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Instruments Published: 26th Oct 2018 - 0 Comments - Be the first, contribute now!

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Your classic may have a lovely set of dials on the dashboard – but do you know how to use them properly? Many don’t so learn how to read trouble ahead!

Oil pressure gauge

This is largely viewed as the most important supplementary gauge of them all as it can stop the engine going bang – if you look for the danger signs, early. As you can imagine low or no oil pressure is A Bad thing – but there again so is an abnormally high reading as well.

The most critical readings are when the engine is up to temperature because, when cold,the thick oil will always give a higher pressure reading. You need to consult your workshop manual regarding your specific car’s requirements, but in general, a good engine should give a steady reading of around 40/60lbft at around 3000rpm and fall to about 25lbft at idle. On some worn units it can drop to virtually zero when hot but this isn’t the major worry as it first appears.

Here’s what to look for. Ignoring a sudden loss of pressure – which is either due to a massive oil leak or a failed oil pump – an intermittent loss, particularly when cornering or braking, indicates a too low oil level but it could be that the pick up pipe in the engine is loose (sucking air rather than oil) or blocked. A gradual drop in pressure over a long journey points to general engine wear; main bearings/crank usually. But it could be simply that the oil is past it and the filter so clogged up that the lube is circulating via the filter’s by-pass valve. So change the oil and filter before fearing the worst and consider a thicker grade lube which may help a worn engine (try No Smoke Oil).

Low pressure when cold is usually due to a blockage or the pump is wearing (does it take a few seconds for the gauge to work?) while abnormally high pressures hot or cold can be due to a faulty relief valve. A fliickering gauge at idle means a fault in the gauge or its pipework but whatever the symptoms, act on them now before major problems arise.


This gauge has been around longer than the motor car! Its job is to show how electric is flowing. On a classic car with aged electrics it’s the most useful supplementary gauge you could wish for because it foretells of impending trouble. If all is well, after start up, the gauge should display a healthy charge (‘plus’) gradually tailing off to just above zero as the generator does its job. If a discharge shows with the ignition off , suspect a ‘leak’ somewhere. A constantly high reading isn’t good news as it signifies either a faulty regulator unit or a failing battery. Vice versa and it points to a deteriorating generator, loose fan belt, faulty control box or, in the case of the dynamo, too many accessories on at the same time. This is usually supported by a dimly glowing ignition warning light!

Oil temperature gauge

Not many classics had this gauge fitted when new (although many modern Peugeots and the MGF are so equipped) and so it may have been installed by a previous owner. To be honest, unless you take part in motorsport, indulge in track days, have a highly tuned engine or tow a heavy trailer, the temperature of the oil should not be a major issue. As a rule, between 70-90 Centigrade is fine; readings higher than this indicates tired lubricant, extreme driving conditions or a slightly slipping clutch. there again, over-cooled oil will wear the engine out and if an oil cooler is fitted then ensure that its thermostat is operating correctly.

Where this gauge can work to your advantage is on classics fitted with automatic transmissions that tow; an overworked auto box won’t change gear properly and long term damage may occur. Fit a gauge to the ’box to keep an eye on things.

Water temperature gauge

Most classics have this basic gauge and it’s as simple as they come. Too cold and the needle barely moves – too hot and it could soar off the scale! What you want is a happy medium which is a speedy move up from the cold sector to the normal operating range, which is around the centre of the dial.

If the gauge is sluggish to move up to running temp then it means that the thermostat is faulty and probably stuck slightly open; a new one can work wonders on many old engines. A hotter than normal reading is okay (presuming there’s no fault with the system that is!), so long as it’s not near the danger zone – this can happen on old engines suffering from furred up waterways or a clogged up radiator. A sudden move up the scale into the red is usually due to a massive, sudden leak (burst hose, core plug etc) and accompanied by a loss of coolant! But if the coolant is still there then the head gasket is likely to be the culprit.

If the engine boils up but the temperature gauge reads fairly normal then it is probably due to you missing the initial danger signs and that there’s so little water in the engine that the level is below that of the sender unit. Ah the sender unit! these can play up and give false readings so check it is okay. Higher than usual readings can be due to a faulty radiator cap, sticking thermostat or a blockage or airlock in the system or a collapsed top or bottom hose that may even look okay externally.

Battery condition meter

This is gauge started to replace the old ammeter back in the late 1960s on many cars as it’s simpler to fathom out. The main difference between the two is that the ammeter reads amps while the battery condition meter shows volts. They broadly do the same job although the ammeter is much more informative and useful.

Essentially, if the battery and the charge system is in good order then the needle should be in the green sector; switching on the lights and wipers should only cause a small needle deviation.

A poor battery, even if it’s fully charged, will have the needle always in the red once the vehicle’s lights and wipers are in use as they are taking out more than the battery can deal with. Before discarding the box of sparks, check the terminals for tightness, poor earths and so on.

If the needle stays in the upper red sector then it points to over-charging – perhaps the charging system or regulator is at fault? Constant topping up of the battery is further evidence of this.

Vacuum gauge

Very few classics were fitted with this busy little instrument but it was always a popular fitment with enthusiasts during the 1960s and 70s as it was a huge aid to engine tuning, judging the unit’s inherent health and even help with economy driving. Unlike other instruments, which largely remain, constant, the vacuum gauge has a frantic time, flicking around the dial as it registers the state of the engine on the move via depression in the inlet manifold. The gauge is usually made up of colours complementing a scale of 0-30 ‘Inches/Hgs’, the later signifying mercury which is used to determine the state of manifold depression and so a vacuum. There are sometimes written indictors on the dial’s face ranging from POOR, FAIR, IDLE, DECELERATE.

Essentially, this gauge it all about showing a vacuum or sucking effect and the more or better an engine can do this then the healthier it is! A good well-tuned engine will idle at around 17-21in Hg; a quick rev will see the needle fall to just above zero before reverting to the original reading after peaking at around 25in Hg. If the gauge only registers 10-17in Hg at idle then it points to wear in the engine and the piston or bores in particular.

Confirm this by smartly flooring the throttle while on the move (with the clutch depressed of course!). The needle will go to zero then rise to ‘25’ but anything below figure this means that the engine is getting on a bit.

A vacuum gauge is a superb aid for engine fault finding and tuning purposes. If the needle continually falls when idling, a sticking valve or an out of sorts ignition (perhaps even the camshaft timing) are the likely causes for this.

You can fine tune the engine’s ignition timing with greater accuracy with a vacuum gauge than even a strobe light as it sets up each engine for its specific needs (mileage, wear etc). Basically, for optimum timing, you tweak the distributor so the gauge gives its highest reading before just starting to fall back. Fuel mixtures can be set more precisely in a similar fashion.

Finally, a vacuum gauge can save you money by helping you drive with economy in mind. By trying to keep the needle at its highest practical reading for the road conditions, with a light throttle, real gains can be found. In the early 1970s when petrol was scarce and more expensive than now, one DIY motoring mag found a 10mpg improvement in a Capri 1600 simply by keeping an eye on the gauge and driving accordingly at its optimum!

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