Carrier 750MT Evaporator fans not working

[Chris2005]

Hi All

I'm having problems with a carrier 750mt not bringing on the evap fans.

The unit has 3 compartments with 3 separate evaps which are all freezing but the fans will not operate.


There is no output from the generator but there is no alarm displaying on the screen.

I've checked all fuses and capacitors and everything is o.k, I've also changed the generator off another lorry that is running to test but still no fans or power from the generator.

I thought I had found the problem as the bottom capacitor on the door had a terminal which loose and had been arching and changed that with no effect.

I have tried flashing the generator with a small battery but there is no life.

Is there anything else I can check?

[IRMechanik]

Do your units have a separate generator or do the fans run off the standby motor?

[Chris2005]

Hi

Thanks for the reply

The evap fans have a separate generator that is located under the standby motor

[sonkax]

Generator has a capacitor alternating relay, check that relay is not overheated or has no bad contacts, usually it's a transparent one

[Chris2005]

Hi Sonkax

The clear relay was in quite good condition as it was changed about a year ago, I replaced it anyway just incase but still nothing.

I thought the unit would be showing some kind of alarm like alt aux but it's showing nothing and just carrying on as normal.

[clivemtk]

have you tried running it on standby

[charlie patt]

ive had um before no alarms just icy coils,useually relates to genny fault but if you have checked genney check voltage on jackplug by left of door if u have checked the fuses caps an d relays the wiring is next on provision belt is there i have had a unit surgeing and take all the caps out fire it up ice the coil and manually spin the fan and sea if it spins up

[sonkax]

evap fans and generator won't bring up alarms, check pm!

[Chris2005]

Hi Clive

Tried standby and engine but wont work on any.

[climaliolios]

hallo i have a xarious 350 mt .the controller flasing always in it

[IRMechanik]

hallo i have a xarious 350 mt .the controller flasing always in it

completely unrelated, maybe try starting a new thread.

Chris,
The 750mt with a generator is a bit of a different animal than I see over here. The NA units have deleted the generator and run the fans off the standby motor. So take my advice with a grain of salt.

There should be two 8 uF (might be 6 uF for a 750)caps wired through the CAR relay. Disconnect these and check them with a meter. Most times when they fail they blow up real good.
There is also 3 larger 115 uF caps for the standby motor, these are unrelated.
Check your engine speeds are correct (1800 / 2200)
Make sure your belt is tight, needs to be really snug.
There is a flat 4 pin diagnostic connector, you can check generator voltage across pins 3 and 4. Should be 240-250 VAC.

[sonkax]

Did you get the gen manual in pm? Had an unit with faulty diode on gen rotor.

[clivemtk]

why check engine speeds as the generator is driven by elec motor on standby and he says they dont work on standby niether

[IRMechanik]

why check engine speeds as the generator is driven by elec motor on standby and he says they dont work on standby niether

Good point, you are absolutely correct.
It is good practice to check the engine speeds. Over time the speed linkage wears where it hits the stop screw and the speed slowly creeps up. This increases the voltage and will eventually cause the fans to fail.

[UKengineer]

Carrier supra mt units have a four pin diagnostic plug for the generator you should get 240v on pin 1 and 4 of the DC plug.
If you have 240v you need to look at the EFB board.
If you don't have voltage on the DC plug check the two capacitors on the door if they are ok the generator is U/S....

[Chris2005]

Hi All

I will be going back to the unit to check out a few things.


I've tried a generator from another lorry in the yard that's working but still no fans operating.

Does the unit need the generator on standby?

[clivemtk]

yes the standby motor drives the generator belt check the generator loom after the plug its usally cable tied to frame maybe rubbed and broken power wire

[Chris2005]

Hi Clive

I did briefly check them wires and they looked ok from what I could see but I will go back tomorrow and pull them into the box and check them better.

[clivemtk]

its usally between the gen and the control panel you will find it cable tied to frame also clear your pm inbox

[Chris2005]

Inbox cleared

[Chris2005]

Was back today and check all the wiring back to the generator and all is in good condition.

I watched the clear capacitor relay for about 10 mins after I started the unit up and it does nothing just stays in the one position with no voltage going to the white wire at any time.

[sonkax]

Again, if capacitors look ok, does not mean they are good, swap caps from the other unit before anything else.

[IRMechanik]

Was back today and check all the wiring back to the generator and all is in good condition.

I watched the clear capacitor relay for about 10 mins after I started the unit up and it does nothing just stays in the one position with no voltage going to the white wire at any time.

The CAR relay (clear) should be engaged in high speed. It's wired to the speed solenoid circuit. It will drop out one of the capacitors in high speed.

Do you have a meter that will read capacitance? Your capacitors are your most likely culprit.
If you don't have a meter sonkax suggestion is good.

[clivemtk]

you fixed yet

[clivemtk]

you fixed it yet