Blown ammeter shunt - now won't start. And must-read safety tip on brakes!!!! - Rolls-Royce and Bentley Forums
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post #1 of 17 (permalink) Old 03-01-2016, 01:24 PM Thread Starter
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Blown ammeter shunt - now won't start. And must-read safety tip on brakes!!!!

I had a (near) total electrical failure in my '76 Corniche the other day: the engine, lights and steering died on me whilst driving - this was quite scary. Fortunately, I was just slowing down on approach to a lighted village and there was a wide low pavement for me to pull safely onto. The RAC mechanic diagnosed a blown ammeter shunt and, unable to fix it, trailered me home (see VERY IMPORTANT note at bottom!!!!).

The salesman at Flying Spares said that ammeter shunts get frazzled when owners jump start their car or attach booster packs. (He suggested always turning the headlamps on when jumping RRs.)

Now, for all the years I've owned this car, the headlamps have always had a slight flicker and the ammeter has always had a small but noticeable waver. It would appear that the connector between the heavy-duty brown and white wire and the ammeter shunt inside the car got frazzled at some point and has been arcing ever since - finally blowing on me the other day. That is my reading of the situation.

Anyway, I have replaced the ammeter shunt but now nothing happens when I turn on the ignition switch! The battery is charged, the interior and exterior lights work, the battery isolator is switched on, and I can hear the fuel pump working. I have checked all the fuses in the cabin but can find nothing amiss. i have searched this forum, but can't identify anything specific to this. I'm just wondering if there's some other 'blowable' component that needs to be checked?

Any thoughts will be most welcome.

IMPORTANT SAFETY TIP
When I pulled the car up, the brakes were working fine. I left the car pointing slightly downhill so that i could roll onto a trailer. When the RAC guy turned up he told me to cover the foot brake when I took off the handbrake. I did, but it made no difference. The car had no brakes at all and despite standing on the footbrake so hard I gave myself cramp, the car continued rolling!!!
I thought that the residual pressure in the accumulators would be more than enough to stop me, but there was nothing. fortunately, I just drove up the slope of the trailer and stopped, but I was shocked at how quickly this could have got out of control. Please be aware of brake failure when you have an engine problem and be ready with the handbrake and, in extreme situations, taking the car out of neutral and selecting 'park'!

1976 Bentley 1A Corniche (Rolls Royce conversion)
CRH21998
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post #2 of 17 (permalink) Old 03-01-2016, 02:29 PM
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Hi Stumbler
I think there should be 2-4 wires on one side and 2 the other (of the shunt) ,are they all on and no insualtion trapped between them?or knocked off another.
Supprised how quickly you lost brakes I thought even with quite depleted accumulators you would get 20+ applications, I think you need a hydraulic service!
And putting an auto in "P" while moving just chews up thr pawl and won't stop you, do the quick accumulator check when you get it running!!!
good luck
Mike

chassis 27650

Last edited by mikebentleyturbo2; 03-01-2016 at 02:33 PM.
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post #3 of 17 (permalink) Old 03-01-2016, 05:05 PM
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The brake issue is definitely very badly depleted accumulators, malfunctioning ACV, or a combination of the two. You should have plenty of applications of the brakes in the event of a stall. The accumulators in many cars will (even when fully charged) lose their accumulated pressure over fairly long periods of time (days to weeks, at a minimum) as the seal of the ACV may not be perfect and a very slow depressurization with fluid returning to the reservoir can occur.

You should be getting a brake pressure (or two brake pressure) warning light(s) if your system is so low that braking system pressure is either way below usual or non-existent.

Brian

The opposite of a correct statement is a false statement. But the opposite of a profound truth may well be another profound truth.
~ Niels Bohr


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post #4 of 17 (permalink) Old 03-01-2016, 10:31 PM
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Hi Stumbler

When you say nothing happens when you turn the ignition switch, do you mean absolutely nothing i.e. do the oil and generator lights illuminate. If you turn the key fully, does the engine turn on the starter or is it all completely dead i.e. no lights - completely dead?

Geoff

PS What is a Rolls Royce conversion - just curious.
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post #5 of 17 (permalink) Old 03-01-2016, 11:52 PM
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Quote:
Originally Posted by Dounraey View Post
What is a Rolls Royce conversion - just curious.
Geoff,

I'm actually surprised you've not heard of this practice. For many years it was not uncommon to find "Rolls-Royces" that had originally started out their lives as Bentleys but that had their grilles and other exterior badging swapped for what had been on a Rolls-Royce that had met it's demise.

