I'm struggling with getting the proper voltage to the A/C clutch relay. I have a 1988 Delanair Mk III unit. I also have the manual for the unit that has been floating around this forum. The ECM seems to work as it should in all modes. When I call for A/C, I should get system voltage (~12V) on pin 20, the GN wire which goes all the way to the clutch relay. However, I only get ~4.2V if no relay is in the socket. Once I install the relay, voltage drops to ~1.45V. Has anyone else experienced this?

I've tested these values at the relay and at the ECM. I've also isolated all parts of the system to ensure there was no leakage to ground and the wires have a minimal amount of resistance. So, I'm confident the problem is at the ECM or the relay/relay socket.

There is a protection circuit at the relay socket that consists of a resistor and a diode. I found the diode to be shot, so I removed the entire circuit from the system to isolate the issue. Still only 4.2V. So, according to the manual, my ECM is bad.

I don't have the cash to replace the ECM at this point (especially one that works in every other aspect). The ECM also looks very clean and undamaged. All the pins are clean, solder joints look good, etc.

Is there a way I could put a voltage booster on the GN wire to operate the relay or is there a relay out there that will operate on the lower voltage I'm getting? Is there something I completely overlooked or missed?

I'm so close to getting my A/C working. Everything else in the system is good. I just need the proper voltage at the A/C clutch relay.

 

A used Mk III ECU shouldn't cost much. Finding a good one might be a problem

I'd be sorely tempted to abandon the existing GN wire at the relay and run a new wire together with a manually operated switch

Cheers
DD

 

The driver transistor that switches the AC clutch on is TOAST. If you are a handy you could have a go at replacing the transistor. I have a section on my Blog where I replaced a ZTX550 driver for one of my vacuum solenoids.

I have no idea which transistor it will be. I have tried to source a circuit diagram for the controller but no luck. There are a few good Youtube videos on how to test transistors with a DVM, so I won't go over this.

There are a few ZTX550 drivers test all of these as well as the large to220 package transistors bolted to the heatsink.

If you find one of the ZTX550's faulty cut the legs and de-solder each leg individually DO NOT try to de-solder the complete transistor. These PCB's are multi layer and damaging the internal tracks will render the PCB un-usable. I always use as large a tip on my soldering iron as will fit where I am soldering, with my soldering station set on Maximum temperature, this allows you to solder quickly, reducing the total heat on the PCB. You need to be quick and DO NOT hold the tip too long on the PCB or the tracks can de-laminate from the fiberglass.

If you do get it fixed or replace the ECU you need to test the AC clutch before you plug the ECU back in, just in case there is something drawing too much current, and POOF your new ECU goes BANG again. I use incandescent light globes in series with the unit under test (UUT), these will light up if there is excessive current draw and protect the ECU from damage.

 

Warren,

Thanks for the help. I found one of the ZTX550's toast. It was in slot T7, do you remember which one you replaced on your ECM? I ordered a new transistor today. As far as the testing the clutch, I'll double check it to be sure. However, the clutch should be fairly isolated from the ECM since it's operated by a relay with a built-in diode in addition to a resistor-diode protection circuit. So, I suppose the only thing that could draw too much current from the ECM would be the coil in the relay?

Doug,

The farther I journey down the rabbit hole, I'm wondering the same thing. Then, I would also have to redesign the A/C system so the evaporator doesn't freeze up.

Thank you both for your input!

 

Warren,

I replaced the ZTX550 in the T7 slot, and now I have system voltage to the relay! Thanks again, everything works as it should!