PRE HE Air con blower motors not working

I am trying to get the AirCon working on my 76 Pre HE. All looked good , the clutch kicked in the air con was starting to cool then all quiet. The blower motors stopped working and the clutch on the compressor was not kicking in. I also noted that the left hand electric radiator cooling fan kicked in when the Aircon was working. I have tested this and its fine, although presumably the relay that controls the radiator fan takes a feed from the Aircon system . I have located the large relay for the AC down on the front side of the tunnel. So , given no fuse failure and the fact that it was working at first, does anyone have any ideas where to look for the culprit? The Operators Manual doesn’t actually give me much of a test schedule for the blower motors as such. It looks a pretty complex set up down there with amplifiers and micro switches. All help much appreciated.

I must have flummoxed you all! Well since the post, I have been carrying out more diagnostics. If I apply live 12v to the Harrison Compressor with the main lead still connected then everything works fine. The radiator fan starts up, the blower fans start up and the clutch engages. Going the other way, the most I get is a momentary engagement of the AC clutch and then it stops as do the blower fans. I have tested the thermostat up on the side of the AC unit and its not even receiving any voltage so is not the culprit. Mine has the very early set up with a large square relay and an additional round Hella unit next to it. I have got power to the relay and now need a test plan for that relay to test whether its good or not. Its looking like it could be that or something else that is telling the whole aircon to switch off. I am only praying that its not the thermoswitch located right on the top of the AC unit as its a dash out job to sort that. Again, if this rings any bells with any of you, advice much appreciated, especially a test schedule that is somewhat easier to understand than that in the early Repair Manual which is very ambiguous to me. Thanks once again

Good primer for testing relays. Hope it helps. Hope all thatbiscwrong is a relay

Thanks Jim, I shall look into this right away. As you say, lets hope its just that! Cheers Andrew


My first knee jerk reaction…is either a fuse problem or the power micro switch on the mode control.

The fact that you lose power to the compressor and the fans at the same time lead me to the previous two issue points.

I recommend removing the left-hand transmission vent panel…separate the white amplifier plug and check for 12 volts on the brown wire on the female side of the plug…switch the mode control knob on and off seeing if voltage is present and remains consistent with switch action. If voltage fails with ignition and climate control system on…main power micro switch would be suspect. If you have no power to the pink vacuum solenoid wire ( just ahead of the relay pack) I would suspect a fuse panel issue ( melted or heat deformed fuse panel). While in the area…also check for good earth connection on the left lower corner of the climate control box…stud with several earth connections attached

Main system power from a fuse in the main fuse panel and power feed thru the mode control assembly is the only place the fan relay coil trigger, compressor, and system sensing circuits have a common link… High amperage fan relay power is provided by a separate fuse from the auxiliary fuse panel.

Lose mounting hardware for the main power micro switch on the mode control assembly have been reported.

Just my quick thoughts…I would check those points and report back. We can dig deeper if necessary into several different tests which can be performed from the left panel…

Hope that helps



Thanks for this Guys. I have had a check and have power on the brown lead female side of the white plug. Also checked if I add 12v to Green and slate lead off the relay, This fires everything up ok all fans, clutch blower etc. However, nothing is live on the relay G/S terminal. Still guessing. Yet to check the earth and pink lead as advised.

That all means it’s a bad relay, doesn’t it?

I am not sure to be honest. If you look at the diagram here, its looks like the resistor pack controls current to the various relays.


Heavy gauge brown/yellow is power from fuse block to relay pack common high amperage feed to fan motors.

This is fused from auxiliary fuse block…50 amp british fuse (25 amp US fuse) it is common to have a 50amp US fuse installed…which can heat up and cause distortion to fuse block holder. Check connection carefully.

The resistor pack is controlled by the relays…the relays engage “legs” in the resistor pack…more resistance…lower fan speed. The resistor pack is made of NiChrome wire…it usually does not fail…but check for loose connections.

The early Delanair MKII uses a separate relay for the LOW fan speed.

What fan speed are you getting? There are 4…LOW, MED1, MED2, and HIGH.

Do you now have consistent AC compressor operation?

I agree with Kirby…if you have no power on G/S the relay is highly suspect.