Now the trend has actually reversed itself, to an extent, and there are "Bentleys" that left Crewe as Rolls-Royces. Since Stumbler's car has chassis number CRH21998 it definitely left Crewe as a Rolls-Royce. At some point someone stripped it of much or all of its Rolls-Royce exterior dress, at a minimum, and replaced it with Bentley dress.

Really dedicated converters try to carry off a complete transformation, but that's very hard to do in practice.

For obvious reasons, these cars do not sit well with the purists.

Brian

The opposite of a correct statement is a false statement. But the opposite of a profound truth may well be another profound truth.
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post #6 of 17 (permalink) Old 03-02-2016, 12:39 AM
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Brian

Thanks for the explanation.

Regards

Geoff
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post #7 of 17 (permalink) Old 03-02-2016, 08:07 AM Thread Starter
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Many thanks for the replies. I've had another look and the situation is as follows:

First, all the wires are correctly fitted to the new ammeter shunt. The headlamps, windows and interior lights work with the ignition switched off. When I turn the key to the RUN position, the generator and oil light come on, and the both brake warning lights on the emergency lighting panel also light up. Nothing more happens when I turn the switch to the START position. The gear selector is in PARK and I can hear it clicking under the bonnet when I move it through the gate.

I'm not sure what other info I can offer at this stage to help with a diagnosis...

Stumbler

Regarding the brakes: no one was more surprised than me to discover there was no pressure left in the accumulators! The car has just been with a very competent RR technician and we specifically checked the accumulators and they were fine. I would normally expect there to be enough pressure for at least 20 brake pedal pushes - I think we tested it to 38.

1976 Bentley 1A Corniche (Rolls Royce conversion)
CRH21998
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post #8 of 17 (permalink) Old 03-02-2016, 11:06 AM
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Prior to the current state of the car, would you please describe what starting the car has been like over say, the last year? Has starting behavior changed in any way? Were there times where you'd turn the key and hear the click of the starter solenoid but the car didn't crank, but if you tried again it would? Has moving the gear selector from Park to Neutral had any influence on whether the starter will crank?

This would be a good starting point, as would checking the starter relay itself to see if it may have gone bad.

Brian

The opposite of a correct statement is a false statement. But the opposite of a profound truth may well be another profound truth.
~ Niels Bohr


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post #9 of 17 (permalink) Old 03-02-2016, 11:11 AM
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Quote:
Originally Posted by Stumbler View Post
Regarding the brakes: no one was more surprised than me to discover there was no pressure left in the accumulators! The car has just been with a very competent RR technician and we specifically checked the accumulators and they were fine. I would normally expect there to be enough pressure for at least 20 brake pedal pushes - I think we tested it to 38.
Then, once the car can be started and the system properly pressurized, you need to start doing some testing to see how quickly, without any application of the brakes or moving of the car, your brake system lights come on.

If the accumulators themselves are even in the OK range (and 38 pedal presses is still OK, not spectacular, but OK) the situation you describe should not occur unless the pressure is being released from the accumulators via an internal leak in the system. Pay attention to how quickly the fluid levels rise in the sight glasses to well above the normal maximum mark, which they will as fluid is pushed out of the accumulators, regardless of which route it's taking to get back to the reservoir. You have got to have an internal leak where pressure is being allowed to rapidly bleed off of the accumulators in order for the situation you found yourself in to have occurred.

Brian

The opposite of a correct statement is a false statement. But the opposite of a profound truth may well be another profound truth.
~ Niels Bohr


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post #10 of 17 (permalink) Old 03-02-2016, 11:24 AM
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Hi Stumbler

The clicking starter motor is a very common fault on these cars. It is possible the ammeter shunt is unrelated to this new problem. The fault can be caused by anything between the battery and the starter motor. Usually it is caused by a high resistance in the (40 year old) wiring, but can be simply the starter motor needs replacing.

Try starting the engine many times. Sometimes the engine will suddenly start to crank. Also, with the ignition on try moving the gear selector to it's other positions and then back to park. A faulty connection here may be causing the problem, as the ignition is wired through the selector mechanism to prevent it from cranking in the D or R positions. Also, if you still have access to the shunt connectors, feel them to see if they are getting hot.

Geoff

Last edited by Dounraey; 03-02-2016 at 11:27 AM.
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