Testing for a short to ground - 03 RHD Seville STS

[rhdsts]

16 minutes ago, Logan said:

 

Bear with us.....nothing like this on the US cars. It's new to everyone here.

You have all been really helpful - thank-you. I am close to sorting this one out, mainly thanks to the people taking the time to respond to this thread. I am very grateful.

I've had one of those crazy weeks that can happen from time to time. First the Cadillac's brake power assist failed, then the lawn mover stopped working (think I have fixed it now), the sewer line at home collapsed and had to be dug out and new pipe installed, then I have had to see a heart specialist about chest pain. I just didn't have the money to throw at an auto eletrician to figure out what was going on. I thought bad things happened in threes. That four!!

I'm not complaining - all good, and everything is working out. But thanks to you guys fixing the Cadillac has become much easier.

[rockfangd]

Ok so your pump does run. When it runs how does it sound?

As for it building too much pressure it should not. It should have a bypass to prevent such a problem.

Please enlighten me as to what the blue wire is. I know it goes to the relay but what is its purpose. Is it the input supply?

Being that you mention the evap being on the same circuit this makes me wonder if the car has a air pump for the evap... If it does do you ever hear it cycle on?

If it has one and you do not hear it run I would suggest unplugging the pump and possibly the valves, then replace the fuse and test drive it. If the fuse no longer blows then plug in the valves and try again. If it still does not blow then the air pump would be the issue

also please give me the fuse description as I am thinking the input to the relay is not the issue but rather the circuit that feeds multiple relays

Sorry my work schedule is just nuts. 

[rhdsts]

1 hour ago, rockfangd said:

Ok so your pump does run. When it runs how does it sound?

As for it building too much pressure it should not. It should have a bypass to prevent such a problem.

Please enlighten me as to what the blue wire is. I know it goes to the relay but what is its purpose. Is it the input supply?

Being that you mention the evap being on the same circuit this makes me wonder if the car has a air pump for the evap... If it does do you ever hear it cycle on?

If it has one and you do not hear it run I would suggest unplugging the pump and possibly the valves, then replace the fuse and test drive it. If the fuse no longer blows then plug in the valves and try again. If it still does not blow then the air pump would be the issue

also please give me the fuse description as I am thinking the input to the relay is not the issue but rather the circuit that feeds multiple relays

Sorry my work schedule is just nuts. 

The pump buzzes like an electric pump - similar to the self-levelling rear susp compressor.

The dark blue wire runs from the relay connector to the pressure switch on the booster pump assy.

 

[OldCadTech]

@rockfangd in the initial post, of the schematic the OP provided, it shows the dark blue wire goes from the coil side of the relay and is the brake booster sensor signal wire. Initially, I thought there was a normal vacuum booster and the pressure system was a back up system. Now, I'm not sure. I do know it is not a hydro-boost system operating from the P/steering pump pressure.

The schematic also indicates a vacuum sensor signal as well, but as @Logan indicated it may be a mis-print, if in fact it does not have a vacuum brake booster.

BTW @rhdsts airplane crashes come in 3"s

[rhdsts]

I put 12v power to a 10A fuse connected to the dark blue wire (to the pressure switch on the pump) and the fuse blew straight away.

I also put a DMM set to Ohms on the dark blue wire to check again for resistance. Again, it fluctuated, going up to 12 Ohms or so, then setting on around 3 Ohms, then showing 0.00 Ohms. 

I did the same test on the new pump and it showed infinite resistance for the same pin on the pressure switch. In other words, no fluctuation.

I'm at the point where I suspect the pressure switch circuit enought to replace the pump assembly. Fingers crossed that this will fix the problem.

[rockfangd]

keep us posted.

Also check the other side of the pressure switch. I have a feeling that is a ground fed circuit. 

should be open normally, then closed when pump is needed. 

I am going to assume that this vehicle does not have a brake booster and  that is the reason for this setup. Probably not enough room on the engine side for it. 

