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I have a P1632 DTC: "ECM (PCM) judges battery voltage below 8 volts" and one of the listed causes is a damaged ROOM fuse.

So, what is the room fuse, where is it, and how can a fuse be damaged without being blown?

I thought fuses either have continuity and work, or they're burned and don't?

I looked in the fuse box under the hood, and didn't find it there, and the fuse panel under the dash doesn't have any labels, so I don't know which fuse is which.

I have a US Haynes manual and the wiring diagram shows it in the "joint box" together with the engine fuse, but I don't know where the "joint box" is and if it's the same on European GF models ( mine is a 98 2L GF ATX ).

I also have a scan of the 97 EU WSM, but other than the DTC description I couldn't find any reference to it's location.

EDIT Aug. 8th 2017

OK, so I've made some progress. I found two fuse under the dash that are hot ( B+ ) with the key off, a 10 amp and a 30 amp. Both fuses visually looked just fine. I then disconnected the ECU / PCM harness and started checking the voltage on all the pins ( hooked to the negative battery post ). I found multiple grounds which were all about 0.01V, although a few started at 0.1V and dropped to 0.01 after a second or two.

I also found three pins on the ECU harness which where hot, 1. with B+ ( 12.66V ), 2. 3.55V and a 3. with 3.13V. ( just labeling them for reference )

When I pulled the 30 amp fuse it didn't do anything, when I pulled the 10 amp fuse, 1. went down to 1V and 3. went down to 0.04V, and they both dropped steadily over about five minutes, with 1. down to 0.29V and 3. down to 0.01V.

The way they slowly dropped like that makes me think there is a capacitor in the path somewhere.

Neither fuse affected pin "2", which always stayed at about 3.55V.

Under p1632 in the 97 EU wsm it says pin 55 should show 10.5V and if it's not then to check the wiring harness between the battery positive terminal and the PCM for a short to ground.

According to what I could discern in the WSM, I should have VPWR B+ on two pins and then the aforementioned 10.5V backup power on one other pin.

I guess the question is what do I do now? I have no wiring diagrams and don't know which pin is which. I'm not even sure if that 10 amp fuse is in fact the ROOM fuse.

I'm wondering if maybe I'd just be better off taking this to a Mazda shop that will ( I assume ) have wiring diagrams and know exactly where to look to track this down...

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  • Fuses can have voltage drop and still "work". Usually the fuse blew in a way that it barely makes a connection or there is corrosion/oxidation on the prongs.
    – Ben
    Commented Jul 31, 2017 at 11:33
  • @Ben I was paging through the 98 update to the EU WSM and it actually has a illustration of a half blown fuse on this DTC, maybe I'll post it later. Since there's only 15 fuses I'll probably just pull and inspect all of them, one by one of course...
    – Robert S. Barnes
    Commented Jul 31, 2017 at 12:58
  • @Ben I did some further testing and updated the question.
    – Robert S. Barnes
    Commented Aug 2, 2017 at 13:36
  • I'll take a look for some diagrams later. how many connectors on the pcm and what are the first and last wire colors on each connector?
    – Ben
    Commented Aug 2, 2017 at 13:40
  • @Ben I looked in the 98-02 US Haynes manual, but those models use a different PCM I think and have a different wiring layout I'm pretty sure. The PCM connector has 104 pins, but not all are in use. I tried to see the colors of the wires as best I could, the one with B+ was Blue/Pink Or Red, the 3.13V was solid blue, and the 3.55V was I think white/blue.
    – Robert S. Barnes
    Commented Aug 2, 2017 at 14:06

2 Answers 2

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If your battery voltage is good (~ 12.6 VDC) at the battery then yeah, just test all 15. You can do a continuity test, and if any have ohm ratings of any level (even 1 miliohm may be too high) then that would be a flag to check further.

I'm not sure what a ROOM fuse is, but I would infer that it has something to do with some cabin feature...

Anyway, depending on where the ECU takes the battery voltage reading from, you'll be able to narrow down your source of Vdrop.

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OK, this is going to turn out to be one of those "duh" moments. I realized that the reason the DTC kept occurring was because the scan tool I left plugged in all the time was very slowly draining the battery. If I didn't drive the car for a week or two it would drain it enough that the voltage would drop during cranking and set the code.

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