Testing for a short to ground - 03 RHD Seville STS

[rhdsts]

I have also asked this question over at the Cadillac forum, but I know some of you guys are Cadillac techs so I hoping someone can help me.

I apologise for asking what is probably a very basic electrical question. I have tried googling this but I can't seem to find a clear answer.

I have a 2003 RHD Cadillac Seville STS. Unlike the LHD cars, the RHD model has a brake booster pump to supply power assistance for the brake system. It contains an electric motor, a pressure switch and an accumulator.

The fuse to the pressure switch of the pump keeps blowing. I think the pump motor or pressure switch is shorting out.

This is the wiring diagram (attached)

Here is where I need help:

I followed the trouble shooting guide in the factory manual. It said to connect a test light between the brake booster vacuum signal circuit (dark blue wire) and ign 1 voltage circuit at the relay (pink wire), with the relay removed. The test light lit up when I did this.

The next step in the flow chart said to test the booster sensor signal circuit for a short to ground.

This is where my question comes in. According to the factory manual you can test a short two ways:

Use a digital multi meter (DMM) - 
Remove the power feed (i.e. fuse, control module) from the suspect circuit.
Disconnect the load.
Set the rotary dial of the DMM to the Ohm position.
Connect 1 lead of the DMM to 1 end of the circuit to be tested.
Connect the other lead of the DMM to a good ground.
If the DMM does NOT display infinite resistance (OL), there is a short to ground in the circuit.

When I set the DMM to '20k' Ohms, I tested the dark blue wire as per above instructions, and the DMM showed a reading fluctuating between 5.66 Ohms to about 9 Ohms. When I tested the pink wire, it showed infinite resistance ('1').
EDIT: Despite what seems to be a good earth (strut tower bolt, but tried a couple of other earth points) the Ohm reading at the dk blue wire is not consistant. Some readings, it's 0.00, others it'll flash up to 19 Ohms and then settle at '1'. When the DMM is set to 200k Ohms, the reading slowly climbed to 140. other times it fluctuated between 80-120.

There is a second method, using a test lamp:
Remove the power feed (i.e. fuse, control module) from the suspect circuit.
Disconnect the load.
Connect 1 lead of the test lamp to battery positive voltage.
Connect the other lead of the test lamp to 1 end of the circuit to be tested.
If the test lamp illuminates, there is a short to ground in the circuit.

When I did this, for both the blue wire and then the pink wire, the test lamp illuminated.

Does this sound like a short circuit to you? Using the second method (test lamp) it shows resistance in the ign 1 voltage circuit, which conflicts with the DMM reading of infinite resistance. Of couse ign was off during short to ground tests.

The whole sorry situation with the pump can be found here:

http://www.cadillacforums.com/forums...ster-pump.html

[OldCadTech]

It all depends on where you are testing the blue and pink wires from. The purpose of both tests is to isolate the circuit wiring.

Pull the Booster pump relay and look at the connector end of the relay, find the terminal marked 30 on the relay and match that terminal to the female connector on the fuse/relay block. Connect a wire to that #30 connector (not at the relay-where the relay plugs into the wire) and connect it to the battery ground terminal. At the booster pump assembly, disconnect the connector and with your test light connected to the positive terminal of the battery, probe the pink wire at terminal C (connector end - not the pump side) if the test light illuminates, disconnect the wire at the negative battery terminal and IF the wire is good the test light will go out. If the test lamp stays illuminated you have a short in that circuit. Physically trace the wire (pink circuit 173 in this example) completely back to the relay connector if necessary to find the short.

If the circuit 173 (pink) tests okay repeat the entire process for the DK BLU circuit at relay connector 86 and at booster pump connector A.

If they BOTH test good let me know and I can run you through the pump test.

These under hood fuse and relay blocks are notorious for corroding, so when you remove the relay check the condition of the connectors. If they look corroded, pull the fuse block so you can see the bottom of it and check it.

If the wires are shorted, and are impossible, or seem to be impossible to trace ( without taking half the car apart) and the fuse/relay block is in good condition, remove the faulty circuit connector from the fuse block and run a replacement wire directly from the relay, through the fuse block and to the booster pump connector. If the booster pump connector is in good condition, cut the faulty circuit wire about 4-5 inches from the connector and solder and seal the replacement wire and the old wire together or use a heat sealed butt connector.

 <<<----- Harbor Freight Watertight Heat-Shrink Butt Connectors

Your relay #30 terminal may not be in the same location as the one pictured, but it will be marked.

[OldCadTech]

I responded above and then read the "Rest of the Story".... 

Anyway, I am old school. I love using DVOM's for testing components etc. but plain old wiring I like to use the above method or in very long circuits I will use a circuit breaker rated the same as the fuse and a short finder. Short finders are great for checking along door sills and any longer wire runs.

If you don't have one, I highly recommend a terminal test kit. They make life a lot easier.

They're especially nice for doing connector and component drag tests.

