Odd AC Problem

[mieczkow]

After a recent complete rebuild of my 1989 non-SE Esprit I went to have my AC serviced by a local shop which has done work on my Esprit in the past. When I left after the service (they determined that the fuse in the rear boot was bad, keeping the compressor from running) they also recharged the system with R12. Things were fine and the AC blew very cold.

 

Several days later I discovered that the AC was operating continuously - you simply could not shut it off. The AC temp control switch in the cabin was ineffective. I returned to the AC shop and they started checking things. They concluded that the AC demand signal to the ECU was defective, keeping the relay closed constantly, and thus powering the compressor continuously. The ECU replaced. No change - the compressor will still not shut off except by pulling the relay. The service shop seems stumped. They state that all other electrical connections seem fine and seem to still suspect the ECU.If I understand them correctly they are saying that the ECU when it receives a demand signal (via the cabin temp switch I presume) then sends a signal tot he relay, which closes the circuit to engage the AC clutch. They say that this signal is a ground - in effect the ECU closes a ground loop to the relay in order to activate it. They are saying that the ECU is continously grounding the relay.

 

Can anyone advise me on where we might look next in order to determine why this "continuous on" is occurring?

 

Tom Mieczkowski

89 Turbo Esprit

St. Pete, Florida

[fflyingdog]

Ok first things first have you checked the rotary switch on the Hvac panel is actually off when selected? If the switch is grounding out internally then the ac compressor will run continuously, although a low coolant level should command the ac compressor off to save the compressor from eating itself.Disconnecting the fly lead from the compressor should command it off, if its still running then your compressor is goosed.

 

      Nick S4s

[mieczkow]

Well, the compressor appears to be good. I returned the car to the AC shop and they said that the only solution they could come up with was to put in a wire patch by-passing the AC Request relay. I have included a copy of their hand-drawn schematic. A couple of questions. What consequence (in theory) does by-passing the AC demand relay as indicated in the schematic? The AC appears to work fine, and the temperature selection feature cycles properly, etc. However, I am a bit troubled by not understanding this "correction" and in general I am sceptical of "work-around" solutions. Any opinions on this "fix" by the AC shop?

 

Can someone give me a brief explanation of how the ECU and the AC system interact? As I understand it when the AC is switched on from the cockpit, the ECU is signaled and it sends voltage to the AC Request relay. Once this is energized, the request relay closes and send voltage to the AC Control relay. When this relay closes the clutch is energized and the compressor is engaged.

 

The AC shop as the schematic shows routed the ECU signal directly to the control relay, so that power is what closes the relay and engages the compressor. But why should the original set-up not work? Is it that the ground connection through the rear harness connection has failed?

 

Any help from knowledgeable AC folks would be greatly appreciated! 

[fjmuurling]

The function of the AC request relay is to signal the ECU that the AC needs to be switched on. Three items control the relay in conjunction namely the on / off switch on the dash, the temperature controller and a pressure switch. They all have to give a "green light" before the relay gets energized.

From there on the ECU gets in charge, once the the ECU gets the signal for the AC request it doesn't switch on the AC control relay blindly. Depending on the ECT and the engine load (IIRC fanfailure is also taken into acount) it swiches the control relay, furthermore the ECU has debounce function for the control. ie to stop the control relay from hunting.

The patch wire doesn't make sense, the way the ECU energize controlling relays is by means of completing the circuit to ground (going low on the output)

 

Freek

[mieczkow]

Thank you so much for the explanation. What is interesting is the AC does work with the patch - I have reversed the correction - eliminated the patch wire and wired the relay as shown in the diagram. The result is that the AC works, but it will not shut off when the cabin AC switch is turned off. 

 

Can you suggest how I should troubleshoot this problem? According to the tech at the AC shop, he said the ECU was not going to ground when the cabin switch was turned to off position. I have some skill with a multi-meter and am adept mechanically, but I am not certain what I should be seeing in the way of output. Also, when you refer to the "pressure switch" can you be more specific? And can I assume when you refer to the temperature controller is that the thermistor?

 

Again, thank you for taking the time to explain this to me. Unfortunately I have skilled Lotus technician available to me and I will have to solve this problem myself.

[mieczkow]

Oh, I meant to say I do not have a skilled Lotus tech available to me!

 

Thanks again for your explanation.

[mieczkow]

One additional comment. The light green line to the request relay is disconnected and substituted by the "patch wire". That was not clear in the original picture schematic. I have added an arrow indicating which wire was disconnected - please see the updated picture.

 

Again, thanks for your help.

[sailorbob]

Disconnect the pin from the "A/C Control Relay" ecu connector and measure the voltage on the wire. This is to verify whether it is the ecu constantly tying the relay to ground or a fault in the wiring between the relay and ecu. Two ecu's with the same fault seems unlikely.

[mieczkow]

Found the problem. When the wiring harness was put back in the car after the renovation the ground wire to J1C3 on the ECU should not have been re-connected. This pin is only used in non-AC cars. This caused the ECU to continuously ground the request relay as soon as the ECU received power. Consequently the AC ran continuously when the ignition was in an on position. One the pin was disconnected from the ECU and the original wiring restored the AC worked properly. Thanks for the commentary to my inquiry.

Edited July 19, 2013 by Tom Mieczkowski