Fan Fail Module - Circuit & operation

[nkemp]

Here is an attempt to document the fan fail module. I realize there are multiple other options to this module so this may be mostly for posterity.

Regardless, this circuit is interesting. It is not simply logic to determine if 12Vdc are present but appears to be monitoring the actual load. If this is the case, it would sense where 12Vdc is supplied to the motor but the motor(s) (fan 1, fan 2 or both) is/are not operating.

Here is a crude circuit diagram for the fan fail module.
6596


Power enters and using heavy gauge wire makes 4 loops around the coil

After the coil it goes to both fan terminals


The coil uses fine gauge wire and makes hundreds or thousands of loops
Inside the coil is "something" I am guessing that it is a reed relay (normally open)

This controls the Fan Fail light


The unit does not monitor each fan independently

So for those who know electronics (or electrics as the case may be):


What does it do?
How does it do it?

In this module, the case showed signs of overheating and the 2 47Ohm resistors showed signs of overheating. Everything else seems OK. I am currently running DPNW fans (and recommend them). With the module installed the fan fail light would illuminate.


Is the circuit tuned/designed to a specific electrical load?
Would it need redesign for lighter load fans? (again for posterity's sake assuming there are other alternatives)

Here are some images. Note that the 2 47 Ohm +/-5% resistors (in series for 94 Ohms total)) are removed from the module and not shown).
6597659865996600

"DFF Proj 30" ===> ?DeLorean Fan Fail Project #30?

Nick

[nkemp]

I contacted the mfr with a few questions about the fan fail module operation. Here is his reply:
"The DeLorean fan fail unit was very simple, it had counter wound coils in the 2 fan supply line which were wound round a reed switch. The current through each coil created a magnetic field, if both fans are running the fields cancel each other out and the reed switch stays open.

If either fan failed, the field of the running fan will cause the reed switch to close signalling a failed fan."

I asked more questions about its operation with lower current load fans and here is his reply:
"If you've swapped to lower current draw fans then that is fine, better in fact.
As long as the fans are both the same then the fields will cancel each other and the principal of the unit remains the same.

The unit was originally designed with a lower current fan, but then during the vehicle development DeLorean had cooling issues (it was always marginal on cooling) and swapped to much bigger fans without telling us, that's why the units fail, the fans take much more current than the unit was original designed for.

The resistors are I believe in line with the signal telltale so yes, any resistor of the same value will do.

Often it's the reed switch that goes, it's quite a sensitive reed, 12 to 15 ampere turns I believe."

Reproduced with permission.

Nick

[David T]

Could be that when DMC went into production the fan motors they picked were a higher current draw than the ones they had planned to use. In any case we know what the end result turned out to be. The fan fail relay is probably ideal for the lower current Spall fans. As bad as the cooling system in the Delorean is the electrical system is borderline overloaded. I consider both of these systems in the Delorean as the most troublesome and in need of the most attention.
David Teitelbaum

[Farrar]

Could be that when DMC went into production the fan motors they picked were a higher current draw than the ones they had planned to use.

Ayup:


The unit was originally designed with a lower current fan, but then during the vehicle development DeLorean ... swapped to much bigger fans ... [that] take much more current than the unit was original designed for.

[DCUK Martin]

What a load of rubbish.

The fan fail module fails because of mechanical failure of the solder joins where the pins join the PCB. The pins are too small and are prone to flexing when the module is being plugged in. Every unit I've ever taken apart has "dry joints" where the main three pins join the board.

This has nothing to do with the current draw of the fans and everything to do with poor design and manufacture.

The operation of the circuit itself is very simple and elegant.

[Elvis]

one of the first things I took apart after I got the car, right after the door lock ECU, was the fan fail module - after I got to hear so much BS about it. Nobody could tell me how it works and what was failing. Years later "Gary" showed up and confirmed my theory.
This is like almost 10 years ago, I think this information has been up several times here in the forum.

Attached is the schematic I drew lightyears ago.

There is only one coil (L2) with 4 windings to measure the current of both fans.
The solder joints can't handle that high current and maybe Martin is right and there is also mechanical stress on those joints.
The inner coil (L1) with the many windings is to adjust the magnetic field that the reed sensor sees or that the current thru the fans has to overcome to turn off the reed sensor. One single fan wouldn't turn the reed switch off.

I don't think those were two identical resistors, and if they got hot - that doesn't matter. To adjust it to low current fans you would have to change R1 and/or reduce the number of windings of L1 or increase the windings of L2.

It is a nice tricky circuit but poorly designed to handle the high current. They should have used a two sided PCB with thru hole connections and this thing would have worked fine.

You can repair it by resoldering it and soldering extra wires to the high current leadings pins, but once the housing is open it looks awful and glueing it back together - naaaaa. It can be done, but I don't suggest to reuse it.
Better solutions are out there and have been discussed in many other threads.

[DCUK Martin]

I'm also interested in the talk of cooling problems. The DeLorean radiator is a sensible size for the job - in fact it's very similar to the Alpine GTA unit.

I wonder what the history is of these "cooling problems".

[Farrar]

I wonder what the history is of these "cooling problems".