[rhdsts]

Thank you very much for responding OldCadTech!

Just to clarify, the first test I did (to establish if there is a short to ground) was to, in effect, bridge (if that is the right term) relay terminals 85 and 86. That is, putting the test light between the female connectors on the plug that the relay plugs into (with the relay removed, ignition 'on', engine off).

So because the test light lit up red, it suggests a short to ground.

When I did the short to ground tests, with DMM and test light, it was to the same female terminals at the connector. Of course, this time with the ign switched off.

You have given me a very simple test to do that I didn't really think of doing - makes a lot of sense - thank-you! EDIT: The relay-to-pump wire that is. The relay-to-switch I had thought to do but hadn't yet got my head around the procedure. You explained it very well.

By the way, this car was aone-owner 44,000km example when it came from Japan 3 years ago (now in Australia). In all my examination of wiring so far (including this particular relay connection) the wires and terminals look like new. That doesn't mean there isn't a corrosion issue at, for example, the underhood fuse block, as you said. 

I'll do the short to ground tests and let you know how it goes.

[OldCadTech]

The key on engine off test gave you a false positive. Do all the tests (key off engine off).

Testing 85 to 86 with the test light simply completed the circuit with key on.

Use the battery pos and neg terminals that way you know you have a good ground and battery pos voltage.

[rockfangd]

Ok is there a plug to the pump?

should have 2 wires. 1 ground (likely black) and one hot(+)wire (purple, pink, or red likely) 

If it has a plug unplug it. Then put your test light clamp on the positive battery terminal. 

Then touch the positive pin to the pump. It should not light up.

In my experience that does not mean that the pump is bad as some motors will show ground on both wires until voltage is supplied to the power wire.

If you want to know if the pump motor is indeed shorted you could supply a fused power to the pump itself and see if it pops the fuse, attempts to spin, or makes any noise.

I would typically remove the relay and supply the fused power to the output pin of the relay only. I actually recommend that you disconnect the positive battery terminal when you do this to prevent damage to other components. 

I would say likely your motor is the problem. They are common and when failing cause excessive resistance in the circuit which is why the fuse blows.

could you post a picture of the pump motor 

[Logan]

I was curious about the export pump....I found a couple of pics on Google. Search 'GM 25638963'.

[OldCadTech]

I'm not familiar with it either, other than I know the exports had it. I've never actually seen one but the pic you posted looks like they used parts from the Bosch ABS unit. 

Not even sure why they felt it was needed. Only thing I can think of is if the engine dies they wanted more / longer assist before the brake pedal got really hard....

[rockfangd]

I deal with those pumps as buses that have hydraulic brakes  have them. Just a bigger version and 2 pumps rather than one. One is for the front and the other is for the rear.

the ones on the buses are Common to failure mainly due to the location of them.

they actually give assist when the key is on without the engine running. 

Typically they do a pump test on KOEO then you may hear it  engage when depressing the brake pedal

[OldCadTech]

So that would be a pump test, Key on Engine off, pump should run?

[Logan]

Took an hour to find.....RHD STS car for sale in the UK. 

Master cylinder right side....electric booster over on left side....Even the beauty cover is different.

https://www.gumtree.com/p/cadillac/cadillac-sts-4.6-v8-300bhp-full-spec-295ftlb-torque-cream-leather/1279491631

This car is for sale at 3750 British pounds ($5000US)....The RHD cars were sold in Japan, South Africa and UK.

Update....curious about 2nd reservoir....looks like first one is filled to the rim......also looks like the air pump relay mounted to 2nd one..

 

[OldCadTech]

Looks like the pump and accumulator is mounted on the left strut tower and an additional reservoir is mounted to the right of the M/cylinder.

I'd HATE to do a rear exhaust manifold on that thing.... Looks like plugs would be bad enough.

[rhdsts]

Hello all, I've had a bunch of stuff going on here so had to put aside the brake booster trouble-shooting for a while. 

I did have time to try OldCadTech's suggestion re: test for short to ground for the pump switch. That wire is not shorting (test light lit up with other end of circuit grounded to chassis, then went out when the ground wire was disconnected.

I have a replacement pump now and I am sooo tempted to just install it. But I will hold off, testing the wire from the relay to the pump first. Then also do tests rockfangd suggested.

Just to note that when I replaced the fuse and went for a drive around the block, the pump did run (I usually can hear it prime for about 5sec when ign turned to on. So with a new fuse it did this as per normal).

So the first run around the block the brakes worked fine. The second time around the block, the brakes lost power assist again. Checked when I got home and sure enough the fuse had blown again.

I think Cadillac used this system because they wanted to have the master cylinder on the driver's side. Some manufacturers when engineering RHD versions of LHD cars use a cross beam from the RHD pedal over to the master cylinder on the left side.

Because the transmission is on the left side, there is plenty of room above it for the traditional vacuum booster and master cylinder for LHD cars. For RHD cars, I don't think there would be room there on the right side, above the engine, for a vacuum booster.

