'98 S/V/C70 AC/Cooling fan routine question.

Post by **Chuck W** » 02 Jan 2019, 08:07

Hey, all.

I'm facilitating a project and have a quick question on the cooling fan function routines on the '98MY cars.

Does the cooling fan engage whenever the AC is turned on? Either by the AC switch, or when Defrost is commanded?

I know it does on the 850's, but wasn't sure if the routine was different on the '98MY cars and it only engaged when the AC was on as needed, instead of just all the time.

Thanks!

Post by **abscate** » 02 Jan 2019, 08:10

I’m 90% confident a commanded compressor on also turns on the fan. Remember that the compressor won’t come on in cold weather until the internal car temperature goes above something like 35F and lifts the temp sensor above I the cutoff temperature.

On edit - more complicated than my Bear of Little Brain can comprehend...read on..

Post by **Chuck W** » 02 Jan 2019, 08:34

abscate wrote: ↑02 Jan 2019, 08:10 I’m 90% confident a commanded compressor on also turns on the fan. Remember that the compressor won’t come on in cold weather until the internal car temperature goes above something like 35F and lifts the temp sensor above I the cutoff temperature.

Yes, I'm aware of the cut-off temp. Been a big hassle trying to test AC things in the winter.

Just facilitating trying to get the M4.4 flashing project to work on the '97GLT (Which is the oddball of the bunch). We've gotten to the point of the AC engaging (which was the problem initially) but the cooling fan doesn't kick on when the compressor commanded. Just wanting to verify that it should on a 98MY car.

Post by **Ozark Lee** » 02 Jan 2019, 08:49

The fan is actually turned on by the ECU but the controlling Air Conditioner input to the ECU is the high side pressure switch on the A/C system.

If you have a low charge or an empty A/C system the fan will not turn on even though the switch on the climate unit is turned on. I don't know what the high pressure threshold is in PSI but it is high enough that the system would actually be operational. I have had them with a charge that is just high enough to satisfy the low pressure switch and turn on the compressor but the fan still won't turn on. When they are that low the compressor usually cycles a lot.

...Lee

Post by **Chuck W** » 02 Jan 2019, 09:05

Ozark Lee wrote: ↑02 Jan 2019, 08:49 The fan is actually turned on by the ECU but the controlling Air Conditioner input to the ECU is the high side pressure switch on the A/C system.

If you have a low charge or an empty A/C system the fan will not turn on even though the switch on the climate unit is turned on. I don't know what the high pressure threshold is in PSI but it is high enough that the system would actually be operational. I have had them with a charge that is just high enough to satisfy the low pressure switch and turn on the compressor but the fan still won't turn on. When they are that low the compressor usually cycles a lot.

...Lee

Lee, is this all M4.3/M4.4 models, or just the 98MY cars?

Here is where we are. Stock 97GLT ECU (-936), when the AC is commanded, the cooling fan engages, every time. Even if the compressor isn't cycling (It's been barely warm enough for the AC to engage). AC commanded by the switch OR in Defrost, the cooling fan kicks on.

Same car, with a flashed ECU with a modded bin (-607 with -936 AC routines), the AC compressor engages as before, but no cooling fan (Again, it's barely warm enough outside for the AC to engage).

Put the stock ECU back in, it works like before.

I'm just trying to see if this is a change in the 98MY M4.4 routines, or if they are the same between them and the 850's.
(Basically, checking to see if the problem of getting a flashed M4.4 ECU to work properly in the '97GLT is solved, or whether there is still something not quite right).

Post by **abscate** » 02 Jan 2019, 11:39

Im tapping out of this one for the experts....

Post by **jimmy57** » 02 Jan 2019, 12:11

All M4.3 and 4.4 (Mot 5.xx on P80 for that matter) use A/C high pressure sensor and pressure level to select fan speed (hi or low). Engine temp also is used but usually if fan is running for A/C pressure reason it exceeds the cooling needs for engine.
The separate EFI and Ignition on earlier 850 used road speed and compressor on/off and a higher A/C pressure switch to decide to run fan. When a/c was on and pressure switch was not closed, the fan would come on when you dropped below 8 MPH and go off when you went over 18 mph (maybe it's 24 mph) as air flow over condenser at that speed should be enough. If the high side pressure climbed above 240-ish psi then high fan was selected until the pressure fell where it would go back to the road speed basis.

Post by **abscate** » 02 Jan 2019, 12:13

Thanks as always for bringing the Five Rivers Mechanic Info to MVS, Jimmy..

Post by **Chuck W** » 02 Jan 2019, 12:20

jimmy57 wrote: ↑02 Jan 2019, 12:11 All M4.3 and 4.4 (Mot 5.xx on P80 for that matter) use A/C high pressure sensor and pressure level to select fan speed (hi or low). Engine temp also is used but usually if fan is running for A/C pressure reason it exceeds the cooling needs for engine.
The separate EFI and Ignition on earlier 850 used road speed and compressor on/off and a higher A/C pressure switch to decide to run fan. When a/c was on and pressure switch was not closed, the fan would come on when you dropped below 8 MPH and go off when you went over 18 mph (maybe it's 24 mph) as air flow over condenser at that speed should be enough. If the high side pressure climbed above 240-ish psi then high fan was selected until the pressure fell where it would go back to the road speed basis.

Thanks for the post.

So, for the sake of my question, both AC/cooling fan routines should be the same between the two in this instance? If we have cooling fan running with AC command on the 97GLT ECU, we should have the same function for a (flashed) 98MY turbo ECU? (These are tested minutes apart).

Post by **jimmy57** » 02 Jan 2019, 13:27

Yes, I don't recall there being any noticeable differences in any of the Motronic versions using two speed fans.
If you are seeing differences then check the two pole grey connector (could be black) on the fan relay. Those round female connectors have the rubbery surround material crumble and the connection can come loose. Then you can have low work and then no fan when it needs high or a delayed fan where hi is triggered for a short run and it loses fan operation when it reverts back to low but has no connection to make low speed run.