View Full Version : 2500M Came Originally With Electric Cooling Fans?

05-31-2007, 12:15 PM
A TVR mechanic just told me that my '73 2500M would have come originally with two electric cooling fans on the original radiator. True?

The previous owner installed a larger Griffith radiator but left off the fans. The temp is fine when the car is moving, but if I let it just sit idling, the temp goes to max and I have to shut it down to cool it off. Thought maybe it's just a bad thermostat? But maybe it's the missing electric fans. Mine does have one mechanical fan at the end of the water pump.

05-31-2007, 12:18 PM
Just checked my parts manual and it doesn't show two electrical cooling fans in the Cooling Section of the manual. Hmmmmm.

05-31-2007, 01:27 PM
I'm no expert, but I was told by the previous owner that my 2500M had (and still does) an engine mounted fan, and he added one electric fan to the oversized radiator to stop overheating problems. So my understanding was that they came with 1 engine driven fan.

05-31-2007, 10:56 PM
A TVR mechanic just told me that my '73 2500M would have come originally with two electric cooling fans on the original radiator. True?

The '74 2500M that I used to own (bought in '78) had two electric fans that appeared to be factory installed OEM.

06-01-2007, 06:16 AM
The 2500M original fitment would have been two electric cooling fans. This is also backed up by the parts manual.
Each of the three 2500Ms that I had, all of which were original when I bought them, had the twin cooling fan arrangement.


06-01-2007, 10:46 AM
Thanks Marshall. Were the fans originally mounted in front of the radiator or in back of it closer to the engine?


06-01-2007, 12:58 PM
No, they were mounted in front of the radiator on a bracket that held both fan motors.


06-25-2007, 02:18 AM
Well I just purchased two electric cooling fans, but not sure how to hook them up. They didn't ccme with a relay or switch. I think I need a temperature switch right? But where to I mount it? Do I have to tap into the temp sensor? If so, where shoudl it be located on my '73 2500M?

Also, where do I buy the switch and relay? I checked with our local Kragen and they just pointed me to a regular toggle switch. I want a temperature activated switch right? I guess I should also put a fuse in line somewhere? Also, since I'm installing two fans, do I need a special relay that can activate both at once?


06-25-2007, 10:58 AM
Some sensors have contacts that can handle the high currents of fan(s), usually these are the mechanical type with the small diameter capillary tube from the sensor to the control unit, often with a temp adjustment feature. I don't like these, or even the electronic sensing types that require a tube or wire to sneak past the radiator hose clamping area and into the water flow - like where the hose goes from thermostat housing to radiator. Too much risk of corrosion and leaks.

Relays. Any controller that doesn't specify current capability (Amps) that is at least the total of your two fans, should not be used without a relay. The relay is just a magnetic controlled switch. The magnet actuator in it (a coil) draws a relatively small current from the sensor unit and translates this to high current capable contacts that switch +12V from the battery (thru a fuse for protection) to the fans. Any number of fans can be controlled by one relay just hook each fan to the other; red to red, black to black (called a parallel circuit), as long as the total demand doesn't exceed the contact rating. Any number of separate relays can be used, one for each fan, as long as the sensor unit output current rating is big enough for the relay energizing coils (total of all relay inputs).

Solid state relays can also be used. They have essentially a light source (light emitting diode) adjacent to a light activated component that handles the higher current demand of the source (fan in this case). There are no moving parts or contacts to corrode. Just be careful of the polarity so you don't let the smoke out (blow it up).

So, I would look for a kit that had a sensor that either screws into the cooling system like your temp gauge, has an adjustable set point so the fans come on after the thermostat opens but before the engine gets too hot (180 or so) and has the current capacity for your two fans combined. If you find a setup that doesn't have the current capability, use the setup as a switch to energize your relay, which actually switches the fans. Many auto stores and electrical supply houses have the relay for the job. Something like a 12v input (coil or solid state type) and output capability of 25 Amps (or greater than both of your fans).

Try googling "Hayden". They make fans and thermostat kits. They have a setup that senses the radiator externally. I have also seen setups on EBay from England that clamp a small sensor onto the radiator hose externally. Again, I would prefer a sensor that taps directly into the water passage in the block or intake manifold area near the thermostat.

Also, let's ask Marshall what the original sensor setup is in the 2500M. We could adapt from there. If there's a place in the plumbing to get at the water, I'll help you find a sensor that will screw in, with the appropriate control unit.

Send me more details of your fans and I can assist further - even buy the relay for you if necessary..

Sorry for the long winded explanation. This is why my kids don't ask me how something works....

06-26-2007, 02:02 AM
Thanks Scotty for the details. Here are the specs on my fans:

Fan Specs: PER FAN

CFM Pull, 1100-1250, CFM Push 1050-1400, Dimensions 9-1/2" x 10-1/2 x 2-1/2 and Amps push/pull 3.5 +-0.5 AMP Volts 12.

Would welcome your assistance in choosing the right switch and relay. Yes, would be great to know what the original sensor setup was on a 2500M.


06-26-2007, 02:09 AM
Just checked JC Whitney. They've got three different switches. Looks like the mount somehow to the radiator core. Scotty, would you recommend one of them in particular?


06-26-2007, 12:05 PM
I didn't see current ratings for the JC thermostats, but I am sure they could be requested. Their prices look very good.

The Hayden site: Hayden Thermostats (https://www.haydenauto.com/catalogs/hayden/2007-hayden-electric-fans.pdf)

I like the model 3651 that screws into a water passage. The design looks good with a relay module that plugs onto the sensor unit. Also no current specs, but we can get this from Hayden. These look more expensive, but Hayden knows this business - is price an issue?