I dunno where the “white plug” is in your diagram. However, I am a bit concerned that applying 12V directly to the green/slate wire fires up everything. It should operate the fans on high speed, but it should not engage the compressor clutch.

I’m gonna ask the dumb question: You’re letting the engine warm up, right? Because until the coolant temp sensor says the engine is warm enough to provide heat, the system is blocked out and won’t run.

I prefer Fig. 13.94 in the Haynes because it shows what’s going on inside the amp and the servo. A couple of notes:

  • Your diagram is in error in showing the yellow relay as going to the 3rd connection on the resistor pack. The yellow relay is low speed; my diagram shows it going to the 4th terminal. However, I think both may be in error. I think the correct arrangement of wires on the resistor pack is that the blue and G/S connections should be swapped on your diagram, and the yellow and red leads should be swapped. That would mean that in low speed current goes in the 4th terminal and out the 2nd terminal to the blowers, going through the 1.275 and the .425 resistors. In M1, current comes in the 1st and 4th terminal, feeding to the 2nd terminal from two directions. In M2, current comes in the 3rd terminal, effectively bypassing the 1.275 resistor. And in high speed, the entire resistor pack is bypassed.

  • Your diagram shows a hatched line around that first relay and another hatched line around the other three. On my '83, all four relays were in a single package. When one fails, it’s a simple matter to break out the wiring and install a separate relay to replace it.

  • The main power to the blowers is a completely different circuit than the rest of the control system. That N/W wire powers the blowers, the G wire powers everything else – including the relays that control the blowers. So, do you have power on the G wire?

I just fixed what I think is a very similar problem. No fans working and no power at relays. If you know where the control switch microswitches are, test the switched power from load power wire where it connects to the microswitch. You should have voltage here. If so your problem is the microswitch. If no power, pull the power wire off the microswitch and test voltage on the wire. If you get power on that wire, then the problem is the ground for the climate control.

Ok thanks for the replies chaps. Mine is a very early car, and has the separate relay as you detailed Gary. reference the Relay. Perhaps I am not making myself clear, not surprising as I find it pretty confusing down there!. There is power to the relay bank via the Brown/White from the fuse. This is fine. None of the terminals on the relay have any current at all what ever you do with the switches. There are two live wires into the relay. Brown and Brown and white as per my diagram above. When I apply 12v to the G/S wire everything fires up. It must as you say, be back tracking. Anyway, what stimulates the various switches within the relay to operate? Thats where the problem will be in my view. the relay terminals cannot be live unless they are instructed to by other controls or else the fans would be on full time .

hi Kirbert. The white plug is illustrated as a flat junction just above the amplifier. Yes I let the car get well above minimum 40C operating temp. The diagram is from a UK specialist which I found on the internet. I cannot vouch for it being accurate, but looks very likely to be so. Remember, my car has 2 relays, one and the other with three within its box. Later cars had just one 4 relay box. I have not checked the Green wire at the round white plug but I do have power on the brown terminal on the female side of that plug.

hi Muttony. Was this on a Delanair 2 system or a later one? Remember I do have power to the relay but nothing coming out of the relay. The micro switches are hidden within the servo unit which looks a pig to get access to on the other side of the AC/Heater unit.

I have heard the amplifiers can cause trouble. Is there a check I can carry out to see whether that’s at fault anyone?

Mine was a Delanair 2 on a 1984 Series 3 XJ 6. Keep in mind that there is battery power AND switched power at the relay block. IIRC the green wire at the relay block is the battery feed power. It will provide power that allows you to test the fans by jumping terminals. Is the brown/yellow wire at the relay block giving power with the key turned to the accessory position and the control switch turned to lo, auto, hi or def?

Also, microswitches aren’t in the servo, they’re part of the climate control fan selector switch. All power to operate the fans funnel.through that switch.

I did a lot of troubleshooting on this issue, so I’ve torn into this aspect of the Delanair 2 alot finishing most recently, yesterday.

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Please do so. Need to confirm power to the controls.

Amplifier does not affect fan coming on or not. Amplifier affects temp setting. Fans should work even with a shot amplifier.

Correction. I just checked my wiring diagram. The brown/white wire carries battery power to relay and won’t be affected by a bad switch. The power to the relay from the brown/yellow wire is the switched power for the fan.