Fuse location on engine a/c compressor

Bird Freak wrote:
Have you checked the low pressure switch itself to see if you are getting power to it? If you haven't it may just be a bad switch.

Doug also suggested this...so that will be my next step. Thanks.

Have you checked the low pressure switch itself to see if you are getting power to it? If you haven't it may just be a bad switch.

Thanks guys for the knowledgeable replies on diagnosing an a/c system.

Interestingly, they did not charge me for any freon! I will call them on Tuesday and find out why "no charge" if they had actually added freon. This was a diesel repair facility and likely didn't have an experienced a/c tech.

I only did a visual inspection on the inline fuses{nothing melted} and will need to check continuity now. Other than that, I will have to find a shop that specializes in a/c only.

hotjag1 wrote:
I found 4 inline connector fuses in that compartment, but they all looked good and they are not labeled as to what they are for.

hotjag1,

I know nothing about your specific chassis or motorhome, so I apologize if what I say may not be applicable, but I'm going to try to stay as general as possible.

When you state "...but they all looked good", what does "looked good" mean?

Did you actually test each fuse for continuity or just give a visual inspection? Fuses may "look good" but in fact test bad.

Also, make sure to clean up any electrical connection you investigate. It's possible on older rigs for corrosion or vibration to loosen connections enough so that they fail under load, even though visually they might appear to be good.

Someone had mentioned that it might be controlled by a relay, and if that's true, then you'll need to trace down and find the proper relay. Relays can either fail on the input (coil) side or the output (switch) side. If a relay is used, then there are probably at least two fuses involved. One for the input coil circuit (probably combined with other things) and another for the output compressor clutch circuit (probably 20A?). If you find that a relay IS used in the circuit (maybe Doug knows?) I can give you some additional hints about relay troubleshooting. Oftentimes, if there's an identical relay used elsewhere on the chassis, you can swap them to note if the problem corrects itself.


Again, while having nowhere near Doug's caliber of expertise, I believe his response to the statement from the service is on target on all points. It puzzles me that they would do ANYTHING if the compressor wouldn't run? It almost sounds like they had a test procedure they used and gave it to a tech who doesn't really understand how the system works. The tech just did what the procedure stated and noted the results. They didn't have any diagnostic experience to question anything that was done or discovered. For instance, if the system was, in fact, only 1 pound low in refrigerant the low-pressure system shouldn't have disabled it, so that would indicate something else is going on. And like Doug stated, if they DID jump the low-pressure switch, why not make note of that? And if their tests indicated no voltage appears at the compressor clutch, I would think they'd have the wire tracing equipment to locate where the problem is.

IF they wanted to.

I'll try to be kind and just say that you might have had a tech that was not well-versed in A/C systems. Or the shop didn't want to get too deep into this project.

Again, Good Luck,

~Rick

grldst wrote:
I have a son in the business. The new AC machines can evacuate and recharge a system without the compressor running. They record what they evacuate and the difference between that and the recharge would tell you how low the system was. Full recharge info is usually printed near the ports.

However, to verify all this you need high and low pressure readings which require a running compressor. It seems odd that a company would do all this though in a system that has a compressor that doesn’t work.

Well, I also am IN THE BUSINESS. 43 years worth. I guess I did not make it clear, but the recovery system does NOT need the compressor at all. And not all AC shops have the very expensive recovery systems that would state exactly how much was recovered. We don't. Not needed. Also, is your son in the RV business or Automotive business side. BIG difference. Doug

I have a son in the business. The new AC machines can evacuate and recharge a system without the compressor running. They record what they evacuate and the difference between that and the recharge would tell you how low the system was. Full recharge info is usually printed near the ports.

However, to verify all this you need high and low pressure readings which require a running compressor. It seems odd that a company would do all this though in a system that has a compressor that doesn’t work.

2003, Should be a Freightliner chassis. Doug

hotjag1 wrote:
Rick jay and Doug, thanks for the feedback about no compressor running.
This is a quote from the invoice.
"Checked A/C. Recovered system. Was about a pound low. Charged to 4.2 lbs. Test ran. Clutch is not engaging". I have no idea if they bypassed the low pressure switch.
Tiffins suggestion for fuse location was no help. I'm going to have to check every fuse that I can locate.

Complete BS from that Service center.
1. There is NO WAY to charge to 4.2 lbs without the compressor running. NO WAY to charge at all. They LIED
2. Clutch Not engaging. TRUE. BUT, If they DID BYPASS the Low pressure switch, do you not think they would have TOLD YOU?
3. Recovered System? Another LIE. Yes, they can remove the existing 134a by a recovery system, then charge the correct lbs/ozs.
4. A POUND LOW? Another lie, there is NO WAY to determine how low the system actually is.
5. So, I assume you paid for a bogus AC checkout/repair and ALSO paid for some 134a?
6. You do NOT have to check fuses. The Low Pressure switch is located either up front by the Evap box or at the rear on top the engine on the hoses to the compressor. Either check for continuity of 12 volts at the switch, or do what I do. Install a jumper wire between the 2 wires at the switch. It is VERY RARE to have the compressor fuse blow.
Doug

I am assuming the dash blower blows.

Might be the control switch.. Or might be the through wall if there is a connector at the firewall.. or a relay .. Some systems use a relay to power the compressor controlled by either a thermostat or the swich on the dash.

Only solution is to trace the wire.. Harbor Freight sells a great little cable tracker.

Rick jay and Doug, thanks for the feedback about no compressor running.
This is a quote from the invoice.
"Checked A/C. Recovered system. Was about a pound low. Charged to 4.2 lbs. Test ran. Clutch is not engaging". I have no idea if they bypassed the low pressure switch.
Tiffins suggestion for fuse location was no help. I'm going to have to check every fuse that I can locate.