Engine bay belts - who, what, when and why?

[tigger]

Hi, my first post for ages due to my 15 year old Bongo (2.5td 1995) just being plain old reliable...

Driving up the M5 last night when I heard a fast slapping sound coming from the engine bay, sounding exactly like something getting stuck in something else that's spinning really fast and catching on saomething. Slowed down, still sounding dreadful, then just before I pulled in it cleared. No warning lights, engine still running.

Hmmm, I thought, sounds like a belt breaking...

Got out on the hard shoulder, unloaded all the cr*p from the back, flipped the seats to have a look. Sure enough, evidence of bits of broken belt scattered around the engine bay. But here's the thing. The alternator still had a belt on it and was still spinning. Something below it - power steering pump? - was the same story. No belts or pulleys visible on the other (driver's) side of the engine except of course the cam belt, and unless I have a magic Bongo it wasn't that that had broken!

So I decided to carry on at reduced speed, and got back home (another 200 miles) with no problems. Question is, which belt broke??? What no longer works????

All thoughts appreciated!

Dom

[widdowson2008]

May be a load of crap but...........
Air con compressor drive belt?
Is your air con working?

[scanner]

Hi, my first post for ages due to my 15 year old Bongo (2.5td 1995) just being plain old reliable...

Driving up the M5 last night when I heard a fast slapping sound coming from the engine bay, sounding exactly like something getting stuck in something else that's spinning really fast and catching on saomething. Slowed down, still sounding dreadful, then just before I pulled in it cleared. No warning lights, engine still running.

Hmmm, I thought, sounds like a belt breaking...

Got out on the hard shoulder, unloaded all the cr*p from the back, flipped the seats to have a look. Sure enough, evidence of bits of broken belt scattered around the engine bay. But here's the thing. The alternator still had a belt on it and was still spinning. Something below it - power steering pump? - was the same story. No belts or pulleys visible on the other (driver's) side of the engine except of course the cam belt, and unless I have a magic Bongo it wasn't that that had broken!

So I decided to carry on at reduced speed, and got back home (another 200 miles) with no problems. Question is, which belt broke??? What no longer works????

All thoughts appreciated!

Dom

The second alternator belt - the Bongo has two of them (don't ask me why, it just does).

Yours now only has one.

[tigger]

Hi widdowson, I wondered about air con, but it still seems to be working OK. Scanner - I think you're onto something here! I'm going to hunt out some belt diagrams to see what belts run over what pulleys.

And as to "don't ask me why" it has two belts - so it keeps on working when one breaks???

Cheers
Dom

[haydn callow]

It won't last long on only one belt....suggest you replace both Alternator/Waterpump belts and do the A/C one whilst your at it (it has to come off to do the others anyway)....then you'll be good for another 15 years.....Well !! for quite a while anyway.

[widdowson2008]

It won't last long on only one belt.....

Spot on Haydn
Belts are designed to transmit a specific load. If Mazda put 2 belts on there, then it is because one alone isn't strong enough to take the load.
If you have been running with only one belt, then that one is being over-worked and could snap any time.

[munroman]

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

I agree, also make sure that any debris is cleaned out, I don't think you should be able to see your cam belt, which should be covered.

Also, it would be worth replacing all your drive belts as if one has gone the others might not be far behind.

A friend with a Scenic diesel wrecked the engine after an Alternator belt broke, the garage who replaced it did not clean out the debris and this then got into the pulley and threw the cam timing.

Thanks to the wonders of Google (showing that the engines had a serious design flaw and Renault UK had failed to recall vehicles with bad pulleys which shredded alternator belts), my schoolboy French, and a carefully written script the initial offer of a refund of £25 for the belt increased substantially to a brand new engine fitted and warranted.



Derek

[widdowson2008]

Hi widdowson, I wondered about air con, but it still seems to be working OK. Scanner - I think you're onto something here! I'm going to hunt out some belt diagrams to see what belts run over what pulleys.

And as to "don't ask me why" it has two belts - so it keeps on working when one breaks???

Cheers
Dom

These may help when you come to refit belts

[tigger]

Wow, thanks everyone for the prompt and fullsome replies. I'll certainly change all the belts asap - particularly as I see that the water pump is driven off the alternator belt.

So just to be clear - the alternator is driven by two belts, and there's another a seperate belt to drive the aircon pump? If so, are both alternator belts the same length and routed over the same pulleys? I'm sure I don't remember seeing an 'empty' pulley on the alternator next to the one that still had a belt on it when I checked last night, but perhaps I'm getting the wrong end of the stick?

Thanks in advance

Dom

[haydn callow]

Both alternator belts are the same and go over?round the same 3 pullys.....the A/C in a differant size

[tigger]

Thanks Haydn. Is it a tricky job to to replace them all? I've been doing DIY car maintenance for years, but it looks like access is a bit tricky here? Also, when it comes to tensioning belts I don't have a gauge - in the past I've just gone on what 'feels right'...

[scanner]

Hi widdowson, I wondered about air con, but it still seems to be working OK. Scanner - I think you're onto something here! I'm going to hunt out some belt diagrams to see what belts run over what pulleys.

And as to "don't ask me why" it has two belts - so it keeps on working when one breaks???

Cheers
Dom

A Bongo has 3 belts - 2 identical and running side by side for the alternator and one different one for the air con.

As to why 2 for the alternator and me not knowing why - yes you are right, if one goes the other keeps things working.

But WHY? - other cars don't have 2, so why does the Bongo? and if one that size isn't strong enough, why not fit a stronger one like other cars?

[munroman]

Hi widdowson, I wondered about air con, but it still seems to be working OK. Scanner - I think you're onto something here! I'm going to hunt out some belt diagrams to see what belts run over what pulleys.

And as to "don't ask me why" it has two belts - so it keeps on working when one breaks???

Cheers
Dom

A Bongo has 3 belts - 2 identical and running side by side for the alternator and one different one for the air con.

As to why 2 for the alternator and me not knowing why - yes you are right, if one goes the other keeps things working.

But WHY? - other cars don't have 2, so why does the Bongo? and if one that size isn't strong enough, why not fit a stronger one like other cars?

The old Volvos used 2 belts too, I always thought it was a failsafe, so that if one snapped the other was still working, so that the vehicle could still keep going, and with a working heater, which could be a lifesaver in cold weather, given Japan has similarly cold weather in certain areas I thought the logic may have been similar.

[widdowson2008]

May be wrong, but I don't think so - prepared to get shot down on this, but......
A belt drive relies ENTIRELY on the friction between belt and pulley, and that is entirely dependant on the contact area between the two.
If you take a look at the diagram, you will see that the 'wrap' over the water pump pulley is less than 90 degrees due to the limited space available and the close proximity and sizes of the pulleys, so the contact area at the pump pulley is relatively small.
To increase the frictional area required to drive the pump, the only option available to the designer is to double the number of belts.

Agree with munroman when he says 'replacing all your drive belts'. Particularly important when there is more than one on a particular pulley for the following reason - as belts are used, they 'stretch' as can be seen from the table showing belt deflection. A new belt will deflect between 7 and 9mm, but has a limit of 16mm. This is belt stretch in action. When the belt has reached the 16mm mark, the correct tension (and hence friction) cannot be achieved. If you put a new belt alongside an old one, yes they will work, but the old stretched one will not provide as much friction as the new one.

[haydn callow]

It's the side of the belts that give the "drive" thats why they are tapered....the bottom of the belt should not contact the bottom of the pulley grove....this gives a lot more contact area than you might think....
It's the big alternator that takes most of the belts effort......on a cold dark morning it can be pushing out 70 amps and this is quite a load on the belts