Archive for category: V8 cars

RELAYS AND OTHER RELATED ISSUES

INTRODUCTION

 Recently, it was reported on various Bristol Forums, that owners of at least one model, the 411, were experiencing Ignition Switch failures. Initially, these parts were replaced, only to have a similar issue a short time later. Understandably, owners were less than pleased after the replacement failed again, and this article attempts to illustrate the reasons why, and to recommend a remedy, that is well proven.

 

Having been employed at a leading company in the Motor Industry in Coventry for over thirty years, reliability testing was a major part of my engineering activities. This involved the logging of prototype vehicle test data to prove out the various electrical components. The information thus gathered then gave an indication of the vehicle lifetime expectancy of each item, and this could be used for warranty investigations later on, if required.

Major electrical loads were monitored during the tests, including air conditioning, cooling fans, external lighting, heated rear screen, heated front screen, electric windows, seat heaters, and in-car entertainment ( radio/ amplifiers, TV etc.). Data from component suppliers was also analysed, to ensure that, for example, the ignition switch, was being operated under electrical load conditions appropriate to the supply company’s recommendations. Also, back emf (the spark that appears across the contacts) at switch-off, did not exceed the limit outlined in DIN57-879. This Europe-wide specification relates to radio frequency interference by induced transients from inductors, ie, motors and solenoids, and requires sufficient suppression to be fitted to eliminate the effect.

The table below, illustrates some of the issues involved, with measurements taken before and after suppression components had been added, clearly demonstrating an improvement and compliance with the DIN regulation, (36V max @ 1mS duration).

 TABLE  A

Component	Transient  at switch-off  before suppression	Transient  at switch-off  with suppression
Seat motor	– 232v @ 1 mS duration	– 31v @ 0.5 mS duration
Cooling fan	-183v @  0.5 mS duration	– 32v @ 0.4 mS duration
Air Con fan (interior)	– 247v @ 0.4 mS duration	– 34v @ 0.35mS duration

mS = mille-second

Over time, the energy generated across relay contacts at switch-off from unsuppressed components, will burn away the contact faces, and lead to switch/relay failure. Adding the suppression improved the reliability of the circuit components, and thus reduced failures to a minimum. Good for customer satisfaction.

A similar approach was taken with external lighting, where values obtained for the switch-on current of filament lamps were also measured. These included sidelights, head and fog lamps. The following table gives some of the results that could be applied to the wiring systems of Bristol cars:

TABLE  B

Bulb	In-rush current at switch     ON – AMPS	Illuminated/rated current – AMPS
H4 – Dip beam 55 watts	45 peak @ 10mS	4.58 @12v nominal
H4 – Main beam 60 watts	50 peak @  10mS	5.00 @ 12v nominal
Indicator 21 watts	17.5 peak @  10mS	1.75 @ 12v nominal
Sidelight 5 watts	4.1 peak @ 10mS	0.41 @ 12v nominal

Also, the value of current that could be reliably passed by the ignition key switch was determined to be approximately 20 amperes. To switch more than this limit, several load relays were introduced to power up the various vehicle systems controlled by “Ignition ON “. These included headlamp flash, headlamps, fog lamps, heated rear/front screens, air conditioning, seat motors, In-Car entertainment systems, and fuel injection/engine management.  The circuitry included fusing for each relay controlled load, and reduced the amount of current directly switched by the ignition key. This reduced the probability of electrical failure of the ignition switch by a considerable margin. Also, the impact of Vehicle Construction and Use Regulations, and Federal (USA) NHTSA legislation, led to circuit evaluation for compliance. For example, all cigar lighters at that time, were required to be switched by the ignition key, and driving/ fog lamps if fitted, were to be switched in pairs, and not individually. These legislative requirements were subsequently incorporated into the circuitry.

BRISTOL SPECIFIC ISSUES

Ignition switch

This is sourced from a Range Rover application amongst others for the 411 model, but the following set of conditions leading to failure, would apply to most other vehicles produced by Bristol with similar wiring/circuitry. Excessive loading leading to early contact failure, especially with the cigar lighter sockets being used to power up various items such as games and DVD players for the rear seat passengers, combined with heated rear window current, plus the usual driving aids such as demisters, windscreen wipers, and cooling fans, will cause the ignition switch contacts to fail after a short time. This is exactly what happened previously.

The following re-routing of the circuits is applicable to most of the later cars from 408 onwards, as the wiring is terminated in the right hand wing locker, and the circuit colours can easily be identified and modified. Of course, when designing circuitry for a new vehicle, one relay would supply two or three additional loads, however, when modifying existing wiring, individual load relays are easier to include without a major strip out of the wiring looms.

See Appendix 1 for the wire colour designations, these follow the old Lucas wiring colours, but for some applications, owners should consult the vehicle handbook/workshop manual for specifics.

