View Full Version : Prestcold Condensing Unit

25-02-2005, 11:08 PM

Wondering if anyone knows the following:

Upon arriving at site found compressor cycling on klixon. checked the capacitors and found none of them (start and run) to be original, the start was rated 160-200uF and the run at 5uF. As there was a capacitor on the fan motor rated at 3uF, I tested the run capacitor on that it and it didn't work. I didn't have another capacitor to test for the bigger one.

The question is what should they be? I thought the run was usually about 7uF?

The unit is single phase (240v 50hz) and is a K75

If anyone can help let me know

Many thanks

25-02-2005, 11:36 PM
Hi Sighman,

It may be more expensice to go through them but why no go to NRS how sell the spares for these units

Start Cap NRS Pt No 676450 44.85 list
Run Cap NRS Pt No 676457 13.13 list
Relay NRS Pt No 676856 44.65 list

or Complete Electric pack (all of above in box)
690333 67.08 list.

have had loads of probs with these, check over the pot internatl elecs because the first sign the compressors is on way out is it keeps blowing capacitors. The amount of times i have known engineers to spend hours looking for an electrical fault, changing elecs and then finding pot u/s is riduculas



Alan B
26-02-2005, 09:37 AM
Hi Rdocwra,
How would you confirm your assumption that the compressor is on its way out?
Alan B.

26-02-2005, 10:05 AM
found compressor cycling on klixon

This is a good sign that it is on its out. Especially if the starting gear still connected.
These parts hardly ever go out.

Chemi :)

26-02-2005, 10:06 AM
1) get a megger and do an earth leakage check
2) Shut the pot off, reclaim gas and wip the head off and have a look inside to see how stiff the pistons are



26-02-2005, 10:43 AM

I was thinking more that someone had put the wrong capacitors onto the system, and perhaps these have caused a failure of the compressor?

From the above part numbers I can see that it should be 160uF and 5uF

The windings seem ok on ohm testing. The relay didn't have any scorch marks on it, just didn't have a big enough capacitor with me to try replacing start cap..........nearest supplier was 60 miles away!

26-02-2005, 11:55 AM
Can you give the compressor hp size and manufacturer?

Chemi :)

26-02-2005, 05:06 PM
It's a Prestcold K75 Chemi or a 0.75 HP.
Tried to find my old Prestcold catalog but throwed away.

26-02-2005, 06:40 PM
Thanks Peter.

We don't get them here. (Prestcold)

In that case, its not worth playing with capacitors.
Just change the compressor and this time, put a new starting unit, usually comes with the compressor.

Chemi :)

chillin out
26-02-2005, 07:51 PM
Hi Sighman

Ive got a data sheet from NRS that I was going to scan in for you but unfortunatly I used the printers transformer and blew the scanner`s pcb :mad:

the comp on this sheet says

K/PK 75 start 60 mfd run 5 mfd and replace with the same

(make sure you check the contactor)

26-02-2005, 08:00 PM
use a clamp meter and check the starting and running amps,if the wrong capacitors have been on for a while it may have damaged the winding.

26-02-2005, 09:06 PM

Interesting that you say 60uF as thats a big difference to 160uf, was unable to check running amps as compressor didn't start! hence why I was asking about the correct start capacitor

If 60uF for the start is right then I think there could possibly some damage to the windings........

27-02-2005, 11:11 PM
This is a good sign that it is on its out. Especially if the starting gear still connected.
These parts hardly ever go out.I have to disagree with you here, Chemi. I change many more relays than compressors. Just consider a set of points opening and closing against an inductive load up to six times an hour for a few years. Compare them to contactors on a 3-phase unit. Can you say that contactors hardly ever go out?

the first sign the compressors is on way out is it keeps blowing capacitors.Only if the relay was changed each time with a factory spec. replacement. Blowing capacitors is the sign of a faulty or mis-applied relay. Never change a capacitor without changing the relay or you'll be back. Even if the relay works great while you're there, a failed capacitor points to a failing relay.

I hired an experienced air conditioning tech once and was surprised to learn he was unable to draw a start diagram for a CSIR (capacitor-start, induction-run) type compressor. He had always worked with 3-phase or PSC (permanent split-capacitor) units. (I discovered this when he called me at two in the morning because the walk-in compressor he had been called out on had fused wiring and he didn't know how to reconnect to run a start test with spares.)

Get yourself a capacitor tester! They are inexpensive, save time (especially on a freezing rooftop in the rain at night!) and last a loooong time.

Since I do light commercial service with lots of single-phase units, I carry standard capacitor sizes, both start and run. True, some of them I have had on the truck for years, but they don't take up much space, they are relatively cheap and they can save a room full of perishables, not to mention your reputation.

I carry the highest voltages, since they can be used on lower voltage, and they can be combined in parallel to sum their capacitances when needed. Likewise I carry the most common factory relays and a few universal relays for emergencies, but I change them for factory spec at the first opportunity.

I have probably gained a new customer or two every year when called out for a second opinion on a condemned compressor because the original company sent a guy who "didn't seem to know what he was doing." Usually I find a bad relay, blown capacitor and normal compressor. Avarice or ignorance?


28-02-2005, 05:56 AM
I agree with Rog,


28-02-2005, 04:41 PM
I agree with Rog,


yes quite true but there are times this comes back and bites you on the bum as the compressor goes out on it's klixon after a week and you have to stand there and tell the customer that he's now lost 2 FREEZERS worth of food on top of the fact that he just blew $200 dollars on a new relay and now he's got to stump up $1500 for a new compressor!!




