Thought it might be worth putting this info under a simpler to find heading, as given the usual thread drift tendencies that which may be thought to be valuable information in the case of a failure the info on pyro fuses is buried in a thread under a non related heading.
Apparently without anyone noticing a pyro fuse has been introduced to the CX Range of Morgans which can isolate the battery from the electrical system, and is designed to do so in the case of an accident.
Might be worth carrying a spare same spec Pyro fuse, more so if heading out on a European trip, and of course it would be handy to know where it might be located...?
As for cost, if anyone knows the part number on the fuse, it may well be a standard BMW part on Morgans with BMW related control systems and as such may be both less expensive or more easily acquired through BMW related sources, as opposed to trying to buy one from the MCC..?
Originally posted by John Harris :-
I thought I might tag this posting from DevaMog member on their forum....Worth checking the battery leads.
Mon 07:23
Msg from Tony Gibson Devamog
160826
If you have a CX model, check the tightness of your battery leads under the rubber covers.
We refuelled at petrol station 3 miles from home. Car lumpy , tried to turn over then all electrics went off. Called a friend out to jump it. Car didn’t register the new charge. The dash clock stopped at 4.10. Even that should run on fine current. So, call the RAC. Sunday in August with petrol station not happy with me blocking the forecourt. 😀🥸. Friend took Jo to our home. I waited ………. RAC diagnosed an extremely slightly loose battery lead to battery connection which couldn’t supply sufficient current to the starter resulting in the 300 amp main fuse blowing!!!! He tested the battery which was spot on. He has a 150 amp fuss on his car which he fitted and the car fired up immediately. I’ll fit a new 300smp tomorrow. Take 2 minutes to check yours!!!! No one got hurt. Otherwise a top day out .
Followed up by Luddite :-
John, I am somewhat confused re your post above.... I must admit that I am in no way up to date on vehicle electric systems though have heard of more modern vehicles having items described as pyro "fuses" which are of high enough amperage to carry the current of everything in the electrical system including the starter motor... I think the idea is that in a crash the "fuse" disconnects the battery from the rest of the electrical system reducing the risk of fire....? I have no idea whether the idea of high current "fuses" have become part of the of construction and use regs..?
In my limited electrical experience I have not come across a fuse that blew as the result of a loose battery connection, a loose connection usually creates the generation of heat under load causing signs or the aroma of burning, thus unlikely to "blow"the element within a fuse...? However using nothing more than perhaps flawed logic, I am guessing a pyro fuse may have a third low current input connection from an ECU which effectively acts like a relay control in that a small switched current can control a much higher current switching device, and in the case of a pyro fuse, instead of switching the device on, it switches the high current off as a one time operation...?
If my logic might be in any way reasonably accurate, then I do wonder if the loose battery connection, caused random fault codes to e generated and in this case provided a spurious output to that which may have been a pyro fuse...?
I really look forward to be proven to have been entirely incorrect..
A couple of other posts again followed up by Luddite :-
I went off to the interweb in the hope of finding some info on pyro fuse functionality and found this one which is rated for EV usage, in terms of it`s higher current capacity circa 2k amps as opposed to the 300 amps suggested as fitted to the Morgan in question... The pyro element of the name seems likely related to the explosive charge.
Now whether the fuse fitted to the Morgan in question was a pyro fuse or not, I have no idea, though it would be interesting to find out if it was, and where it might be located for those owners of similarly equipped Morgans... If the 300 amp fuse is other than of the pyro variety, I am left to wonder quite how it might have blown and quite how a loose battery terminal could cause it so to do...?
Hopefully the link to the very short vid below may help provide a degree of understanding as to the mechanical operation of the "fuse"
https://www.youtube.com/shorts/vkGsjETxisE?feature=shareAfter which John consulted A. I. and posted the following :-
AI explaination
A pyro 300 fuse (pyrotechnic safety fuse) is fitted on modern CX-generation Morgan Plus Four models.