The following Key-In Ignition ON loads supplied directly from the ignition switch should be removed, and powered up via an additional relay

1)     Heated rear window;

2)     Electric windows;

3)     Air conditioning(where fitted);

4)     Cooling fans

5)     Cigar lighters (where applicable)

 

This then leaves the ignition circuits, wiper motor plus the radio, (without external/ additional amplifiers) at about 12 – 14 amps, that is well within the load capability of the key switch.  .

 

HEADLAMP FLASHER

 

This is a function of the indicator switch stalk. However, the contact rating is about 10 amps on my car (a 408 MkII), and looking at the current in-rush values for the main beam filaments in Table B, it can be seen that flashing four headlights results in a combined in-rush current of 190 amps for 10mS duration, after which the steady current is 19 amps, almost twice the rating of the switch. Also, the voltage at the lamps is reduced, and hence the light output, because the cross sectional area of the wiring to the flasher switch is too small. This is another circuit that requires a relay to switch the filament load.

COOLING FAN

The supply to this circuit should be removed from the ignition switch as previously discussed, and the following additional modification ought to be carried out. The cooling fan circuit utilises a thermal switch manufactured by Otter, situated in the radiator header tank. This switch controls the fan relay, and will fail if the relay coil is not suppressed. The back emf (spark), generated at switch off on an unsuppressed relay, is typically as high as 92 volts for 2mS duration. This appears across the thermal switch contacts that open relatively slowly when compared to similar relay contacts. The slow opening speed draws an arc across the air gap as the contacts move apart, and erodes the mating faces. Eventually, with carbon build up, the contacts when made will be of a high resistance, and the relay will fail to operate because of it. Therefore, a 0.1 microfarad capacitor should be wired in parallel with the relay windings to eliminate the issue. These suppressors are readily available from accessory suppliers

 

CONCLUSION

The only modifications that I have carried out to my own vehicle, a 408 MkII, are the suppressor modification on the cooling fan relay, a headlamp flash relay with a fuse in the power feed, fog lamps paired up with their own relay, and changed the starter relay for a plug-in type, as the original had burnt contacts. Additional fuses have been fitted in the headlamp circuit, the front fog lamp pair, and the rear fog lamp pair. As my vehicle is without a heated rear screen, the cooling fans remain wired through the key-switch as is, the load current is about 10 amperes. All of the applications discussed previously, can use relays sourced from Jaguar parts suppliers, See Appendix 2.( NB The cigar lighters on my vehicle are powered directly via a fuse from the battery, and therefore are not controlled by Ign-ON.)

When attempting to modify the circuitry, it is recommended that the cheaper alloy type of lucar spade terminations are not used, as electrolytic corrosion will ensue in damp conditions, owing to two dissimilar metals being brought together in the cable crimp, ie copper and aluminium. Always use either brass or tinned brass for these crimped terminations. Again, these are available from companies that advertise in classic car publications and the Internet.

Regarding the 411 ignition switch failures. These occur because the original circuit design did not take into account the maximum current that could be reliably switched by the contacts. Having the air conditioning on, with the wipers and demisters running, plus the heated rear screen and other accessories being powered from the cigar lighter socket(s), would probably produce a back emf at switch-off, that would easily erode the contact faces of the switch element in a relatively short time. Changing the switch again is only a short term solution, and the only effective method to eliminate premature failure, is to apply the circuit changes discussed in this article.

 

APPENDIX 1 – Wire colours

 

Many British designed vehicles use colour coded cables to assist in identifying the various circuits in use. This is an extract from BS-AU7a 1983 Colour Code for Vehicle Wiring, from the British Standards Institution, 2 Park St., London W1A 2BS.

(Note that these colour codes may not apply directly to older cars. For example, the wipers on cars up to 1980 (at least) are not on a separate fuse circuit, so they are not orange, but green. Check the schematic for your car to be certain.)

 

	TRACER	PURPOSE
Black		All earth connections
Black	Purple	Temperature switch to warning light
Black	Green	Relay to radiator fan motor
Black	White	Brake fluid level warning light to switch and handbrake switch, or radio to speakers
Black	Orange	Radiator fan motor to thermal switch

 

	TRACER	PURPOSE
Blue		Lighting switch (head) to dip switch
Blue	Brown	Headlamp relay to headlamp fuse
Blue	Red	Dip switch to headlamp dip beam fuse
		Fuse to right-hand dip headlamp
Blue	Light green	Headlamp wiper motor to headlamp wash pump motor
Blue	White	a) Dip switch to headlamp main beam fuse
		b)Headlamp flasher to main beam fuse
		c)Dip switch main beam warning light
		d)Dip switch to long-range driving light switch
Blue	Yellow	Long-range driving light switch to lamp
Blue	Black	Fuse to right-hand main headlamp
Blue	Pink	Fuse to left-hand dip headlamp
Blue	Slate	Headlamp main beam fuse to left-hand headlamp or inboard headlamps when independently fused
Blue	Orange	Fuse to right-hand dip headlamp