28-02-2005, 05:32 PM
I always advise my customer of his options and leave the final descision to him.
I tell him that the part that I am replacing be it the starting pack or the condensor fan motor or even a regas, which may have caused shortcycling may have placed a strain on the compressor. The compressor may last one day or 1 year or more.
I can change it now or when it packs up.
When he hears the price of a replacement compressor the answer inevitably is: "change it when it packs up - we will cross that bridge when we come to it".


28-02-2005, 05:51 PM
yes and he will moan like f**k next week and say "I ain't gonna pay for that last call"

unless you in SA has such a different class of customer he comments "can't complain, I DID get another week out of it for my $200 dollars!!"

Customers have the memory of a goldfish when it comes to parting with money.. ;-)



28-02-2005, 08:33 PM

Well I got hold of a full set (start, run and relay) for the unit......
still didn't work , the windings seem sound (check ohms and megger tested them) so I guess in this case it is just mechanical failure...........

I too agree with rog, my meter tests capaciors, however I didn't want to have to unsolder the resistor so I could test it knowing already that the run cap had failed.

Whenever I change a compressor I keep the relay/cap etc (if they are good!) using them to test others just that day I didn't have anything big enough or two small enough to make a correct capacitance- plus I was worried what they should actually be as it was obvious they were not original.

I always say to the customer, I will try new electrics, but warn that it could be a sign of compressor windings failure........I would say on average it makes a lasting repair 60-70% of the time.

I would say that unless you can get positive proof through testing of windings (ohms/megger etc) then it has to be worth replacing electrics?

28-02-2005, 08:33 PM
If the compressor is older then 8 years, then I never try to replace anything but the compressor at his whole.
It's done his service for so many years and it's time now for retirement.
There must also be something more then just bread on my sandwich.

28-02-2005, 09:03 PM
A 12 pin transistor? :confused:
Sure it was a transistor Marc?
But I agree complete your statement, labour hours are to expensive to try something which costs the customers finaly to much time.

Same why I never repair anything on a more than 8 year machine, always replacing.

For example: I never clean the mesh screen in a TEV, replace it always completely, orifice and valve.
And whenever I opened the system, even if I'm sure no moisture came into the system, always replacing the filter/drier.

I argue that they do the same with their cars when servicing the engine; they also replace the oil, fuel and airfilter, so why not the filter/drier.

chillin out
28-02-2005, 10:05 PM
Hi Sighman

Ever heard of an 'ANNIE'

see under; tools and calculators , good old annie

It will stop any messing about with fitting parts to duff comps.

PS I don`t agree with replacing parts just because they are old , the older the part the better made it will (probably) be. The old parts (10 years or older) don`t fail for no reason and you generally get a second chance with them.

28-02-2005, 10:35 PM

I disagree with this,.. ever heard of MTBO and MTBF?
I worked long, very long time ago in a lawnmower factory and we even did test to predict this.

The older a piece becomes, the more chance you have it will fail.
Reason why so many parts on airplanes, in your car, on production machines are replaced before it fails.

The MTBF for most consumer goods is 10 years and for cheaper items things even less.

I didn't said, replacing because they're old: I said or at least I meant replacing old parts when they failed or they're a probable cause of another failure.

Sure, older parts were better but they don't have an everlasting life.

In the case of a compressor tripping on his overload, if the contactor relay is more then 10 years, I should even replace this one. What if one of the burned legs is the reason for the problems?

Suppose, you only changed the compressor and two weeks later, compressors fails again because you haven't changed the contactor, will your customer understand this if all his good are unfrozen?
If he paid a little more because you changed suspicious parts, but you don't have to come again, then he'll be satisfied. But if he paid a little bit less and you have to come again, even for another reason, be sure he will complain and the change is great he will not pay the second repair because he will think you did something wrong the first time which you need to fix the second time.

I's like going to a restaurant: if you ate good and you paid a lot of money, you will probably say that it was expensive but had a least a good meal.
On the other hand, if you ate a terrible meal and paid not much, you will say something more like "it wasn't worth the money, even if it was cheap" we will never come here again.
In the first scenario it will be more something like: "It was not cheap, but it was delicious. We will remember this place for a special occasion"

If you paid a fair price for the goods or service that was delivered, then everybody will be satisfied. And those who complain they have to pay for a good service, well.. those complain always.

I have a good old ANNIE and due to the help of MArkFiddy, it's fully operational again.

chillin out
28-02-2005, 11:30 PM
I disagree with this,.. ever heard of MTBO and MTBF? no

I didn't said, replacing because they're old: I said or at least I meant replacing old parts when they failed or they're a probable cause of another failure.

I know what you meant i maybe didn`t word mine properly.

As for the rest of your reply about the contactor those should be changed with every new comp going on even if they are new , as the comp manufacturer states this.
Because, if in the event of a failure under waranty, the comp being changed without contactor replacement would mean you wont get a new one from them.

Anyway contactors , driers ect. are consumable items .
You said you wouldn`t even try to get an 8 year old comp running if it was down to the electrics. comps are expensive items driven by consumable items that could be replaced to get it working again and as i said you do get second chances.

You wouldn`t change the whole engine in your car if the timing belt broke. (on the assumption that there was compression)

chill out peter :)

01-03-2005, 05:04 AM
With regard to MTBF's does any one have a list of MTBF's
for the compressors of different manufacturere? It would be interesting to do a comparison.