 

[OldCadTech]

If you did the same test on the new pump and the fuse did not blow, I would replace the unit also.

[rhdsts]

1 hour ago, rockfangd said:

keep us posted.

Also check the other side of the pressure switch. I have a feeling that is a ground fed circuit. 

should be open normally, then closed when pump is needed. 

I am going to assume that this vehicle does not have a brake booster and  that is the reason for this setup. Probably not enough room on the engine side for it. 

 

So to check the pressure switch, do you pin both power in and ground out from the switch with a DMM? Set on Ohms to check that it has infinite resistance?

[Logan]

No vacuum brake booster system.

If I understand correctly......I would expect the fuse to blow instantly.....even a 50 amp. 

For the pump to turn on.....the pressure switch must be closed to ground.

The service manual seems to have a misprint showing the pressure switch as being normally open...it should show as normally closed. Not opening until something like 1800 psi.

So putting a +12v test jumper in...even with a fuse...to the blue wire circuit....would be a direct short to ground. The fuse is doing exactly what it is suppose to do.

 

If you go study the schematic in the first post. It is very, very clear that there is a misprint. Tell me how the pump can turn on.

[OldCadTech]

2 hours ago, Logan said:

If you go study the schematic in the first post. It is very, very clear that there is a misprint. Tell me how the pump can turn on.

In the 1st post diagram the pump doesn't turn on until the switch closes then the relay coil is energized and the coil pulls the relay switch closed.

[rhdsts]

2 hours ago, Logan said:

No vacuum brake booster system.

If I understand correctly......I would expect the fuse to blow instantly.....even a 50 amp. 

For the pump to turn on.....the pressure switch must be closed to ground.

The service manual seems to have a misprint showing the pressure switch as being normally open...it should show as normally closed. Not opening until something like 1800 psi.

So putting a +12v test jumper in...even with a fuse...to the blue wire circuit....would be a direct short to ground. The fuse is doing exactly what it is suppose to do.

 

If you go study the schematic in the first post. It is very, very clear that there is a misprint. Tell me how the pump can turn on.

Yes there is definitely no vacuum booster.

Okay so what tests can I do to rule in (or out) the pressure switch (or pump) having a short to ground?

So far, the only 'conclusive' test I've done is when I checked resistance at the power wire (dark blue) that connects to the pressure switch. On the wire in the car, I got some odd readings (alternating between 12 Ohms to 0.00) while the new pump (out of the car, testing the pin on the pressure switch itself and earthing to the chassis with one of the DMM pins) read a consistant infinite ('1') resistance. 

I don't pretend to understand why the schematic shows the circuit as normally open. Other than maybe the assumption is that the system is up to pressure, and that the accumulator has sufficient stored pressure in its normal state. I don't understand enough about such circuits to know.

I'm running out of ideas here. So unless someone can tell me what test I have not done (or done properly) that would give me the answer, it's down to replacing the pump assembly.

[Logan]

For the pump to run.....the P/B relay must be energized and closed. 

For the P/B Relay to be energized, the 10 amp fuse must be good, and the Power Brake Booster Pressure Switch must be closed to ground. 

Hence the Power Brake Booster Pressure Switch must be a normally closed switch for the pump to turn on. 

Further investigation shows Power Brake Booster Pressure Switch opens at about 1800 psi, turning off the pump. When pressure drops below about 1800 psi, switch closes, turning on pump, recharging the system. 

 

The GM service manual has three (known) misprints.

(1) Schematic shows the Power Brake Booster Pressure Switch as normally open.

(2) Schematic shows the blue circuit is a booster 'vacuum' signal circuit. There is no vacuum in a 1800 psi system. 

(3) Step 5 in the diagnostic flow chart has you fix 'short to ground' on the blue circuit. There is no short to ground. Blue circuit is suppose to be ground.

 

It's rare....but GM service manuals can and do have misprints.

