Ffan belt replacement

This might be useful…
Jag fan belt


The difficulties with this operation are a combination of difficult access, lighting and the correct setting up of belt tension. If too loose the belt will squeal, wear quickly, and there will be slippage (eg poor charging). If too tight there will be undue wear on pulley bearings, especially the water pump and alternator.

My aircon is inoperative and I have not used it for years, so in my case I did not need to renew that belt. This meant that the work could be done from above the car.


  • Using appropriate lever, partly loosen fan nuts, using Vaseline to collect all the shakeproof washers and nuts, to avoid dropping. These have to be removed gradually as there is little clearance for the fan.
  • Ease fan forward and allow it to rest gently on radiator cowling.
  • Remove two cowling clips and lift radiator cowling and fan (loose inside), using two hands.
  • Reach down and cut aircon belt clear (if not required as explained above). This will allow access to the alternator belt. (If you do need the aircon or need to replace the belt, you will have to work from under the car, having first removed the fibreglass skirt.)
  • Cut old alternator belt clear.
  • Apply liberal amounts of WD-40 to upper alternator (long) adjustment bolt thread, cleaning with a toothbrush to allow easy turning with fingers only, once loosened.
  • Using short open-ender, box banner or deep socket loosen alternator upper adjustment nut. It should then be possible to turn this easily using fingers only.
  • Using lever on front flange, ease alternator up
  • Apply liberal amounts of WD-40 to lower alternator adjustment bolt thread and nut.
  • Screw lower adjustment nut well down thread; should be OK with fingers only.
  • Ease alternator down sufficiently to allow fitting new alternator belt across the pulleys, alternator pulley (smallest) last.
  • Using lever on flange, ease alternator up so that belt is reasonably tight. (SEE ADJUSTMENT NOTES.
  • Screw up lower adjustment nut using fingers, until snug against alternator flange.
  • Screw down upper adjustment nut, and after re-checking tension (SEE NOTES BELOW) tighten this nut.
  • Using both hands, lower radiator cowling with fan resting inside,
  • (NOTE: This step and the next two may have to be reversed). Ease cowling into lower slots and secure using the two clips, levering these from the curly front of the clips, aiming uncurly (!) backs at an angle towards upper slots. Press into place.
  • Using Vaseline to avoid droppage, place shakeproof washers and then nuts on fan bolts, gradually (little clearance) rotating fan hub as necessary to give optimum reaching angle.
  • Levering round against nuts, fully tighten fan joint.


Making this tool should not be attempted unless you are competent in metalwork.

Always use safety glasses when working with metal or tools.

Jaguar manual: ‘Apply unknown pressure at right angles to the belt in its longest straight line between pulleys and measure the deflection caused in the belt’ Eg: ‘alternator — 4.4 mm (0.17 inches)’.

Via ? manual (quoted by ‘Tex Terry’ on JagLovers forum: ‘…find the longest belt span. For the section you are working on (eg. alternator); a span is measured at the tangents of the pulleys where the belt is still seated; if in a span of 7 to 10 inches, you want approx. 1/4 inch deflection; if in a span of 12 to 16 inches, you want approx. 1/2 inch deflection; the deflection is from a straight line, at the outside of the belt, with the belt at rest, then pushed in towards the inside of the belt; the deflection point is at the midway point, on the belt, between the pulleys; if you put a straight edge guide (ruler or piece of wood) along the belt, reaching from pulley to pulley, at a point where the belt is seated fully, then you can see the distance you are pushing the belt away from the straight edge item’. Converting to metric measurements, we have:

Span Deflection
17.8 — 25.4 cm 6 mm
30.5 — 40.6 cm ….13 mm

This begs the question: What about spans in between approx. 25 and 31 cm? Clearly the figures given are rule of thumb, and for precision you would expect some regular relationship (linear or logarithmic?) for all. For example, if the span is towards the low end of a range the defection ought to be smaller and vice versa. Showing the rule of thumb figures and the substantial gap graphically reveals the quandary. However, see later in this particular case.

I designed my deflection gauge to be applied to the alternator belt from the top of the car. The longest span available from the top is that from the water pump to the alternator belt to the water pump.

