11

I drive a 1999 Chrysler 300M, V6 3.5L engine. I am about to start a water pump replacement repair. I've discovered that the water pump is driven off of the timing belt, rather than either of the two fan belts on the front. This obviously complicates the repair quite a bit. What is the advantage to running it off of the timing belt? Why is it designed this way?

10

Engine safety. If you lose an accessory belt driven water pump, you're likely to keep driving, thinking the "oh, I just don't have an alternator" while you're busy cooking your engine beyond repair (normally with no temperature notification/change, if anything it'll read cold). When the water pump is on with the timing belt (or geared to crankshaft as is occasionally the case), if the water pump seizes and tosses the belt, the engine will shutdown. Inconvenient, but rarely dangerous, and saves a very expensive repair.

  • 1
    if you don't mind me asking, why would an engine's temperature readings not be reflected accurately if the water pump is seized up? – Woodrow Barlow Sep 4 '14 at 16:20
  • 2
    The "engine temperature" is actually the "coolant temperature" The coolant is no longer flowing with the pump no longer turning, so the hot coolant is contained in the block, and the coolant outside will be cooler (how much cooler depends on the exact location of the sensor, outside temperature, airflow, etc. It's often near the radiator's hot side entrance where it'll get significant cooling effect at speed. – Brian Knoblauch Sep 4 '14 at 18:35
  • Funnily enough, I had exactly this recently - my Discovery has an aux-belt-driven water pump, so when the belt failed due to a siezed PAS pump, I couldn't drive it home as it would have cooked... – Nick C Sep 5 '14 at 9:12
  • 1
    When the water pump gets seized and tosses your timing belt, your timing will be off. Even though the engine shuts down, there is still rotation happening in the engine. Just another point to mention. – HasH_BrowN Sep 8 '14 at 23:04
  • 2
    This point is only valid on the assumption that you have a non-interference engine. – Steve Matthews Sep 13 '16 at 14:51
13

I think the main reason for this is convenience. It's an easy place to run the water pump. If you ran it out to a fan belt, it would be in the way of the timing belt while doing it, or it would be a really awkward mess trying to work around it. The second reason is for compactness. With the water pump stuck out of the way, it physically makes the engine shorter, which means it will fit in the hole which is the engine bay a little easier.

Most overhead cam engines which run a timing belt use this method to propel the water pump. It has been done this way for a long while.

On a separate note, since you are in there replacing your water pump, replace the timing belt and any tensioners which may be there with it. No sense in having to take it apart in another couple thousand miles to get it done. This would be the same reasoning behind getting the water pump done when you do your timing belt. It's just cheap insurance and beats having to do the same work twice.

  • 3
    i don't know about "cheap insurance", since a pump alone would have cost me half of what i paid for the belt, tensioners, and pump... but fear not, i'm replacing the whole shebang. thanks for the advice. – Woodrow Barlow Sep 4 '14 at 1:51
  • 3
    @WoodrowBarlow ... It's a lot cheaper than replacing a motor due to timing belt failure. Good on yah, btw. – Pᴀᴜʟsᴛᴇʀ2 Sep 4 '14 at 8:38
  • 1
    @WoodrowBarlow basically those are done at the same time, if the water pump is being replaced you replace the belt, and vice versa. It's always cheaper when you factor paying labor at a shop. When it's your own labor it's just your time. But if the belt breaks it's at a minimum a huge inconvenience and you still have to do the labor again and at worse major engine damage. I have done only belts many times at the customers request, only to have the water pump start leaking soon after on many of them. I wouldn't even consider doing a pump without the belt, it's just too much risk otherwise. – Move More Comments Link To Top Sep 4 '14 at 22:20
  • @WoodrowBarlow. Had my timing belt replaced, water pump was ok for 3000KM then started leaking. May have been more due to over tightened timing belt which caused the failure. – TheLegendaryCopyCoder Aug 19 '16 at 10:12
3

Historically the water pump was at the front of the engine in close proximity to the vehicle radiator, when vehicles were mostly rear wheel drive. There is no real advantage with todays vehicles but they continue to be at the front of the engine. Using the timing belt to drive the water pump lends itself to compactness at the front end of the engine. Some special purpose vehicles have remote or supplementary water pumps.

2

Having snapped alternator and water pump belts in the past, I feel the most likely reason is engine safety. If it's a non-interference engine, having the timing belt drive the water pump ensures that the engine safely shuts down in the event that the water pump belt breaks. That way you're just replacing a timing belt instead of rebuilding the engine after it overheats.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.