[OldCadTech]

The pump is an on demand pump. If the PBB P/Switch is normally closed the pump would run constantly. Completing the circuit and supplying power and ground. If power is supplied to the Pink wire circuit #173 or connector C at the pump motor there is a ground circuit at the connector B terminal without the pressure switch being closed. I would imagine this circuit is the pump run circuit at key on.

Yes, there are mis-prints esp the vacuum sensor signal. I don't have the FSM. FSM errors were a problem year after year. It was not that rare in the past, hopefully there have been improvements.

[Logan]

It is a on demand pump. The pump turns on/off to charge the system while vehicle is being used. 

You need to go look again. There is no ground path to the P/B relay pin 86 unless the pressure switch is closed. No ground means no closed relay. It is impossible for the pump to run and pressurized the system unless the pressure switch is closed.

Pressure switch must provide the ground path to relay....before relay will close....sending power to pump. 

So....pressure switch must be a normally closed switch. Then opens under pressure. 

Pressure switch does open under high pressure...turns off pump...pressure goes down...switch closes...pump turns on...pressure goes up....switch opens...turns off pump....repeat.  

[OldCadTech]

Please look again. The terminal B at the pump is a direct path to ground G-106 from the pump motor. With the pressure switch open the PCM or BCM, whichever has control, can still command the relay and run the key on pump test through the 25A fuse at the rear fuse block. The 25A fuse is hot at all times and only needs the relay coil to complete the path

The pressure switch is another path for power when needed by pressure demands, or as a parallel circuit for fail safe.

[Logan]

Look again. 

Unless the P/B relay closes. Nothing happens. 

For the P/B relay to close. It needs 12v + and ground. 

Pin 85 on P/B relay is 12v+ (pink). 

Pin 86 on P/B relay is ground (dark blue). The ground circuit is only complete IF the pressure switch is closed.

 

No way on earth will the pump will run unless the P/B relay has +12v at pin 85 and ground at pin 86. 

 

Service manual system description: 

"The power brake booster pump supplies the hydraulic pressure needed for power assist braking. The power brake booster system supplies hydraulic pressure to the hydraulic brake accumulator after the accumulator is discharged. When more hydraulic pressure is needed in the accumulator, the power brake booster pressure switch closes, energizing the power brake booster pump relay. The power brake booster pump motor runs to supply the additional hydraulic pressure to the accumulator."

In other words. Pressure is low at start up. Pressure switch is closed....pump runs...pressure goes high....switch opens....pump turns off....pressure goes down...repeat over and over. 
 

 

[OldCadTech]

LOL - Looking again,

If you have service manual access look for the ground distribution. There has to be another control of the ground, perhaps on splice 130.

Fuse 11 10A is only hot in key on and start, not run, unless you consider  that is a FSM error also.  There is a key on run test, if the switch is normally closed the relay would not provide power while the key is in run if the pressure was above the low limit of the switch.

[Logan]

I consider key on as run.

 

Service manual system description: 

"The power brake booster pump supplies the hydraulic pressure needed for power assist braking. The power brake booster system supplies hydraulic pressure to the hydraulic brake accumulator after the accumulator is discharged. When more hydraulic pressure is needed in the accumulator, the power brake booster pressure switch closes, energizing the power brake booster pump relay. The power brake booster pump motor runs to supply the additional hydraulic pressure to the accumulator."

 

In other words. Pressure is low at start up. Pressure switch is closed....pump runs...pressure goes high....switch opens....pump turns off....pressure goes down...switch closes....pump runs....repeat over and over. 

 

Being a former GM lifer....I doubt they are going to fuss too much about PCM diagnostic controls for a RHD export car. I figure there can only be maybe 2000 2003 RHD cars ever built for export. Maybe 1-1.5 % of production....not many. 

 

[OldCadTech]

5 minutes ago, Logan said:

When more hydraulic pressure is needed in the accumulator, the power brake booster pressure switch closes, energizing the power brake booster pump relay.

This means the switch is normally open.

[Logan]

No....your looking at it wrong.

Low pressure= switch closed. 

High pressure= switch open. 

