Thar She Blows!

   The radiator fan system in many TR8s is comprised of two electric fans, two thermostatic switches, two fuses, three relays and a pressure switch if you have air conditioning. Just to confuse things, the engine starter solenoid gets a piece of the action, too.

   The fans are supposed to work like this:

   Low speed - FI: The thermostatic switch screwed into the left side of the radiator closes at 199F (93C). Relay 11 pulls in, placing the fan motors in series. That causes them to run at reduced speed, and draw one fourth the current demanded at full speed.

   High speed - FI: The thermostatic switch screwed into the intake manifold closes at 226F (108C). Relays 10 and 13 pull in, placing the fan motors in parallel for high speed operation. High speed will also be initiated if the pressure in the air conditioning system rises high enough. The high speed circuit will operate properly even if the low temp thermostatic switch or Relay 11 is broken.

   Low speed - carbs: The circuit works the same way as the one in the fuel injected cars. There is a diode added "backwards" across one of the relay coils. It prevents the high voltage spike that is generated when a coil is de-energized. The older clip-in style radiator switch may need it to live long.

   High speed - carbs: The circuit works the same way as the one in the fuel injected cars. See above.

   The fan system has some interesting quirks. Note that the coils of the three fan relays are are not grounded normally. Instead, the relay "grounds" are returned to the hot side of the starter solenoid coil. The starter solenoid winding has very low resistance - about a quarter of an ohm, to allow the high current flow necessary to provide the strong magnetic field required to move the starter pinion. Each fan relay has a coil resistance of around 80 ohms. So, the starter solenoid winding is just a fairly good ground wire as far as the fan relays are concerned. That changes when the engine is cranked. Battery voltage is applied to the starter solenoid and since the fan relay coils now have twelve volts at each end, they drop out, stopping the fans.

   High speed operation also turns off the coupe's rear window defroster in a valiant effort to bolster the alternator's often pathetic attempt to keep the battery charged.

   If you have a continuing problem with blown fuses, don't assume that it's always caused by a short circuit somewhere. Over the last 20+ years the fan motor bearings have probably dried out and gotten dirty. The increased friction causes the motors to draw more current.

   In addition, the clips in the fuse block may have lost some of their spring tension. Passing high current through a poor connection can generate a lot of heat. The clips can get hot enough to blow the fuse element right at the end, where it's not visible. Not only can they blow the fuse, the clips can get hot enough to melt the fuse block itself. If your fans stop working, check the fuses with an ohmmeter, since they can be blown while still looking okay. Tighten the fuse clips by squeezing them together a bit before replacing the fuses.

   If you have an engine overheating problem, do not short the thermostatic switch to make the fans run at high speed all the time - that will probably cause the fuses to blow when you least need trouble! I'm speaking from personal experience, here. If your fuses do blow, don't replace them with higher current ones. You'll wind up with a melted fuse block and maybe a car fire. Remember, British and American fuse ratings are different by about 2X. A 35 amp British fuse is about the same as a 17.5 amp American fuse. The British fuses are also a little shorter - forcing the wrong fuse into the holder may break it.

   I finally replaced the fan fuses in my TR8 with modern ATO blade type fuses. They still get a little warm, but never anywhere near hot enough to blow out when they shouldn't.

   If you're looking for exact replacement fans, good luck! About the only other cars sold in America that used those exact fans were Fiat 124s and (I think) Stradas. Lately, I've heard thet Porsche Boxter fans are pretty much direct replacements.

   Since the TR8 fans are set to turn on at a minimum temperature that is somewhat above the temperature where the radiator thermostat is fully open, the temperature gauge tends to bounce around if you are in slow traffic or stopped. New air conditoned TR8 owners often think that something is broken. Nope. If it drives you nuts, you can fix it by making the fans come on at a lower temperature.

   Here are some posts on the subject from the TR7/TR8 mailing list:

From:Jack B.
   The radiator switches from the 1985 to 1995 Saab 9000's are direct screw-ins and have temp ranges (cut-in/cut-off) as follows:

1985-1988   92/88C single range switch.
1998-1994   90/86C & 106/102C. Dual range. It's a three prong unit - pick your range.
1995           100/96C & 111/107C. Dual range It's a three prong unit - pick your range.

   200F is around 93C, so the 1985-1988 single range is perfect. I know I don't have one of these, but any Saab dealer can provide it. Bring your sensor in for a comparison, if you like, but I'm definitely running a SAAB switch in my '8.

   [There is also another] lower temperature switch, which is from a VW Dasher.

From: A List Member Whose Name I Have Lost
   I had a recomendation for a Standard Parts # TS-151. This has approx. the same figures as the stock high speed switch. Also, there is a part for an unknown model of VW which I am using as it is more available. This is a MTE # 823959481. The box has a /77 after this part number. I believe this to be the temp setting. It says it comes on at 75 and goes off at 70. There is another with a /J, this is supposed to have a higher setting.

From:Mark A.
   During my email thread about my TR8 overheating I mentioned I used a fan switch sensor (recommended by a LBC mechanic) that turns the fans on at a lower temperature. I've had several emails asking me what type of switch it is? It's a Jaguar part used on several models of the XJ line. The part number is DAC3061 and it costs about $50. I'm not sure of the exact temperature difference between the original one and the Jag but watching the gauge does show the fans turning on at a lower speed.

   For my TR8, I bought an adjustable aftermarket capillary-tube type switch and mounted it to the left of the radiator. I made a little metal clip to hold the probe end of the tube snugly against the bottom of the radiator without drilling any holes and used some thermal heatsink grease from Radio Shack to improve the thermal coupling between the probe and the radiator. I wired the switch in parallel with the 'Ranco' pressure switch (since it was easiest to do) and adjusted it to turn on at about 3/8 scale on the temperature gauge. No more gauge bounce, and the factory switch screwed into the radiator acts as a failsafe backup.

   I originally planned to use my new thermostatic switch to control a ten inch auxilliary fan. Attaching the new fan using its long plastic mounting pins was just too creepy and I didn't want to drill any holes or modify the car in any permanent way, so I built a frame from 'L' and 'U' shaped aluminum channel I got at a hardware store. It's designed to bolt in without any mods to an air conditioned TR8, and it will hold one or two fans. Your dimensions may vary.


   I never did install it since the stock setup seems to be working pretty well these days. I'm keeping it in the basement in case I need it, though.

   P.S. I have heard that a single 14" aftermarket fan will also easily fit in front of the radiator.