The question I have is where there's access to the water passage on the engine. Perhaps the Triumph guys have an idea and I will look at my buddies GT6. Sorry I sold my TR6 or I'd look there. Also, I have a message to Marshall for info on the stock setup - at least for a starting point.

If we can't find a hole for the sensor, any of the other products will work. The trick is to get a good thermal coupling to the radiator or hose. My 280i uses the "otter" switch (as many other cars do), which mounts directly to the radiator for a solid coupling to the H2O temp. I assume you would rather not modify the radiator. Does your radiator have any plugs in it (below the low water line point)??

06-26-2007, 12:45 PM
A cruise in EBay Motors proved interesting - searching on "fan thermostat".

For the radiator sense type - lets hold out for a more positive connection, like the second link:

radiator sense (https://cgi.ebay.com/ebaymotors/DERALE-16738-Single-Stage-Electric-Fan-Thermostat-180-d_W0QQitemZ110080161751QQcmdZViewItem?hash=item110 080161751)

Better sensor in H2O jacket (if available) - but not adjustable. $40:

water jacket sensor (https://cgi.ebay.com/ebaymotors/DERALE-16739-Single-Stage-Electric-Fan-Thermostat-180-d_W0QQitemZ110077404151QQcmdZViewItem?hash=item110 077404151)

adjustable, $50 - but I don't like the looks of the temp adjustment pot - I am sure it isn't sealed and could be problematic to adverse conditions (water leakage). It is the square thing with the round center - just below the wires in the upper left module photo. They pot the module, but it sure looks like a printed circuit board type of potentiometer which isn't very robust for under the hood.

adjustable (https://cgi.ebay.com/ebaymotors/DERALE-16749-Electric-Fan-Deluxe-ADJUSTABLE-Thermostat_W0QQitemZ110037877739QQcmdZViewItem?has h=item110037877739)

The relays look correct for a couple of fans, but we'll have to keep an eye on the 25Amp fuse that feeds it. Do your fans have a startup or spinnup current spec? Fans will draw considerably more current at startup. Contact the vendor if possible to see if two fans would be good for a single 25A fuse.

06-27-2007, 09:42 AM
According to Marshall the 2500M also used the otter switch. Mine is on the upper left (US drivers side) side of the rad. I wish I could get a better look at mine - on the 86 & 87 280i's the radiator is moved forward, under the wedgie nose. I was barely able to remove the bypass wire from mine and properly connect the fans (rather than being on continuously). Anyway, look for something on the rad with two spade lugs on it and I will look at the 280i manual when I get home. I don't know if the otter switch actually enters into the radiator. I seem to remember a gasket of some sort so it may be clamped in place rather than screwed. If not screwed, and assuming there's a place to put the switch (it may be there and not connected), you may want to pick up an otter switch. I can show you how to use it to turn on the relay and power your fans...

06-30-2007, 10:37 AM
Wow - I am not a very good listener (just ask my wife). I looked at the first posting to see that you don't have the original radiator (no chance of an otter switch there). One of the kits above should do just fine. Let me know if you need help with the wiring.

07-09-2007, 07:00 PM
Thanks Scotty. I just placed an order for the "radiator sense" switch. Looks like it's correct for dual fans and my radiator doesn't have a threaded port anywhere on it. So this looks like my only option. I can't remember if I asked this, but can both positive terminals from the two fans be connected to the same 12V supply line from the relay?

Also, any recommendations on mounting methods for the two fans? They are light plastic fans with four 1/2" holes around the circumference of the each fan body. Doesn't look like anyone sells mounting kits. Long zip ties seems to be a popular way to go, but doesn't seem like a very secure method to me.


08-16-2007, 11:54 PM
Well, I finally completed the installation of my cooling fans. The only thing I'm not sure I did right is I mounted both fans on the engine side of the radiator and they are pushing air through the radiator toward the front of the car. I also still have the engine driven fan hooked up. Wouldn't this configuration still work? All engine driven fans are pushing air toward the front of the car, through the radiator. So doesn't it make sense to have the electric fans also push air to the front?

But I'm reading some online instructions that state "push" fans should be mounted at the front of the radiator to push air toward the engine. So technically, I think I'm supposed to rewire my fans to "pull" air through the radiator. But still doesn't make sense to me.


08-17-2007, 11:31 AM
As you know, I responded to your personal mail, but at the risk of being beheaded by this forum I thought it a good idea to state my response here.

I have never heard of engine or electric cooling fans oriented to impede the natural flow of air thru the rad. For almost every car this would mean engine and/or electric fan pushes or draws in the direction of front to rear of car. Opposing the natural air flow direction as the car moves forward would decrease (or completely null) the cooling effect of air ducting from the front of the car.

Check the engine fan. It should be moving air from front to rear - remember it will be turning clockwise from the front (except modern serpentine belted systems that may be counter clockwise due to being driven from the back side of the belt). If it isn't drawing, I would question its source. If it is in fact pushing the air forward and far back from the rad and un shrouded as you indicated earlier, I would remove it and let the electric fans do their thing (in the correct air flow orientation).

Swap the electric fan direction (following the directions - some swap polarity) so air moves from front to rear of rad (and car).

08-17-2007, 12:16 PM
P.S. Turning the engine fan around won't change the air flow direction. Like threads, props are built (threads cut or rolled) as pushers or pullers (right hand or left hand threads).

08-17-2007, 03:19 PM
Thanks Scotty. Dummy me. I always assumed an engine fan on any car was pushing air toward the radiator. Totally makes sense that it would pull, not push. I'll swap polarity on my electric fans. Yeesh. Good thing I asked!