996 ABS Pump with PSM constantly running

Hi everyone, this is my official first post. I wish it was commenting on someone else's car but unfortunately it's for my own problems. I can be a bit long winded so please bear with me. I picked up my first 996 a few months ago. It is a 2000 Carerra 4 with a 6 speed. It needed some rehab to get going but I've been driving it for about a month now and I like it. However here's my main issue I'm hoping everyone can shed some light on. When I first started driving the car I had an ABS light on and no speedometer. After some rather quick investigating it turns out the ABS unit was unplugged. I will add this is an ABS with PSM so getting one easily hasn't been much of an option. Ok simple enough, I plug it in. It immediately starts running and makes sense why it was unplugged. So now I'm using some serious google-fu and looking up everything I can about why this happens. With all the fuses pulled for the unit, which I found online to be 3 (B9, E9, E10) the pump still runs. After using a multimeter on the plug I have a constant power to pin 2 which is labeled as "Voltage for solenoid valve relay/Voltage for return pump relay". So my here are my questions, Where are these relays? Should this have constant power? As of right now I depinned that constant pin 2 and plugged everything back in and no constant running and my speedometer work. At this moment I'm leaning toward the pump being ok and perhaps one of these relays has shorted out being constantly open. However if that power is supposed to be constant then yes, it could be my pump. I just need a definitive answer which is where everyone here comes in to play. I've read about other pumps constantly running but everyone I read about just sent the pump out and had it rebuilt. I'd like to have that be my last resort. Thank you. Bill

Since its new to u and it seems like u want to work on it yourself get a Bentely book and it will have all the wiring diagrams. That way u can trace it all out as wiring advice over the internet with no schematic is tough.

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I have downloaded a set of wiring diagrams for this car but I am not good at reading them. I can look into having someone help me go through them but my questions were pretty simple and strait forward which is more than likely knowledge someone will have without having to look in a book or decipher wiring diagrams. An amazing user on here could also meter that pin out and tell me if they have constant power with the key off. All very simple things that would help me immensely. Thanks again.

To answer one question....the relays r above the fuse panel in the drivers foot well. Sorry i cant help with the pin checks as my car no longer has its y2k harness or the psc system.

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The relays are there.... they are also in the back on the shelf near the DME. From what I have read there is no mention of one for ABS at all so I'm hoping someone will have some more insight...

Hey Bill - those "relays" are built into the ABS unit electronics module. It sounds like there is an electronic fault in the ABS electronic module. There are several places that you can send the electronics module to that will repair this kind of problem. Look up "Bosch ABS Repair" to find them. This should not be too expensive. You would need to remove the electronic module from the ABS unit and send it to them, which is pretty easy. Be careful about letting the ABS return pump run. That motor is not made to run constantly and will overheat and burn out. Then you will need a new/used/rebuilt ABS unit!

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As soon as I found out that it just wanted to run I depinned it. The power on pin 2 is what was causing it to run constantly and if nothing comes out of this hopefully that could help someone in the future but the pump is what I was worried about. I've seen a few places that should be able to repair the module. Can you verify that pin 2 is constant power? Also exactly what fuse is that if you or someone else knows that would help a great deal. I would find it hard to believe that it isn't fused somewhere. Thanks again. Bill

I'm not exactly following your "pin 2" - is that on the 2-pin connector to the return pump motor - the motor on the ABS unit itself? Or is pin 2 somewhere else. The 2 pin connector to the return pump motor should have constant GROUND on one pin and SWITCHED 12V on the other. The ABS unit is fused by a 50 Amp fuse located in the "current distributor" which is a white box on the firewall, behind the heater air box, in the passenger footwell. I don't have the wiring diagrams for the NA Carrera so I am going off the wiring diagrams for my 2001 Turbo.

Relays are normally "always powered" and only turned on with a ground signal on the input side. In your case, something inside the module has shorted the input pin to ground and is causing the relay to be energized and motor to run all the time. It is also in line with the comment that the ABS unit is faulty.

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It's pin 2 on the 42 pin ABS plug. I believe the mine is the same as the turbo since I have the PSM as well. Apparently it was an option. Let me look to see if I can find that fuse. All of this is pointing to the unit being faulty. Thanks everyone.

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I understand they are "always powered" but the wiring for what I was asking is labeled as a relay. I thought it was the trigger wire for it not realizing it is an internal relay. Now that I have that information I check that off my list and move on. I've noticed a lot of the Pcar relays have a negative trigger to "turn on" if you were referring to these cars specifically but a ground signal isn't the case for every relay, it's pretty common to have a 12v positive trigger especially in american vehicles. You are more than likely correct about it being shorted inside, if that's the case I can't imagine this is actually very difficult to fix without sending the unit out. Thank you everyone again.

U should be able to follow that wire back in the schematic but u did say u r not that good at reading the diagrams. Ill see what i can trace back in my book this evening and verify where the pin 2 comes from.

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theprf mentioned it's fused inside the cabin on the passenger side. I'll look for that fuse and probably just pull it for the time being. Getting some snow here tomorrow so I'll see if the ABS is still working or not.

So this is why these problems r hard to help with over the internet. I can look at a schematic and figure out what is going on but its hard to atriculate in a post. There is a 50amp large current fusable link. It also goes to the "engine relay" and "valve relay" (terminal 30)both r external relays but i cant find them in the bentley so i cant give u a location. They may be on or in the controller but the drawings shown as external. it appears that the pin 2 of the psm feeds the signal side(coil) of the 'engine relay" (terminal 85) no indication if it should be the full time + or ignition + as its comes from the circuitry in the controller. Labled as "UR" ? There is no induvidual fuse for this wire. it looks like the ground trigger (coil) for the relay (terminal 86) is pin 7 of the psm. Labeled as "acuation engine relay". (Termal 87) output of the relay feeds pin 20 of the psm and its labeled as the "motor controller" so at this point if pin 20 does not have power then the motor has an internal issue, if it does have power u need to find the "engine relay" to test itfor functionality.