 

The system requires pressure. When the high pressure system drops too LOW....the switch closes....the pump turns on. When pressure is back high again....the switch opens....the pump turns off. Repeats over and over.

[rhdsts]

4 minutes ago, Logan said:

No....your looking at it wrong.

Low pressure= switch closed. 

High pressure= switch open. 

 

The system requires pressure. When the high pressure system drops too LOW....the switch closes....the pump turns on. When pressure is back high again....the switch opens....the pump turns off. Repeats over and over.

Okay you've lost me. Whether the circuit is normally open or normally closed (I can't can't see how it can be either, because as you say, it opens and closes all the time to keep brake fluid pressure at the correct level), how does one work out a short to ground? Because I seem to have found a short in the pressure switch circuit that might explain why that 10A fuse keeps blowing.

Maybe I have missed the comment, but can anyone confirm that my tests have been conclusive? If not, what do I do?

[OldCadTech]

Whether it is a normally open or normally closed switch makes diagnosis of a short to ground important. 

Logan is correct in the operation of the switch, but my point is, the switch is normally open unless the load is needed and then the circuit is energized, then the circuit opens back to a "normal" state.

I'm not sure that the test you performed indicated a faulty pump or a faulty switch.

[Logan]

My diagnosis is a bit different.....based on the GM service manual schematic.

 

The RHD brake system runs uses hydraulic pressure. The pump is 'on-demand' and only runs when needed. 

The hydraulic system pressure varies as the car and brakes are used.  

When the hydraulic pressure gets low, the pump turns on to boost the system pressure higher. When the pressure is back higher, the pump turns off. 

This cycle repeats all the time while the car is being used. How often the pump cycles would be directly related to how often the brakes are used. 

 

Looking at the schematic....the service manual shows the pressure switch as 'open'. To me....the system will not function that way. 

For the system to operate. The pressure switch must be closed at low pressure....open at high pressure. 

If my diagnosis is correct. The switch in the schematic really should show 'closed'. The system will function as described above. 

If the pressure switch is 'open' as shown in the schematic....the system would never turn on. A obvious misprint or lack of system operational information.

 

Originally you posted about confusion related to step 5 in the service manual about fixing the short in the circuit. To me....that circuit should be ground. It should act like a short to ground all the time except under running, high pressure conditions (1800 psi).

When you put the fused jumper in....it blew the fuse. To me...this would be the expected result. Being if the pressure switch is closed to ground..a fused 12v jumper to the blue wire would be a direct short to ground...blowing the fuse. This would support the theory that the switch is in reality...closed to ground.  

 

As for the posts here....anytime now someone will chime in about have we checked the brake booster vacuum hose or tail light bulbs....What?..

The schematic is as clear as night and day...it is basic automotive electrical 101. A relay needs both power and ground. The current GM schematic for this system shows a open relay ground at the pressure switch. That, of course, cannot be correct. When folks can't read a schematic...we can tell. 

Update: But the rant continues. There are no magical PCM / BCM drivers or secret hidden ignition switch positions. Now they're talking about NO and NC relays...way off course...

The posts following this one prove my point....it truly seems as if they are making it up as they go....they cannot explain how the system relay works.  

 

My final suggestion would be to save the old pump assembly forever. You may need it in the future to have it rebuilt somehow.  There may only be a thousand of the RHD cars worldwide left with this pump...if that.

 

 

 

[OldCadTech]

13 hours ago, rhdsts said:

So far, the only 'conclusive' test I've done is when I checked resistance at the power wire (dark blue) that connects to the pressure switch.

This test can be misleading, doing an ohm check,  as the DVOM supplies a small voltage to the circuit. The tests at the pump and switch connectors should be done with the connectors disconnected. An ohm check of the pump itself would be valid. Do the check between circuits B & C at the unit with the terminal connector disconnected.

[OldCadTech]

@rhdsts This may help you understand the importance of normally open or normally closed. It all depends on the energized state.

 

http://www.galco.com/comp/prod/relay.htm