[OldCadTech]

Refresh my memory - which fuse is blowing the 25A or the 10A

[rhdsts]

The 10A fuse that protects the pressure switch circuit (the MAF and EVAP purge valve are on the same fused power circuit). There is a chance that it's the MAF or EVAP circuits shorting out. I'll rule out the brake booster first. The booster is known to fail, but I have not heard of the failure causing a blown fuse for the pressure switch circuit.

[OldCadTech]

Neither have I..

Test the pink wire. If it tests okay, do the pump test that @rockfangd suggested. He knows these systems very well.

As an experiment I would put in a 10A circuit breaker and see what happens or try a 15A fuse and see if that lasts any longer. The switch side of the relay ( can't remember if you replaced the relay) might be faulty. You could swap another relay of the same part number and see what happens also.

 

[Logan]

Brake Booster Pump Description and Operation (Right Drive) 
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.

 

 

So it sounds like the system turns on/off as needed. It does not continuously run when the car is running. 

So is there a minor misprint? I think the booster pressure switch is closed until the the pressure reaches a certain point....opens....and turns off? So the pressure switch is NC (normally closed)?

Not really what the diagram shows. 

I think the blue wire should show ground all the time.....until the pressure builds up enough to open the switch.....turning off the pump. 

 

[OldCadTech]

If the diaphragm vacuum switch  is faulty it could be causing the pump to run too long, overheating the pump motor, then goes the fuse...

[Logan]

Possible service manual misprint. Pump will never turn on unless blue wire is ground. 

Service manual step #5 says check brake booster sensor signal wire (blue) for short to ground.....umm....it has to be ground for the pump to turn on. 

Service manual schematic shows pressure switch as 'normally open'.....should be 'normally closed'.

[OldCadTech]

The way I'm reading the circuit, the Booster vacuum switch is normally open, when vacuum falls to the minimum the switch closes which energizes the relay coil, which pulls the relay switch closed and activates the pump.

Which to me raises the question(s) why is the vacuum booster losing enough vacuum to activate the pump, OR, why is the accumulator losing enough pressure to demand the pump to run?

[rhdsts]

Okay so I tested the relay-to-motor (large pink) wire - the test light lit up when I had earthed-to-chassis  one end of the wire, and went out when I removed the earth. So it looks okay.

I then reconnected the motor plug and fitted a 10A fuse between the pink wire (at the relay connector) and 12v power. The motor ran - I gave it about 5-6sec and stopped because I was worried about building up too much pressure in the system (given that do this test, I had by-passed the pressure switch).

Then I reconnected the pressure switch connector to the pressure switch. Then I fitted one blade of the 10A fuse to the dark blue wire at the relay connector. To the other blade of the 10A fuse I applied 12v power. Wow, talk about a spark! The fuse blew straight away.

So am I right in thinking that there is a short to ground within the booster pump assy, somewhere in either the pressure switch or between the pressure switch and the motor?

[rhdsts]

33 minutes ago, OldCadTech said:

The way I'm reading the circuit, the Booster vacuum switch is normally open, when vacuum falls to the minimum the switch closes which energizes the relay coil, which pulls the relay switch closed and activates the pump.

Which to me raises the question(s) why is the vacuum booster losing enough vacuum to activate the pump, OR, why is the accumulator losing enough pressure to demand the pump to run?

The accumulator is new (six months ago) as the old one was losing pressure. (symptom was that the brake pedal went to the floor when car had not been used for a few hours, then would resume normal operation once ign on and pressure restored).

You can hear the pump running inside the car. It only seems to run on start-up (for about 5sec) and then occasionally I think I hear it when driving (it is a lot harder to hear it with the engine running).

[Logan]

I don't think it's a vacuum pump.....service manual description is 'supplies the hydraulic pressure'.

 

"Brake Booster Pump Description and Operation (Right Drive) 
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."

 

[rhdsts]

5 minutes ago, Logan said:

I don't think it's a vacuum pump.....service manual description is 'supplies the hydraulic pressure'.

 

"Brake Booster Pump Description and Operation (Right Drive) 
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."

 

I have been wondering that myself. I think it produces hydraulic pressure, stored in the accumulator. I'm not sure really, on a big learning curve with this. Anyway, I think I have found the problem, as described above. Unless someone can tell me it's impossible for the pressure switch circuit to have a short to ground, I'm going to replace the pump.

[Logan]

Been looking at GM manual......did find info.....it's not vacuum......it's pressure.

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

Back to the original problem you had while testing. It appears the service manual is misprinted. For that pump to run at start up.....that pressure switch must be closed to ground. I think that is why you were getting the goofy readings on the blue wire.....and the service manual was saying you had short to ground. 

Adding to the confusion....the service manual schematic shows it incorrectly as a 'brake booster vacuum sensor signal'.....should be 'pressure sensor signal'.