Its Core Purpose
The pyro fuse serves as a critical emergency safety disconnect designed to minimize fire risks during a severe accident. It works through an integration with the car's crash-sensing systems:
Instant Power Cut:
Upon a collision severe enough to deploy the airbags, the Engine Control Unit (ECU) instantly sends an electrical signal to the fuse
.Explosive Disconnect:
The fuse contains a tiny, controlled pyrotechnic (explosive) charge. When triggered, it physically blows apart the main heavy-duty circuit connection inside the car within milliseconds.Isolates
Major Electrical Lines:
It completely cuts the primary electrical power flowing from the battery to high-draw systems like the starter motor and alternator.
Fire and Restart Prevention:
By instantly cutting the power, it eliminates the risk of short-circuits sparking near ruptured fuel lines and prevents the vehicle from being restarted after a crash.Important
Ownership NotesNon-Resettable:
Because it uses a physical explosive charge, once a pyro fuse is deployed, it is completely destroyed and must be replaced entirely to restore electrical power to the vehicle.
No Crank Condition:
If a CX Plus Four has been involved in an impact and has completely dead electronics or refuses to crank despite a healthy battery, a blown pyro fuse is usually the culprit.
BMW Architecture Integration:
Because the modern CX-platform Morgans rely heavily on BMW electronic architecture and powertrains, they utilize these modern, BMW-standard safety components rather than the traditional, simple glass or blade fuses found in older classic Morgans.
The pyro fuse on a Morgan CX Plus Four will fail or trigger for two reasons: it was intentionally fired by the car's safety computer, or it suffered a physical or electrical hardware failure.
Here is the exhaustive breakdown of exactly why this happens.
1. Intentional Safety Triggers (The System Worked)
In these cases, the car's computer actively tells the fuse to blow
.A Severe Accident:
This is the primary design reason. If the car undergoes a severe impact, the airbag control module instantly fires the fuse to isolate the battery.
Minor Jolts or Curb Strikes:
The crash sensors cannot always tell the difference between a fender-bender and a harsh mechanical shock. Hitting a massive pothole, striking a high curb hard, or a severe underside jolt can accidentally satisfy the sensor's threshold, triggering the fuse without deploying the airbags
.Electrical Short Circuits:
If a major power cable (like the main line feeding the starter) chaffes against the aluminum chassis and causes a massive, dangerous current spike, the electrical system may force a shutdown.
2. Hardware and Environmental Failures
Because the fuse is an electrical component, it can stop working or trip accidentally due to age, wear, or installation issues.
Loose Chassis Earth (Ground) Connections:
Morgan CX models are known to occasionally suffer from loose earth leads where the battery connects to the alloy chassis panels. Bad grounding causes massive voltage fluctuations that can confuse the system or cause localized heat.
Moisture and Corrosion:
The lightweight Morgan chassis exposes various components to the elements more than a standard enclosed steel sedan. Moisture seeping into the battery junction area can corrode the delicate signal pins that connect the fuse to the computer, causing a false trip or a broken internal connection
.Manufacturing Defects:
The physical explosive charge or the internal metal bridge can simply fail internally over time due to heat cycling and vibration, leading to an open circuit with zero external trigger.
Battery Replacement Spikes:
If a battery is changed incorrectly, or if the car is jump-started with backwards polarity, the sudden electrical surge can accidentally pop the trigger mechanism.
How to Check ItIf you suspect the fuse has deployed or failed, look for these three signs:
The "Loose Cable" Test:
On these BMW-style safety terminals, the thick power cable will often feel physically loose or detached from its plastic housing after firing because the internal piston pushed it out.
Perfect Dash, No Crank:
Often, the smaller wire powering the dashboard remains live, but the heavy wire to the starter is dead. Your screens might light up, but turning the key results in absolute silence.
Diagnostic Codes:
Plugging a scanner into the OBD port will usually reveal a fault code explicitly stating "Battery Safety Terminal - Resistance Too High" or "Open Circuit."If you need to fix a dead car,
I know understand a little more as to the pyro fuse function and likely reasons for a trigger.,even a jolt to the car could cause it to trigger, let alone a loose battery cable electrical spike..........or an earthing issue.
