This thread is not about the SAAB's Oil Pressure.

[Fast_Ed]

viscosityvstemperature400.gif (6.73 KiB) Viewed 536 times

Credit to elephantracing.com

Good news! A 15w40 at 250F is thinner than a 5w20 at 180f...

[kevm14]

You also have to draw bars around those graphs because there is rounding. There is no 5W-32 but calling something a 30 weight at 100°C requires a viscosity range, not an exact value. The behavior outside of the two viscosity measurement temperatures is also not linear or the same across different oils (synthetic vs conventional, etc.). Those are good for relative purposes though.

A high end synthetic 10W-30 may be thicker at 40°C than a conventional 5W-30 but it could very well have better cold pumping performance (like at -10°F) than the 5W-30. The same is certainly true on the hot end. You are probably better off, for example, with Amsoil 5W-30 with oil temps of 260°F than Castrol GTX 10W-40.

I believe there are other characteristics of oil that influence engine protection than just viscosity. Now viscosity is probably the predominant driver of oil pressure...but again, oil pressure isn't the only factor for ensuring proper bearing lubrication.

[Fast_Ed]

True, and I know they don't list brands, etc on this chart. I really just wanted to see what the curves look like. If the viscosity of a 40-weight oil is halved between 200F and 250F, then I would expect a decent increase in oil pressure by reducing my temperature by this amount.

With some loss in pressure due to the oil cooler being in use more, of course....

[Fast_Ed]

So I noticed that the oil pressure drops down to around 9-10 psi during stop and go traffic. It appeared to gain several PSI when the fan would turn on, up to around 12/13 psi. When I got to the gas station on the way home, I decided to plug in my obdII scanner and look at the coolant temp. The SAAB's temp gauge is one of those stupid computer controlled "modern" ones that only goes above halfway if there is a serious problem. The coolant temp was getting up to as much as 235F. The fan would then kick on and drop it to around 220F.

I did some searching and this is a bit high for this car. Should be in the 210F range. It turns out that this probably means that the fan is not working in low speed mode (single fan on this car..). I had wondered about the fan because I don't often hear it kick on. I even thought about turning down the temperature controller somehow using a tech II. But now I have something to work with. I can do the diagnostic and determine whether it is the relay or the fan resistor. If it is the resistor, many saab people use a $7 100w resistor from Amazon to replace the stock one.

At any rate, it seems quite plausible that I could hold a steady 15psi oil pressure in stop and go traffic with the fan kicking on at the correct temperature. Would I then have to start thinking about something other than oil pressure?

I doubt it. I recently acquired an oil temp gauge and an 80C oil thermostat. It's not like I'm not going to try those out now that I have them... And maybe some autorx to clean out the oil cooler and anything else in that engine... And BITOG recommended M1 20w50 or 15w50 for the summer on these cars. SAABcentral seems to agree on that one too. I still have lots of fun stuff to do.

[kevm14]

Interesting. Fix that fan!

[Fast_Ed]

So I have a resistor and a giant heatsink. It's just been too bitterly cold to install them.

I took a quick peak at the OEM resistor and it has three wires going to it. The Hi Fan connection and motor connection go to the same pin on the resistor, and the Lo connection goes to the other end of the resistor, passing through the 0.2 Ohm resistor on its way to the motor lead.

It seems to me that the load on the electrical system is similar whether the fan is on Hi or Lo in this type of set up? A lot of people have issues getting aftermarket resistors to stay cool, ultimately having the resistor blow and losing low fan again.

So.. Should I just remove the resistor from the path, and have the fan run on Hi in both situations? I would just connect all three wires where the resistor currently resides. Could easily be reversed. I assume fan life could be impacted.. but any other issues? Quick fan cycles maybe?

Low fan is on whenever AC is running. Would now be Hi Fan as well. Super cool AC?

[kevm14]

So I have a resistor and a giant heatsink. It's just been too bitterly cold to install them.

The irony is not lost on me.

It seems to me that the load on the electrical system is similar whether the fan is on Hi or Lo in this type of set up? A lot of people have issues getting aftermarket resistors to stay cool, ultimately having the resistor blow and losing low fan again.

So.. Should I just remove the resistor from the path, and have the fan run on Hi in both situations? I would just connect all three wires where the resistor currently resides. Could easily be reversed. I assume fan life could be impacted.. but any other issues? Quick fan cycles maybe?

Low fan is on whenever AC is running. Would now be Hi Fan as well. Super cool AC?

Is that single or dual fans? If dual, the best way to wire that is a 3 relay arrangement that puts the fans in series or parallel. Or just one, and then both.

If single, the reliable approaches seem to be Ford's PWM controller (probably doesn't help you) or a different (older) Ford single fan which has dual windings for high and low speed (could help but annoying).

The whole resistor thing sounds sketchy and I've never heard of that for a factory cooling fan.