Note the definition of span:- distance between pulley tangent points, so the next problem is how to accurately establish the exact position of the tangent of each pulley. This can be done for the water pump (higher) by eye but that cannot be reliably done in relation to the alternator pulley (lower), owing to its position low down. By definition the tangent point is a pulley radius (half the diameter) from the perimeter (at right angles). A practical way of measuring the diameter of the alternator pulley in the dark confined space is to press a piece of clean white paper against its end, forming an impression. I found this diameter to be 73 mm so the radius is approx. 37 mm (obviously we are not attempting to work to fractions of a millimetre!).

We are able to establish the outside start position by arranging that the measuring tool is held against the outside of the pulley. The tangent position will be 37 mm in from that.

I fashioned a tool from a piece of aluminium 25 mm square stock 30 cm long, with a small peg of 10 mm round stock inserted close to one end, the peg long enough to be pressed against the lower side of the alternator pulley. The lower end of the span would be 37mm from the peg position. The upper end of the span can be closely estimated by eye, checking with the water pump pulley. This was marked off and revealed that the span was 18.8 cm. The half way point is therefore 9.4 cm from each span end, and it is at this point that we gauge the deflection.

In my tool, I do that by applying a suitable calibrated plunger made of 10 mm round aluminium stock through the tool body.


Making this tool should not be attempted unless you are competent in metalwork.

Always use safety glasses when working with metal or tools.

Materials: Square aluminium stock 25.4mm section, 30 cm long
Round aluminium stock 100mm diam, I off 7mm and 1 off 8mm
4mm bolt 30 mm log.
4mm nut and shakeproof washer.

Highly preferable: Drill press.

  • Mark, centrepunch and drill a 10mm hole through square stock, 15mm from one end.
    Ream slightly as required and insert 100mm stock 7 cm long so one end is flush with the square stock. This is the lower peg.
  • Holding the lower peg steady, drill a 5mm hole through the square stock on the adjacent side, so that a 4mm bolt can be passed through the stock and the peg, thus securing it.
  • (Alternatively, drill a 5mm hole through the adjacent side of the square, insert the peg and mark through the 5 mm hole using (eg) a thin scriber. Then drill the peg 5mm separately, using the drill press and a V block).
  • Secure the peg to the stock and file down any slight protrusion.
  • By placing the part-made tool with the peg under the lower (alternator) pulley, you can lay the tool along the belt, and mark off the position of the upper pulley tangent, judging by eye
    Take the tool to the bench and mark/re-mark the upper tangent point as just found, plus a line 3.7 cm from the inside of the peg (where it will press against the underneath of the lower pulley). This line will be the lower tangent point.
  • You should find the distance between the two tangent points to be approx. 18.8 cm.
    The centre point (where we will gauge deflection) is therefore halfway (9.4 cm from either end. Mark that clearly.
  • Drill a 10mm hole at that halfway position and ream slightly to accept 10mm circular stock 8 cm long (this is the gauging plunger).
  • Note that the span length falls at the lower end of the 6mm deflection as recommended in the manuals.
  • Our access and lighting problem would make it impossible to reliably guess how far the belt can be pressed (deflected). This is the purpose of the gauging plunger which will be calibrated on the outside! It will also be very clearly marked.
  • So we now mark the deflection plunger so that we can clearly see at the side opposite to the belt, just how far the plunger protrudes from the square stock (which is the deflection). In our case 4.4 — 6 mm is the minimum.
  • With the plunger inserted into the square stock so that the end is flush, mark hard against the square stock.
  • Slide plunger so that 5mm is protruding and mark again on the other side of the plunger.
  • Do the same for 10mm protrusion.
  • Either by eye, or using drill press, mark the 5 and 10mm positions all round the plunger.
  • When satisfied, score those lines.
  • Spray the plunger all round the scored lines with black paint, and immediately wipe it clear, leaving the paint in the bottom of the scored lines. This will provide easily seen calibration marks to the outside of the tool
  • What can possibly go wrong? Experienced Jaguar home mechanics quickly realise that in use, the plunger will fall out of the tool and into the engine bay, onto the floor if you are lucky or even into the inside the radiator cowling! Aluminium is not practically magnetic so you’re out of luck with the extendable magnet we all use. How-to prevent it? Drill through the non-belt end of the plunger and through the square stock near to the hole, to provide tie points for short length of thin marking out cord. If the plunger slides out it will dangle only!
  • What else can go wrong? If you make the string too short you won’t be able to insert the plunger; too long and it will get in the way…


Obviously, hook the lower peg under the alternator pulley, lay the tool along the belt and depress the plunger firmly, reading off against the plunger marks. 5 mm would be fine (it is not critical). Adjust per instruction above and retest.