 

	TRACER	PURPOSE
Brown		Main battery lead
Brow	Blue	Control box (compensated voltage control only) to ignition switch and lighting switch (feed)
Brown	Red	Compression ignition starting aid to switch
		Main battery feed to double pole ignition switch
Brown	Purple	Alternator regulator feed
Brown	Green	Dynamo ‘F’ to control box ‘F’
MAIN	TRACER	PURPOSE
Brown	White	Ammeter to control box
		Ammeter to main alternator terminal
Brown	Yellow	Alternator to ‘no charge’ warning light
Brown	Black	Alternator battery sensing lead
Brown	Slate	Starter relay contact to starter solenoid

 

MAIN	TRACER	PURPOSE
Green		Accessories fused via ignition switch
Green	Brown	Switch to reverse lamp
Green	Red	Direction indicator switch to left-hand flasher lamps
Green	Purple	Stop lamp switch to stop lamps, or stop lamp switch to lamp failure unit
Green	Light green	Hazard flasher unit to hazard pilot lamp or lamp failure unit to stop lamp bulbs
Green	White	Direction indicator switch to right hand flasher lamps
Green	Yellow	Heater motor to switch single speed (or to ‘slow’ on tow- or three-speed motor)
Green	Black	Fuel gauge to fuel tank unit or changeover switch or voltage stabilizer to tank units
Green	Pink	Fuse to flasher unit
Green	Slate	a)Heater motor to switch (‘fast’ on two- or three-speed motor)
		b)Coolant level unit to warning light
Green	Orange	Low fuel level switch to warning light

 

Light green		Instrument voltage stabilizer to instruments
Light green	Brown	Flasher switch to flasher unit
Light green	Blue	a)Flasher switch to left-hand flasher warning light
		b)Coolant level sensor to control unit
		c)Test switch to coolant level control unit
Light green	Purple	Flasher unit to flasher warning light
Light green	Green	Start inhibitor relay to change speed switch; or switch to heater blower motor second speed on three-speed unit
Light green	Yellow	Flasher switch to right-hand warning light
Light green	Black	Front screen jet switch to screen jet motor

 

	TRACER	PURPOSE
Orange		Wiper circuits fused via ignition switch
Orange	Black	Switch to front screen wiper motor parking circuit
Orange	White	Timer or intermittent unit to motor parking circuit
Pink	white	Ballast terminal to ignition distributor

 

Purple						Accessories fed direct from battery via fuse
Purple			Brown			Horn fuse to horn relay when horn is fused separately
Purple			Blue			Fuse to heated rear window relay or switch and warning light
Purple			Red			Switches to map light, under bonnet light, glove box light and boot lamp when fed direct from battery fuse
Purple			Green			Fuse to hazard flasher
Purple			Light green			Fuse to relay for screen demist
Purple			White			Interior lights to switch (subsidiary circuit door safety lights to switch)
Purple			Yellow			Horn to horn relay
Purple			Black			Horn to horn relay to horn push
Purple			Pink			Rear heated window to switch or relay
Red				Main feed to all circuits mastered by side lamp switch
Red	Brown			Rear fog guard switch to lamps
Red	Blue			Front fog lamp fuse to fog lamp switch
Red	Purple			Switches to map light, under bonnet light, glove box light and boot lamp when side lamp circuit fed
Red	White			a)Side lamp fuse to right-hand side and rear lamps
				b)Side lamp fuse to panel light rheostat
				c)Fuse to panel light switch or rheostat
Red	Yellow			Fog lamp switch to fog lamp or front fog fuse to fog lamps
Red	Black			Left-hand, side lamp fuse to side and tail lamps and number plate illumination
Red	Pink			Side lamp fuse to lighting relay
Red	Orange			Fuse box to rear fog guard switch
Slate				Window lift main lead
White					Ignition switch or starter solenoid to ballast resistor
White		Brown			Oil pressure switch to warning light or gauge, or starter relay to oil pressure switch
White		Blue			Choke switch to choke solenoid (un-fused) and/or choke to switch to warning light, or electronic ignition distributor to drive resistor
White		Red			Starter switch to starter solenoid or inhibitor switch or starter relay or ignition (start position) to bulb failure unit
White		Light green			Start switch to starter interlock or oil pressure switch to fuel pump or start inhibitor switch to starter relay or solenoid
White		Yellow			Ballast resistor to coil or starter solenoid to coil
White		Pink			Ignition switch to radio fuse
White		Slate			Current tachometer to ignition coil
White		Orange			Hazard warning lead to switch
Yellow					a)Overdrive
					b)Door locks
					c)Gear selector switch to start