19 replies to this topic

[NLinPEN]

Hello all, my first post here. I could not find a similar topic in the search so I try a new topic.

 

My car has SPi and automatic gearbox. The issue I'm facing is that when I start the car and then push the brake pedal the engine dies. I have tried a few things but those did not help to solve the issue.

(1) I replaced the four vacuum hoses as a previous owner had installed ordinary rubber hoses (which I didn't like). I discovered that the fuel trap was clogged, so I installed a new one. This did not solve the issue.

(2) The vacuum tube from the manifold to the brake servo looks very old and wasn't flexible at all. I replaced this, plus the inline non-return valve. This did not solve the issue.

Then I ran an experiment to see whether the brake servo is leaking: I disconnected the vacuum hose from the manifold to the brake servo. And temporarily plugged the connector at the manifold to avoid a vacuum leak here. The behavior was unchanged, so my conclusion is that it is not the brake servo that's causing this.

I now don't know what else I should check.

 

This is what I am observing: I start the car with the gearbox in P and the handbrake on, without pushing the brake pedal. The car revs normally (initially approx. 1000 rpm, after a couple of seconds approx. 850). I then push the foot brake and the engine stalls immediately. I tried to give it a bit throttle (e.g. 1,500 ~ 2,000 rpm) and then push the brake: the same happens, the engine stalls. Stalling is not limited to low (idle) rpm only.

I am not sure whether this problem is specific to SPi cars, or that other Mini's have it too. As a starter have I posted it in this SPi forum section.

 

Are there any suggestions out there? What could I investigate to find the cause of this problem?

[weef]

You appear to have covered the brake servo side of things and any related vacuum leaks by plugging and replacing the appropriate pipework.

If you are sure there are no vacuum related faults I can only suggest that there may be some electrical problem.

I would first try starting the car as normal, pressing no pedals, if all is fine stop the engine, press the brake pedal and try to start the engine. Depending on what the result of this test is i.e. if the engine starts or not depends on how you move forward. If it starts I will need to give things more thought. If the engine does not start , take the wire off the brake light switch, start the engine again and press the brake pedal to see if it makes any difference.Try this and see what results, if it makes no difference and the engine still stops I will have to give your problem more thought.

Another thing to check is to make sure there are no wires being trapped under the dash  when the brake pedal is pressed.

[coopertaz]

first thought was also servo diphragm but you seem to have ruled this out above, so check around brake pedal and linkage in engine bay to see if it fouls anything when operated

[NLinPEN]

Thank you both for your replies. Today I continued my search. It is indeed pointing in the direction of an electrical failure. I disconnected both wires from the brake light switch (at the brake pedal). Now I can push the brake pedal and the engine won't stall: it keeps on running without the slightest difference in engine sound or revs. However, if I don't push the brake pedal but connect the two wires from the switch together the engine immediately stalls. This gave me the impression that there might be a heavy load on the battery, or a short circuit. I removed both light bulbs from the rear. Now I can connect both wires and the engine won't stall. The (aftermarket) third brake light is still connected and works during this test. This confirms that there is no short circuit in the wiring in the car. Both light bulbs seem to be fine, nevertheless I exchanged them for some spare GLB380 light bulbs. Even with only one light bulb installed the engine stalls. Then I recalled that I have done a lot of testing over the last couple of weeks, started the car many times over, and might have drained the battery. I have put it on a charger and will check tomorrow whether that made any difference. If so, that would make me wonder whether the ECU measures the battery voltage? Does the ECU switch off in case of a too low battery voltage? This car is relatively new to me, and I don't know exactly how the ECU performs.

[weef]

If the engine does not stall when the bulbs are out  points to an earth fault, do both brake lights actually work ?.

The left hand rear lamp cluster shares its earthing point with the fuel pump, so it may be the pump is somehow loosing its earth causing the pump to stop and so the engine.

I could be completely wrong but I would try running a segregated temporary good earth connection to the pump and see what happens when the brake is pressed with the bulbs fitted in the lampholders.

If this solves your problem check all the earths associated with the rear lamp clusters.

 

[68+86auto]

This confirms that there is no short circuit in the wiring in the car.

 

No it doesn't. When you remove the bulbs you've isolated the earth side and any holder issues. It's possibly a poor earth in the boot or something like that. A poor earth could explain your cutting out, it's not a good enough connection to power the lights and fuel pump at the same time.

[NLinPEN]

Today I did some more testing and experimenting. I made some progress but haven't determined the root cause yet. Charging the battery made no difference: the engine stalled when I connected both brake switch wires together. Without the light bulbs installed the car does not stall if the wires are connected. What I forgot to mention in my previous post is that I had determined that the tail light works properly, without causing any incident. The tail light and the brake light are two filaments combined in the same light bulb and share their ground connection. This means that the ground connection from the bulbs to the battery's negative pole is okay. To be sure about this I also measured the voltage drop across this ground connection, which is very small.

So I concluded that there is no short circuit in the wire that goes from the brake switch to both bulbs. I confirmed this also by measuring resistance to ground (result: no connection). And measured the voltage: apply battery voltage at the wire on the switch end, then measure the voltage at the connecting pin in the light fitting. If there were a short circuit this battery voltage would go to zero, which it doesn't.

This made me focus on the fittings and how the light bulbs are mounted in the fittings. I started playing with wiggling the bulbs in their fitting, and repeatedly taking them in and out of their fitting. I think there is something wrong in both fittings: sometimes it results in a short circuit and the car would stall when the brake switch is closed, sometimes there would not be a short circuit, the bulbs would light up when the brake switch wires are connected, and the engine would not stall in this case.

