I/O spots on the right side of the edge connector, but no wires on that side of the harness attached to it.

Any reason why the jumper would prevent the blower from turning on?
 
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Old 11-23-14, 10:05 AM
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Any reason why the jumper would prevent the blower from turning on?
Reviewing the wiring diagram there aren't any wires connected to the 1 and 4 pins. Also, the diagram shows there are two separate blower relays. IDBR1 is for HEAT and IDBR2 is for cool. It appears the black lead from the blower goes to the COOL terminal and the red wire goes to the HEAT terminal. I don't believe cutting the jumper wire between 1 and 4 would affect operation of the blower.
 
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