S Type heater motor, tech question

I recently learned on a forum for the BMC 1100/1300 cars that if one converts the car from factory positive earth to negative earth it is also necessary to reverse the leads to the Smiths heater motor. Otherwise the motor runs in reverse although it will still blow air out but in quite diminished volumn. I proved this with my Austin America which was long ago converted from positive to negative ground. A recent swapping of the leads to the motor on my Austin resulted in a substantial air flow improvement although it was not non existent when wired incorrectly before. A check of these motors shows a label for which way the Lucar spade connectors are to be plugged into the motor depending of the ground set up. Now I am wondering if all of us with Mark II or S Types who have converted from positive to negative earth might also have compromised air flow from our motors? The Jaguar fan is a squirrel wheel type while the Austin is a turbine fan type. Would this make any difference? Are there any experts out there on the motors fitted to our Jaguars? Anyone verified the Jaguar motors are unidirectional irrespective of positive or negative earth? Here are pics of the one used in the BMC 1100 range.

Jaguar used series motors on our cars so they turn the same way no matter
how they are wired. If they hadn’t, I can only imagine how the windshield
wipers would behave with reversed polarity. Come to think of it, they would
probably be unnecessary. All you would need to do is turn on the windshield
washers and they would vacuum all the water off the windshield.

Mike Eck
New Jersey, USA
www.jaguarclock.com
'51 XK120 OTS, '62 3.8 MK2 MOD, '72 SIII E-Type 2+2

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One of the problems I’ve been thinking about on my car for many years is the flow of air; good (uprated) fan; (full Clayton upgrade box etc.), seals in place, yet the flow of air (heat is fine) is poor. I tried to re-check the seals by running the fan and holding my hand around the area between heater box and bulkhead (firewall in U.S. speak) and was getting a draft of air from around the fan aperture seal to bulkhead area. My car is positive earth; I note Mike’s comment but still wonder might it be the fan is running backwards* and pushing the air the wrong way?

*If the Clayton installation had assumed a negative earth installation?

Snicker…:laughing: Eck, FT humor W!

If it’s not an original motor then all bets are off.

Mike Eck
New Jersey, USA
www.jaguarclock.com
'51 XK120 OTS, '62 3.8 MK2 MOD, '72 SIII E-Type 2+2

One of the problems I’ve been thinking about on my car for many years is the
flow of air; good (uprated) fan; (full Clayton upgrade box etc.), seals in
place, yet the flow of air (heat is fine) is poor. I tried to re-check the
seals by running the fan and holding my hand around the area between heater
box and bulkhead (firewall in U.S. speak) and was getting a draft of air
from around the fan aperture seal to bulkhead area. My car is positive
earth; I note Mike’s comment but still wonder might it be the fan is running
backwards* and pushing the air the wrong way?

*If the Clayton installation had assumed a negative earth installation?

that looks the same as the 420G motor.

I replaced one with a GM motor that spins either way, depends on how you wire it,
it has 4 speeds, 3rd is more than stock, and 4th is galeforce.

The spindle diameter must match, and the squirrel cage shaft be suitable fora “true” remount on new motor shaft, there is not much clearance within the cage

one must modify & fabricate new motor mounting ears

the squirrel cage blades must be oriented in the right direction, they are inefficient if run backward

If you are able to dismount the motor from the heater box and hence isolate it from the car you could test with a + and - wire at the terminals of the motor without risk of a short. If the motor were to be a permanent magnet type the rotation would change direction as you swap the polarity. At least this way you would know. Paul

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Typically lazy of me, but is there a safe way to test / try different connections to see if rotation is the issue? I would like to assess the airflow performance with heater box etc still in place.

