2004 v70 limp mode/brake problem

Post by **abscate** » 01 Mar 2015, 05:19

The 2004 v70 owner manual lists fuse 8 as handling

AC compressor
ECM fan
Accelerator position sensor APS

This all makes sense now. On startup everything was fine, but pushing the brake pedal probably triggers the ECM to query the APS, and when it doesn't find a signal, it defaults to limp mode

Faust · Post by **Faust** » 01 Mar 2015, 20:33

abscate wrote:The 2004 v70 owner manual lists fuse 8 as handling

AC compressor
ECM fan
Accelerator position sensor APS

This all makes sense now. On startup everything was fine, but pushing the brake pedal probably triggers the ECM to query the APS, and when it doesn't find a signal, it defaults to limp mode

But, if the accelerator worked normally until the brake pedal was pressed, why wouldn't the ECM find a signal? With a blown fuse, there should not be an APS signal before pressing the brake.

Post by **precopster** » 02 Mar 2015, 04:48

When the brake pedal position sensor is failing on a P2 you will get similar symptoms. Whilst pressing the brake you cannot have throttle action......engine just idles.

I wonder if fuse 8 powers the brake pedal position sensor on the brake booster.

Faust · Post by **Faust** » 02 Mar 2015, 06:38

precopster wrote:When the brake pedal position sensor is failing on a P2 you will get similar symptoms. Whilst pressing the brake you cannot have throttle action......engine just idles.

I wonder if fuse 8 powers the brake pedal position sensor on the brake booster.

What you are describing is "brake-throttle override" federally mandated since (I believe) 2012. This is supposed to overcome the "problem" of "unintended acceleration". I didn't think an '04 would have it, although you cannot shift out of PARK without stepping on the brake.

Post by **abscate** » 02 Mar 2015, 09:09

To really answer this you would have to know the software in the ECM in some detail. It does kind of make sense that, since you can't drive the car (automatic) until the brake pedal is depressed, the ECM stays dormant with respect to a bunch of signals on startup until you wake it up with the brake pedal nd shift into drive.

You probably get low battery voltage when cranking, so the ECM would get all glitchy if you have it interrogating everything on key on.

Post by **abscate** » 02 Mar 2015, 09:13

Mod note - merged with other thread discussing same

Faust · Post by **Faust** » 02 Mar 2015, 13:16

The problem is not fully resolved. Yesterday I went out in the morning and found I had the old hit the brake and no throttle problem. I immediately went to the fuse and found it bad, it looked more broken then burnt, so I ignored the problem. This morning, same condition except obviously burned. Runs fine all day and yesterday, must be something to do with cold start.

Post by **precopster** » 02 Mar 2015, 13:33

Now that fuse 8 is blown test my theory by measuring for voltage at the bpps on the brake booster. I bet you won't have any there. I could be wrong of course.

Post by **abscate** » 03 Mar 2015, 05:01

Ok, but now you just have to source why the Fuse 8 is blowing. You know it's one of three things from the post above and I'm guessing the AC isn't in use in MA right now

Test that ECM for current draw
Test the APS sensor

I'll go with the APS sensor as the culprit.

On edit,...this whole thing started on serp belt failure. Check the ECM wiring for shorts..maybe just unplug the fan since you won't need it for a few months..

Faust · Post by **Faust** » 03 Mar 2015, 12:50

abscate wrote:Ok, but now you just have to source why the Fuse 8 is blowing. You know it's one of three things from the post above and I'm guessing the AC isn't in use in MA right now.

Well, some "progeress". I took the car out on an errand last night and developed a lot of noise under the hood. I pulled into a local gas station and opened the hood to inspect. The engine stalled as a I did that. The battery apopeared dead. I had it towed home and hooked a charger up over night.

This morning it started, but made a lot of noise. I opened the hood and watched the serpentine belt shred itself. With the belt gone I tried the various accessory pulleys. The alternator and p/s spun freely, the a/c compressor pulley was frozen. The a/c compressor/clutch is protected by the now infamous fuse #8. I have to suspect that this is the root cause of the blown fuse and the failing APS. I cannot fathom how use of the brake pedal effects it. I have now posted on unfreezing the pulley.

PS There are positions on the heater/defroster which kicks in the a/c, even in winter. The dried air assists in defrosting.