Hear, hear! This is something else I have seen no proof of. People like to say "the cooling system on the DeLorean was notoriously poor." And yet no one I have ever known has overheated their DeLorean without doing something else that would have overheated any other car, like bursting a hose and therefore losing coolant. It's not like the DeLorean's water pump will fail without warning, and even in summer with NO radiator fans, I managed to keep the car operating at a safe temperature by simply moving at 45 MPH or above. In fact, the radiator fans don't even come on when the ambient temperature is below 80°F unless I am sitting in heavy traffic.

I think the DMC-12's "shoddy cooling system" is just another urban legend.

Farrar

[Elvis]

A little leak in the VOD and you see the difference between Delorean and my MGB.

I once forgot to close the MGB radiator and lost about 3 Liters of water.
I didn't even see the temp go up, nothing.

I had a leak lately on a test drive with my little orphan I have here to fix
some stuff. 2 Liters lost because of litte leaks in the VOD - heat went up,
needle showed me where heaven is !! Outside temp was only ~5 degreeC !!!!
I hopefully haven't blown a head gasket.
I'm installing a low water switch now. Just in case...

oh well yes - back then in Italy when we drove up to San Marino many of
our cars had heat problems, this was before I replaced the 40A (originally 30...35A)
circuit breaker and made the fan circuit bullet proof.

[DCUK Martin]

It's not really a design flaw in the car though is it? (Although I'm sure Bill would have something to say on the subject!)

It's one of simple geography - there isn't a lot of coolant volume above the water pump. And assuming no leaks, this has no effect on the cooling system.

I took #1458 to LeMans and was stuck in traffic in 40 degree heat for a couple of hours without a single issue. My cooling system was 100% original apart from the expansion tank.

[nkemp]

I thought the engineer's comment that the fans were changed at the last minute was the interesting part. Changing to higher load fans explains a lot with regard that whole electrical circuit (breakers, connectors, controls, etc).

Design flaw ... Hmmm...I can't get to that conclusion. I don't recall late 70's car technology being all that great in almost any regard. And yes we now know better ways to do a lot more things. For that matter, how many cars of that vintage are still running or look as good. Most are compressed into a 2 foot cube by now.


Attached is the schematic I drew lightyears ago.



Thanks for the schematic. It is much better than my drawing. I searched for a schematic but with no luck. It may have been lost with dmctalk.com. When I started this I mostly wanted to document the design and understand how it works. It is an "interesting" circuit/approach. Two coils cancelling each other out in normal operation and closing a reed relay when a fan fails.

The engineer's comments were interesting.

[Ron]

The reed switch/relay was almost half a century old when the De Lorean was made...

I think Elvis was talking about a flaw in the "our" implementation, not in general.

[Dracula]

It's not like the DeLorean's water pump will fail without warning...

Mine did, actually. I had just left DMCMW for some fairly simple fine-tuning of the Stage II setup they had just installed for me and was on my way to an Anime convention. The car sensed my nerdyness and the pulley on the water pump wouldn't have any part of it and backed off; the belt disconnected and, since the cooling fans had been running due to the stop-and-go traffic I had been stuck in, the car overheated. I was with my soon-to-be ex-girlfriend at the time, so that made things even worse.

[DCUK Martin]

That's a rare failure, and not really a "fault" in the cooling system (ie one that'd be fixed with bigger radiator fans!)

Bad luck with that one, hope your engine survived!

[nkemp]

Let's remember that the original problem with the D's cooling system was that it did not come with a method of automatically bleeding the air from the cooling system. Mine got pretty hot one time due to an air bubble in the thermostat housing. When I tried loosening the bleeder it was frozen to the housing. Once I installed the bypass, it runs cooler than normal (likely due to bypassing the thermostat with just enough flow to keep the temp down... I've been meaning to restrict the flow).

Maybe the original cooling issues he mentions were related to air in the system....Or maybe not... it could have been a function of the early prototype designs. We may never know.

[Dracula]

That's a rare failure, and not really a "fault" in the cooling system (ie one that'd be fixed with bigger radiator fans!)

Bad luck with that one, hope your engine survived!

I know it's a rare failure, thankfully. It's also not something I'd consider a design flaw with the car; perhaps the manner in which the water pump was constructed, but nothing else.

So far, my car's had about 15,000 miles of problem-free driving ever since.

[DCUK Martin]

Let's remember that the original problem with the D's cooling system was that it did not come with a method of automatically bleeding the air from the cooling system.

Is it? I believe most cars have manual bleed points.

The Alpine GTA has a bleed on the main hose exiting from the stat so it performs exactly like our "self bleed kits".

[David T]

Many cars have bleed points. The biggest problem is once the system starts to leak you have to continually bleed the system and fill it. If you do not bleed it and keep it full eventually it can get so low that even if you fill it (and do not bleed the air out) the system can become "airbound" and no longer pump coolant. Small quantities of air can "self-bleed" out of the system. On the Delorean, with all of the plumbing to get the coolant from the back of the car to the front and back again, there are several places for air to collect and a lot of potential leaks. Especially if the car is not maintained.
David Teitelbaum