To check wether your ABS is working pull the fuse for the pump and replace it with a long wire that has a fuse and a (push button) switch inline of the wire. Drive the car, get up to 10mph, brake on a slippery spot as hard as you can and press the switch at the same time. You should feel the brake pedal pulsing and none of the wheels locking up. Edit: after looking/studying the wiring diagrams in the Bentley manual, I agree with dgmark's post below and the above method most won't work. Only if you remove pin 2 (of the 42 pin plug) and connect it to the psm control unit via a switchable lead you can used the test I described to see if the pump running continously is the only defect. It's a BOSCH 5.7 ABS/ESP(=PSM) system in your MY2000 Carrera 4. It's used in many contemporary (german) cars so searching for your symptoms might lead to the cause. For instance BMW calls their ESP/PSM DSC and had the 5.7 Gen in their 7 series: Symptom - The pump fitted to the rear of the DSC hydraulic unit runs continuously, even with the ignition off, this causes a flat battery. Cause - The MOSFET drivers for the return pump are damaged and one or both of them are short circuit. Source: https://www.bimmerfest.com/threads/bosch-dsc-5-7-abs-module-diagnosis-and-repair.822139/

There are no power relays your probably mistaking the valve actuator coils for relays in the diagram You basically have only three power inputs to the ABS unit, F1 50amp, hot all the time E9 25amp hot all the time and b9 15 amp hot with key on. The control unit grounds the pump when activated. If its running all the time with no other codes its junk. I get good units used all the time for less than 200 bucks look on Ebay. the only way your going to get the pump to quit running is to disconnect the 50 amp lead not easy to do as its behind the AC heater box and you will still have an ABS lite.

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In the bentley under the 1999 drawings they clearly show relays and r labeled as so. Note lower left of page. Maybe they actually do not exist? Not disagreeing with the pump being bad, but the po wanted to diffinitively test the unit. I was trying to give him the info to do so. Now my '00 car did not have psm so i have not been into that system. Also the '99 were 100% match to my '00 harness for my gt3 swap.

@Will, Id like to establish whether your MY00 C4 has Bosch 5.3 or 5.7 Gen PSM. Please look at the hydraulic unit and tell us whether you see a pressure sensor alongside the pump motor (black plastic socket on a brass colored bit, bolted into the alumium block). I'm that case it's 5.7. If the pressure sensor sits at the back of the so-called pre-charge pump (located underneath the master cilinder) then you have 5.3 and the wiring diagram for your car is identical to MY1999.

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very valid point as the psm in the 2002 diagrams is very different and the relays r internal. It could be that the change was made on the e-gas cars and not as a mk2 change? I do not have access to y2k schematics. The later psm has pin 2 directly from the current distributor, it has no fuse to pull and it should be hot at all times.

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The ABS units without the PSM are very inexpensive on eBay, however the units with PSM are around $1,000 usd. I'm leaning toward it being shorted out internally but I should mention I do not have an ABS light on with no power to pin 2, just the light indicating PSM is off. We never got snow here Saturday so I have not tested the ABS to see if its actually functioning.

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I'm not totally sure I'm following your description but I do have a black sensor coming from the back of the aluminum ABS block. The part number on the unit is 996.355.955.55 but I really don't know if that will help anyone. When I test the ABS I will get back to everyone here. By the way thank you so much the combined knowledge here is comforting. I knew I came to the right place. Thank you everyone!

Looks like u have the 5.7 unit. May not be available new. There r a few places that repair these units. https://einmaligparts.3dcartstores.c..._p_176857.html

Does anyone in P-Car world use Module Masters? I've had Bosch ABS/DSC modules rebuilt by them. Very satisfied. https://modulemaster.com/

OK, so 5.7 I'm convinced the hydraulic unit, I.e. the 'ABS block' itself is fine, it's the ECU that is defective. Based on the description in the link I posted earlier it seems the MOSFET (labeled as 'Motor Relay', T1 and T2 in the Bentley Wiring Diagram for PSM 5.7) are no longer working and provide the pump with +12V from terminal 2 all the time instead of only when required during intervention of ABS or PSM. If you can find a way to fit a separate wire with a press button switch between terminal 2 on the wiring loom and the ECU you can check if that's the only problem/defect. Do the ABS check (at your own risk obviously) I described earlier and press the button at the moment you've stepped on the brake pedal and release the button as soon as the car has come to a stop. If you experience regular ABS stuttering and a pulsing brake pedal then it's the only defect in the system. If the wheels lock and any ABS/PSM warning lights light up, then something else is amiss in addition to the MOSFETs

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I should mention even with the pump running all other functions seemed totally normal. No pulsing in the brake pedal or anything however I didn't try to activate the ABS. The car has been sitting for a bit now I haven't had the opportunity to drive it lately but when I do I will definitely report back on it's function. I appreciate the break down electrically from everyone, I like to know how things work and why they're broken exactly. I found a thread somewhere about the part numbers for these units changing over the years and the new updated one is available but it was between $5,000 and $6,000 and I don't know about anyone else but that is just WAY to much for this type of repair for me. I will look into some of the companies that repair them, I think modulemasters was mentioned on another thread as well but they didn't specifically say on their website that they repair this module with the PSM but I will give them a call. Thanks again.