Tony, I know this is a while ago, but what GM motor did you use please? Thanks Chris

Thats a good question, which I was unable to answer from searching the archives

I have a strong notion it may have been VT-VZ circulation fan, but unfortunately may be wrong

best bet is to take your Jag fan to the Commodore wrecker and ask them if its the VT-VZ one that looks fairly similar

The first guy I dealt was a top fella, instantly grabbed one off the shelf and practically threw it at me for about $20, didnt try to rip me off when I told him it was for a Jag. Other owners are not so helpful. They probably dont fail that often.

I thought I did a writeup with pics for this, but cant find it

anyway, check the spindle diameter is the same as yours, as you must re-use your squirrel cage, the GM one is too big, also only 2 of the 3 mount studs line up, I worked around that with a bracket, or something. Its got to be right as any misalignment means the fan blades will strike the blower housing. The motors are very close to being the same size, the GM one is a few mm bigger

I think I ran a separate fused wire, with relay, as the GM fan pulls a lot more current on max (and moves a gale of air)

I meant to fit Pulse width modulation DC motor control, for ultimate control over fan speed, but alas, have not done this experimental mod yet !

Can’t imagine what the gale force speed sounds like. Hi speed on my 3.8S sounds like a small jet engine winding up behind the dash!

Does this look like it? Its a VT,VY commodore. VZ is a totally sealed in plastic job… off the Pick a Part and grab a couple… cheers

$40 aud on ebay! tested and delivered…cheers

that does look like it.

I cant assure you your fan motor is the same size as MKX/420G, good chance it is though

I will have a peak at mine tommorow to see how I mounted it

On my car its stupid how many things are a super tight fit for such an enormous car

admitedly its mainly when you do mods like alternator, fans, radiators etc

setting 3 is similar to the maximum Jag 4 setting

setting 4 on the GM unit is like having a 747 in your duct, it roars, and air output is very high, I recall it draws twice the maximum current, hence the new fused wire & relay

I only fitted this experimentally as part of an aircon system upgrade, and Its not needed for heating alone (imo)

Its a reasonable solution if you have a failed OEM unit I imagine, I have some failed ones, but you can sometimes repair them yourself

I dont know whether “new” ones are available, their would be used ones on Ebay etc, probably they would be expensive, and unreliable if used

will confirm for you that I painted the item black, so it does not stand out as different to the original Lucas motor

I wouldnt neccesarily do it again ( ie to the other side yet), but its also true that sometimes, for a short period of time, there is no substitute for gale force air movement

Tony, I bought a fan from pick a part… and the Commodore resistor. I will have a go at using that too. maybe use speeds 2 and 3… Anyway, to mount it I wanted to keep the fan motor as standard as possible so I could just buy another if needs be. Firstly, the commodore shaft is bigger so I drilled the mark 2 fan barrel out to 7mm. Now a nice snug fit but it does not use the flat shaft drive… just press on and tighten the standard not on the end of the shaft. Then I cut down the plastic shroud off the commodore fan. The fan mounts to this and its very assymetric. Only goes on one way… different angle and different heights!. Then I used an old tin of the right size (130mm OD) to make a housing for the plastic shroud, and riveted the tin to the plastic. Had to do some measuring with the fan in place to work out how high the tine needed to be. A few holes later, its on the heater and works a treat. Blows VERY well. Attached a re some pics of my proto type. I have glued up the gaps between the tin and the plastic. When it dries I will pull it off the heater and re “clock” it. You can see from the pic of the heater sitting on the ca that it is VERY close to the cam covers at the 8 o’clock position. I will move that to 9 o’clock in V2.0!! I had left that gap at about 2 o’clock to try a fit the commodore resister there. will have to re think that.

So total cost so far… $23, a few rivets and an old tin! I need to pretty it up a bit in V2 but I reckon it will work fine. cheers




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looks like a good job, just paint it black, and its hard to notice

on the 420G, there is 2 fans at either end of a heater box

I think I used the commodore resistors, wasnt able to look at my setup today

one thing for sure, it definitely does move more air on max

looks a bit tight to your cam cover, so lucky it fit

think I cut one leg down