1987 SIII sedan probably needs an HVAC system amplifier replacement or repair. The car is stuck in defrost mode one day, then the servos work the next day but the fans don't.

And, at no point will the system get hot. It's cold air, all the time.

I'm going to attempt to do this work myself. How hard is it to get the old amplifier out and put in a new one? Is there a step-by-step out there on this? I did try the "search" feature first but never found exactly what I was looking for.

Jess

 

Its been a long time since I was that brave.

It is basically loctated behind the stereo, but only accessable via the LH console cheek. The 4 pack relay unit for the fans must be removed first, and then with heaps of patience it can be unclipped from the DUMB hooked clip and withdrawn.

Only issue is that clip is STRONG, and access it IMPOSSIBLE, and lots of foul words will be said.

If it has been replaced with an aftermarket unit, luck may on your side, as they are mostly smaller, and just sitting in the hole, not clipped in, whoopee.

RHD we have slightly better room to get in the general area, LHD, good luck.

On our S2's, I found that the RH switch pack retaining setscrews were loose by 1/4 turn, and once they were tightened, CAREFULLY, everything worked sweet. Access to them is by taking the radio out, the fascia off, and then with nibble fingers and a 4mm spanner, the 2 long setscrews that hold that multi switchpack together and in place can be tightened. It is a task equal in difficulty/stupidity to the amp removal.

 

Don't bother trying to remove the old amplifier, not worth the trouble. All you have to do is disconnect / separate the round, white plastic, 7-wire connector, and just connect the new amplifier to the proper plug half. How do you know the amplifier is the problem? If there is no heated air, that points to the Heater Valve or a vacuum issue, since the DEFAULT action of the climate system is hot air all the time when the amplifier fails. Try this: warm engine up. -Heater does not operate until engine is warm enough-. With someone else at the climate controls, tell them to place the Mode switch in AUTO, them tell them to rotate the Temp switch from 65 to 85 as you watch the linkage under the heater valve moving up and down in response to the rotation of the Temp switch. This will confirm if the Heater Valve is operating, opening and closing as it should. The Heater Valve is located at the center of the firewall.

 

Right. Just tuck the new one in there however you can





Sorry, but that simply is not true



I agree that check the heater valve is a good idea....but even if it has failed it would explain the erratic operation described in post #1


Cheers
DD

 

Doug, what is not true? You don't explain....

 

You said:

".....since the DEFAULT action of the climate system is hot air all the time when the amplifier fails. "


That is not the case. Amplifiers can and do fail without the system defaulting to heating mode....because there is nothing in the design of the amplifier to accomplish that.

In fact, amplifiers often signal that they've failed by giving erratic operation....as described in the first post. I've replaced a couple because of that. Or, sometimes they fail by becoming ''stuck' at the least setting/reading the were in, even if it was a cold setting. No default to heating mode in any of those cases.

However, the heater valve is designed to default to heat (default to 'open', that is) if vacuum supply is lost. Perhaps that's what you were thinking of.

Cheers
DD

 

Thanks for the suggestions. It's good to know I don't have to actually remove the thing. If the plug is accessible as it is in my XJS, I'll go ahead and do the work myself.

This is a rundown of some of the behavior I've had from it over the past couple of weeks:

Scenario #1: No operation whatsover, no matter the setting. No servo noise, no airflow, no nothing. Like a fuse was blown (which was my first thought).

Scenario #2: Stuck in defrost with the compressor on, churning out frigid air right into the windscreen. This was a nice touch considering the outside temp was around 25 at the time.

Scenario #3: Same as #2 above, except "LOW," "HIGH" and "AUTO" produced nothing, no servo sounds, etc. It would function in defrost but on no other setting.

Scenario #4 (this is where we are now): Servos seem to be working properly (and noisily, at that). "LOW", "HIGH" and "AUTO" produce nothing in the way of fan speed, however. It's like the system is forever waiting for the engine to warm up before it cuts on the heater system (think 80s/90s Ford HVAC unit operation in cold weather). Defrost works as advertised, but I still can't make it warm up.

Given I live in the American South, I can just wait about a month and I'll be fine. Our "winter" here ends around March 1. But it's bloody annoying right now.

Jess

 

Not sure of access on the LHD cars.