I have attached two photos of the LH rear lamp (RH side looks identical): one of the whole thing, and one closeup of the fitting for the brake&tail light. It looks like that the contacts for the filaments are made with a pieces of PCB (printed circuit board) material. But I see some cracks on the side. I don't know whether this is an original construction, or that a previous owner has made some modifications. I wonder whether wiggling the light bulb could cause a short circuit to ground on the back side of this PCB.

Attached Files

•  LhRear.jpeg   42.03K   1 downloads
•  Closeup.jpeg   56.84K   1 downloads

[viz139]

If the problem is the rear earth the engine should cut out when you operate reverse lights, hazards or heated rear screen , may not be faulty brake circuit. Does the rev counter slow down or drop dead?

[NLinPEN]

Does the rev counter slow down or drop dead?

Drop dead. If the brake light bulb causes a short in the fitting it is immediate: the battery voltage is shorted to ground when the brake switch gets engaged and the engine immediately switches off.

[bpirie1000]

I would suspect the brake sensor which is located on the pedal arm. White box with a button. (Lucas part number SMB421)

 

These Have been an issue for me previously. 

 

 

Do the brake lights work fine with the ignigtion on and the engine running? 

[Ethel]

See if any other circuits cause the same effect, hazards, wipers...

 

You could also rig up an alternate supply to the brake switch. If that makes a difference it may be worth looking at a wiring diiagram to see if the brake circuit shares a supply with something on the engine engine management. Then see if that bit has any poor connections.

[viz139]

If the problem was fuel pump the revcounter would drop slowly  but the fact it is dropping dead means ignition. I think you may find a short between  the  brake light wire green/purple and rev counter white/black  somewhere behind the dash.

[NLinPEN]

I am going to mark this topic as "solved" although I'm not fully satisfied. I found a workaround that makes the car run again. This will please my wife as she is now able to use the car again. Why am I not satisfied? Because I have not found the real root cause of the issue.

In my previous messages I reached a conclusion that there must be some short circuit to ground between the brake light switch's green&purple wire and the lamps in case the bulbs are installed. Without any bulb installed the voltage at the rear end of the car reaches battery level. But if at least one bulb is installed that voltage goes to zero, and at the same time the engine stalls. So I tried to find the root of the short circuit. I could not find any. Also the light bulb fittings are good, do not cause a short circuit when any light bulb is installed.

I have now replaced the tail&brake light bulbs with LEDs: the problem has disappeared. The LEDs light up clearly when the brake is applied and the engine does not stall. The voltage remains high, at battery level. I am aware that LEDs draw a smaller current than conventional light bulbs. With LEDs installed my wife can drive her car again. An additional benefit is that a lower current will flow through the brake light switch (one 21 Watt bulb draws about 1.6 Amps at 13 Volts). I hope that will help its longevity.

My conclusion therefor is that there is no short circuit "downstream" from the brake light switch to the rear lamp assembly and its ground connection. What I suspect is that there is a bad connection "upstream" between the battery +ve terminal and the green wire that supplies power to the brake light switch. A bad connection leads to higher series resistance and thus to a larger voltage drop if the current gets high. I have seen from the schematic diagram that there are a few connectors in between. But I don't know how the wires are actually routed in the car, and therefore in which area of the car to look to inspect these. I did verify that all fuses are okay, both in the passenger compartment as well as in the engine compartment.

[weef]

MMMM solved ???, kind of think it will only be solved once the fire brigade have paid a visit.

From reading your post you seem to have a decent approach to fixing things, so don't give in.

Use a temporary "jump wire" with an ammeter in circuit to see if you can pinpoint the fault.

Firstly, all standard bulbs fitted, disconnect the brake light wire feed at both rear lamp clusters , then running the jump wire from the brake light circuit supply fuse directly to the rear lamps, check that the current drawn is as expected and the lights operate as design and if the engine runs. 

If all fine, reinstate the rear lamp clusters to normal, and connect the jump wire to the wire that goes from the brake switch to the rear lamps and check the current draw is the same as the previous test and the light operate and the engine runs.

Again if all is as expected, at the fuse that supplies the brake light circuit, C6 I think, remove the wire that feeds the brake light switch, short together the wires at the switch completing the circuit to the rear lamps, connect the jump wire between the fuse and the switch feed wire and again check the current etc.

Breaking down the circuit into these bite size pieces may help you detect where your fault lies, and repair as required.

[Ethel]

I suspect there's some resistance upstream of the brake switch and it's creating a voltage divider when you operate the brakes.

 

 

Looking at a wiring diagram there's nothing on the same feed, via the top fuse in the fuse box (white to green). Tracing it back through the ignition switch (first suspect) there's a circuit via a line fuse that feeds the instruments: Ignition & oil lights plus voltage regulator. There might be an earth short, but you'd expect the lights & gauges to show something too. Another branch off that line fuse operates the fuel pump relay, the emissions purge valve (switched to ground at ECU) and connects to pin  11 on the ECU, presumably just as an ignition on signal - that would be my 2nd suspect, including everything back to the line fuse. You could bridge the fuel pump relay terminals to eliminate that - then swap in another relay to test for relay or wiring if required.

 

...and,

 

The reverse lights do share the brake light supply & have the same load, so they ought to cause the same fault.

 

...and, and,

 

I guess it's possible a short on the reverse light (also heated rear screen) circuit could be added to by the brake lights to steal just enough the voltage off the engine electrics.