Current draw on the electrical system would be less in low speed mode with the resistor in series, because it is limiting current. Make it a dead short and the fan draws as much power as it needs (full 12V operation). With a little bit of resistance, it becomes, say, an 8V fan. Certainly the resistor is dissipating some power but probably not enough to make the combined draw as much as without it.

Say it is a 200W fan natively. At 12V, that is 16.66A, call it 17A steady state.

In high speed mode, the fan draws 17A and thus pulls 200W on your electrical system. The effective resistance of the fan is 0.7-ohms.

In low speed mode, there is a 0.2-ohm resistor in series. That means the electrical system sees a 0.9-ohm resistance in steady state, and the total current flow is 13.33A or 160W. 13.33A through the 0.2-ohm resistor is 32W (voltage drop about 2.66V), meaning the fan draws the remaining 128W (voltage drop about 9.33V). I guess "similar" is a matter of semantics but it's not the same. Because V=I*R.

[Fast_Ed]

So I have a resistor and a giant heatsink. It's just been too bitterly cold to install them.

The irony is not lost on me.

It seems to me that the load on the electrical system is similar whether the fan is on Hi or Lo in this type of set up? A lot of people have issues getting aftermarket resistors to stay cool, ultimately having the resistor blow and losing low fan again.

So.. Should I just remove the resistor from the path, and have the fan run on Hi in both situations? I would just connect all three wires where the resistor currently resides. Could easily be reversed. I assume fan life could be impacted.. but any other issues? Quick fan cycles maybe?

Low fan is on whenever AC is running. Would now be Hi Fan as well. Super cool AC?

Is that single or dual fans? If dual, the best way to wire that is a 3 relay arrangement that puts the fans in series or parallel. Or just one, and then both.

If single, the reliable approaches seem to be Ford's PWM controller (probably doesn't help you) or a different (older) Ford single fan which has dual windings for high and low speed (could help but annoying).

The whole resistor thing sounds sketchy and I've never heard of that for a factory cooling fan.

Current draw on the electrical system would be less in low speed mode with the resistor in series, because it is limiting current. Make it a dead short and the fan draws as much power as it needs (full 12V operation). With a little bit of resistance, it becomes, say, an 8V fan. Certainly the resistor is dissipating some power but probably not enough to make the combined draw as much as without it.

Say it is a 200W fan natively. At 12V, that is 16.66A, call it 17A steady state.

In high speed mode, the fan draws 17A and thus pulls 200W on your electrical system. The effective resistance of the fan is 0.7-ohms.

In low speed mode, there is a 0.2-ohm resistor in series. That means the electrical system sees a 0.9-ohm resistance in steady state, and the total current flow is 13.33A or 160W (voltage drop about 9.33V). 13.33A through the 0.2-ohm resistor is 32W (voltage drop about 2.66V), meaning the fan draws the remaining 128W. I guess "similar" is a matter of semantics but it's not the same. Because V=I*R.

The math makes sense. In other accounts of this fix, a 50W resistor has not held up for very long ( not sure if they were mounted in an area of enough air flow). People with 100w resistors have better luck. Though one person mounted it to the washer fluid filler neck, and it melted through. Another had theirs melt through the zip ties, come loose, and melt through the battery. A comedy of errors, really...

I guess I'll begrudgingly install the resistor. I don't want the larger draw to kill the battery (low fan runs after the car has stopped sometimes) or make it annoying to run the AC. My plan is to build a small sheet metal bracket to bolt between fan supports. I will sandwich this bracket between a section cut out of the heat sink and the resistor. Should be enough cooling for this molten piece of electronics. Maybe. I will also probably build a short section of wire to keep in the car to bypass the resistor if/when it melts. The resistor has blade connectors, so it would be a simple fix as long as the mount is reachable.

As an aside, loss of this resistor in other SAABs has often been followed by sudden loss of the AC system due to overpressure when the condenser is not cooled. (because low fan is lost.) My AC stopped working suddenly a little before I first took the car apart back in 2011. Perhaps this resistor failure is the beginning of the saab's troubles. No AC, low oil pressure... All due to a fan resistor? Seems a bit too simple...

[kevm14]

So 32W of heat dissipation is well within what desktop-class CPUs have to dissipate. They use small heatsinks with fans. I think the biggest problem is making good thermal contact with the resistor. It is very easy to get heatsink/fan combos that will dissipate 32W, if you can get that 32W of heat into them. But the resistor has 4 sides and only one can contact the heatsink, unlike a CPU where the heatsink actually touches the silicon (where the heat comes from)

[kevm14]

Also that 32W thing assumes a 200W fan. In high speed it could be more than that. I mean normal 94-96 B-bodies used a 150W and a 100W fan and that is widely regarded as a lot less airflow than most 90s Ford single fans on high speed, for example.