Randy V

A new development:

I discovered that there is a broken/burnt circuit run from plug connection 5 to the 12 pin on the relay. I built a solder bridge across the burnt section in that run and plugged the head unit in.

The compressor blew cold air for about 20 seconds then the solder bridge melted.



Looks like this ain't gettin' resolved tonight. And forecast of 98 in the east county tomorrow - fark!

pjg

Solder a small section of wire over the open trace.

Randy V

OK, that helps on the trace repair.

Still looking for guidance on the pin mapping.

dr bob

Email sent with diagram

CraigL

>
>The compressor blew cold air for about 20 seconds then the solder bridge melted.
>

I think you are proving that the original design is still flawed. There is too much current routed through the Climate Control under normal circumstances. When my AC clutch blew, it took out the relay and weakened one of the traces. After the trace blew, I decided to install a remote power relay.

Painless Performance sells a sealed relay kit. I mounted it just in front of the right (front) wheel cavity. I spliced into the clutch control wire using Weatherpak connectors. Power for the relay is obtained at the jump post. If for any reason I decided to restore the original connection, the Weatherpak connectors allow me to do so in minutes.

dr bob

I sent Randy a diagram that shows how to connect the new relay to the controller. Also suggested the wire for repairing the trace before I saw that Paul had done the same.

Then use an Ohm meter to check the resistance in the clutch coil. From that we can decide if it's drawing too much current. Much lower than 3 Ohms is worthy of a closer look, IMHO, since that's 4+ Amps draw. Add a 5A fuse in the compressor clutch loop, per Greg Brown's recommendation.

Randy V

That's all well and good - got any resource details, links etc.?

Dr. B was kind enough to create this map below. I've added the numbers from the original relay - perhaps confirm if I haven't wonked that up.

Correct map now posted - removed the wrong one - rv

If I install the higher rated (10 amp) Radio Shack relay with a 5 amp in-line fuse at the freeze switch location will that prevent the current overload that is burning that circuit board trace?

Alan

No it won't make that any better - but the relay will be better able to handle more power... unfortunately the traces & connectors are still a bit marginal if the loading is more than about the 2A rating for the original relay.

I think a good answer is to add a new standard SPDT bosch style relay on the CE panel before the original relay dies. You need some terminals but its robust and easily changed and takes the high current clutch circuit out of the HVAC head unit - no need to modify the HVAC head unit at all in this case.

If the HVAC relay is already dead - its also possible to swap to a simple miniature SPST relay on the HVAC head unit mounted inside to the circuit board (for the main HVAC control unit only) and still relocate the main clutch power relay onto the CE panel (driven by the same coil contacts). 12V SPST miniature relays with low ratings are quite easy to source and very small and cheap.

If I were fixing it I'd do one or other of these methods - I really don't like the pigtailed relay hanging off the back of the HVAC head unit so much...

Alan

dr bob

The second diagram.


The 5-amp fuse won't absolutely protect the trace, as so much depends on how it's repaired. The 5-Amp fuse needs to be the "weak spot" in the circuit. Fig Newton's Third Law of Electrical Reciprocity states that, in a circuit where a fuse is installed to protect an expensive component, the expensive component will fail, protecting the fuse. Seriously, fuses are installed to protect the wiring upstream of the fuse, so if the trace repair is done correctly -and- the current overload isn't significant, the fuse will do what it's supposed to. Meanwhile, heat is generated at every point in the circuit where there's resistance and current flowing. In a serious zero-Ohms short circuit, the trace will be heating at the same time the fuse is heating. The fuse -should- fail first, but there's no guarantee that the trace heating won't be enough to delaminate the trace and cause failure.


For those playing along at home, the numbers on the original relay pins on Randy's correspond to the numbers on a 16-pin "DIP' (Dual Inline Pin) socket. None of those pins is rated for more than 2-3 Amps if my foggy memory serves me.

Randy V

Hey, Alan - thanks for checking in from the UK - appreciate it.

I hear what you are saying and agree that head unit is on its last legs with the loading we are seeing.

Problem is I can follow along with pichers like this thread, but can't create your solution from the wtitten word - thanks for trying though!

Yeah, I'm askaired that even with the new relay mapped into the board the high current draw will fry that trace again and we're back at square one.

The engine bay relay solution is sounding to have more merit all the time!

Tony

FIFY Randy

Randy V

Fvck you, T!

dprantl

I went with the engine bay relay. I have seen anti-freeze switches blow out before, so to me there was no point in transferring the current load to the next failure point. A weather-proof relay in the engine bay rated for 40amps will be yawning driving the A/C clutch and all the components between it and the car including the head unit will see almost no load at all.

Dan
'91 928GT S/C 475hp/460lb.ft

MainePorsche

+1

Bigfoot928

I would also not power the relay under the hood or anywhere else in the fender from the jump post. it creates an un fused lead that is hot all the time and could short out. Follow Tony's diagram and you will be good to go.