Up on the heater core copper pipes INSIDE the cabin (behind our glovebox and up high) is a "pad style" temp sensor, that ceases all operations of the system, except defrost, until that copper pipe is at a perdetermined temp.

Its Lucas, and fails.

I simply join the 2 wires and move on.

MAYBE.

 

There are four fuses for the system. Checking all of them for continuity and corrosion would be a good idea. Two are in the auxiliary fuse box below the glove box (on your LHD car), one is behind the console's RH 'cheek' panel on the brown and brown/yellow wires, and the last one is on the amplifier ground wire




As an aside I'll mention, in case you didn't know, that the compressor runs in all modes....even when heating is called for




Might be a poor connection, might be a failing amp, might be a failing servo assembly. However, failure of the servo assembly (which is a fascinating device) is rare-ish. Poor connections and amp failures are common.


When you adjust the temp knob from max hot to max cold, and the servo whirs, does the airflow change from footwell vents to the center dash vent? If it does, the system understands your request and is trying to comply




Yeah, if you are calling for heat the system won't come to life until coolant temp is about 150ºF or so....as sensed by a sensor on one of the heater pipes at the heater core. Of course, if the heater valve can't/won't open, or isn't being commanded to open, the pipe won't ever get hot enough to close the switch



Defrost is partially divorced from the 'auto' aspect of the system. It defaults to max fan speed and max heat. But, again, if the heater valve isn't opening......


My gut feeling is that you have an amp failure, or perhaps both an amp failure and a heater valve failure. But, check the heater valve first.

The system is designed to refrigerate all incoming air first and then heat it as required.

The heater valve should be open in all settings except max cooling, when vacuum is then applied and it will close.

Cheers
DD

 

Doug, scary question here, but how difficult is it to get at the temp switch for the heater pipe?

I would prefer to jumper it as Grant suggests, if there's an easy access point for it. I'd rather have control of that function myself.

Jess

 

Fairly easy.

Remove the LH under-dash trim panel. Look up into the dash, roughly above the throttle pedal, and you'll see the heater core pipes. The switch is plainly visible. It's screwed onto a little tab on one of the pipes.

Cheers
DD

 

A couple of small points, one on topic, the other only a bit.


1. A warm up is needed before warm air comes from the vents. Stage one, engine coolant warms. Stage two, warm air warms the duct work.
Each needed before one feels the warm air.


2. My son has been messing with his ancient CNC mills. One is modern computer controlled X, Y and Z movement of the table.
At my last visit, he had the lay out in test mode on the dining room table. Four servo' reacting to a program on a lap top. the fourth is to command the spindle!!! I think???

Yup, Doug, those servo's are truly entrancing....


Carl

 

Could I jump in here with a related question rather than start a new thread.

Assume: Engine warmed up;temperature dial set to 85F; blowers/fans are working at low, auto and high settings, but only getting cold air nothing warm. Would that indicate the switch on the heater pipe is not the issue? If that switch is not closing it sounds like I wouldn't get fan operation.

Thanks, just trying to eliminate that switch as a possible cause and then focus on checking heater valve and ac amp as the problem.

 

Carl,

Thanks for chiming in; the car had plenty of time to warm (like, 20 min) and the temp needle was almost halfway up the dial. Probably about 160-180F if I'm converting numbers properly.

We tested it again today, warm day, after a long (1 hour) drive. Nothing but cold air on any mode.

Jess

p.s.: And the speedo took a dump today. Of course. It has to be a speed sending unit, because the computer also stopped logging miles. Another day in paradise!

 

Right. If that temp switch was the issue your fans wouldn't operate.

If the system sees that heating is called for and the mode switch in auto, lo, or high nothing will happen until that temp switch sees xxx-degrees. When xxx-degrees is reached, the fans will come to life.

But the system is smart. If it sees that *cooling* is called for, it'll turn the fans on right away....with no regard to that coolant temp switch

Cheers
DD

 

I had similar issues when I had my Jag. Here is the link to the thread I had. It has diagrams and great advise from the gurus in it. Hope it is of some use :-) p.s. I never fully solved my issue, but determined it was a faulty servo unit. https://www.jaguarforums.com/forum/x...light=confused