bowtie350_428

After replacing both fan motors, I am Still having problems with Radiator Fans on my 91 ZF6 L98. This time it is the Aux Fan. It never Turns on, even when the A/C is running! It seems as if the ECM is just not sending the ground to the Aux Fan relay!

The only way I got the the aux fan to turn on is when I use a test probe light to test the Blue/White wire on the Aux relay. When use the probe light to test the Blue/White wire, the probe light will glow very faintly, then the Aux Fan will turn on. But it probably turns on because I am just grounding the Fan relay with the probe light instead of the ECM.

Another thing I noticed is that sometimes the aux fan relay would click on and off erratically within a second or two and without moving any wires or without touching the car.

I connected the Laptop and recorded a datalog(DataMasters) and I am still trying to figure out which Bit in the “Status Bit Display” represents the Primary Fan and Aux Fan. “Fan 2 PID”, “Fan 2 Rqst”, “Fan 1 was on”... what the #^@%???

1. Does anyone know which bit in the “Status Bit Display” represents the signal to turn on and off both fans?

2. Everyone says that Both fans should ALWAYS come when the A/C is in use. Can anyone tell me if they have a 91 where it is Normal to only have the Primary Fan come on with the A/C?

Any help or input you can would greatly be apreaciated. Thanks

rick lambert

Don't know if a 91 is like an 87, if it is I'd replace the aux fan temp switch if your relay is working.On an 87 the main fan is controlled by the ECM-the aux is not.

65Z01

Have tried unplugging the A/C pressure switch and running the engine. This should simulate an over pressure condition and may cause the ECM to bring on the secondary fan.

CFI-EFI

I think the aux fan is computer controlled in 1991. But even in your 1987, the A/C activating the aux fan has nothing to do with the temp switch.

bowtie350_428,
It almost sounds as though there may be a pressure problem in the A/C system.

RACE ON!!!

bowtie350_428

I was told that both Primary & Aux Fans are controlled by the ECM. I don't believe I have a temp "switch" for the fans like previous models. It uses a temp sensor to sends signals to ECM, then the ECM decides to switch on the fans.

So 65Z01, if I disconnect the High Side Pressure A/C Switch, this would trick the ECM to turn on the Aux Fan? I will try experementing the A/C pressure switch when I get home.

The thing is I was told BOTH FANS ALWAYS go with the A/C turned on. Is this not True?

I was also told that when you start the car, both fans should turn on for a few seconds just to let you know that they are both working. Is this true or not?

SunCr

If you disconnect the pressure switch, the ECM is going to sense an open in that circuit and get 0 voltage or no pressure on the return or signal line. That will shut the a/c down, but I don't think it's going to generate a ground signal for the fans. If I remember correctly, the 5 volt reference to the switch is Black/Red, Black is ground, so whatever the third color is the return signal. It needs to see about 1.5 volts on the return to ground the relays - otherwise the signal is from the CST. I'd start by swapping relays for the fans (assuming they're interchangeable) - if the secondary pops on, you know what you need. If not, and the secondary isn't coming on at about 221 with the a/c off, suspect an ECM problem, but a scanner would really help you find it quickly.

SunCr

Ok - reread your post - Here's what you should look at: What is the CST signal or temp? Is the a/c request Yes or No? Pressure voltage and actual PSI is what?

With an a/c request and voltage of about 1.5 or 150 psi, check for ground at the secondary Relay. If the CST is about 221, check the same ground. If it's there and/or the Relay is clicking on and off the Relay is probably shot. If not, suspect the ECM, but ohm out the ground or driver wire back to the ECM.

bowtie350_428

I swapped connections on the relays and they both behave the same when connected to the Aux Fan Signal Wire (Blue/White). So I know that the relays are fine.

It just seems like the ECM is just turning on the Aux Fan Relay. That’s why I want to know how to tell if I have one of the Following problems:

1. The ECM is sending the signal but the signal is not getting to the relay OR…

2. The ECM is just not sending the signal at all.

Thanks

SunCr

Put your meter on the ECM side of the Relay and see if there's voltage with the a/c on or a pressure signal of 1.5 volts or with CTS of about 221.

bowtie350_428

I believe I know what is going on. I think I have a loose connection at the ECM.

Yesterday the DataLog recording, taken from TTS DataMasters ScanTool Pgm, says that “Fan 2 PID = 1” and “Fan 2 Rqst = 1” while the A/C was running. So if I am reading the “Status Bit Display”, this means the ECM was sending the signal to the Aux Fan but somehow the Signal never reached the relay.

When I swapped relays, I would get the same results. Pri was working Aux was off. So it was not the relays.

A loose connection that goes from the ECM to the Relay is the only thing that makes sense. This would be the Blue/White wire.

Today, I unbolted the ECM to get a closer look at the wiring, then I bolted it back on. Turned on the car with the A/C running and guess what... both the Primary and Aux Fan turned on. I must of wiggled something and now the signal reaches the relay and the Aux fan works. Next, I will check the Blue/White wire leading out of the ECM for a bad connection.

Thanks for everyone’s input.

65Z01

As mentioned, I would test the Blu/Wht connection at the relay when the scanner shows the ECM is calling for the fan On. At this time the ECM should be pulling the line low; at other times the line will be either floating or at +5Vdc.

If the On command is getting to the relay, check the connector just after the fusable link that powers that relay. On my '88 there is a dual connector near the battery into which go the two fusable links, one for each fan. Follow the two Orn fusable links from the battery Pos. terminal.

Phil stock 91

I have a 91 doing the same thing ECM not turning on the aux fan all the time no it is not supost to run all the time the a/c is on .I could not find anything wrong except the ECM is not providing a strong enough ground to operate the relay every time it called the fan to come on.after figuring out how the system works I spliced in to the wires on both relays so I can turn fans either one or both on when I want with or with out A/c on now I do not have to waite for it to get so hot in trafic with out A/c before fans come on and it beat buying a ECM for now

SunCr

A ground is a ground - it's either there or it isn't. You could ohm out the wire between the ECM and the relay, give it the wiggle test too.

Phil stock 91

yep a ground is a ground maybe shoud have intermitent but one time it is there next time it is not

Phil stock 91

yep a ground is a ground maybe shoud have said intermitent but one time it is there next time it is not

SunCr

That's why you shake it - but no more than 3 times.

bowtie350_428

I soldered new fuseable links when I changed the fan motors. I have good power at the relay coming from the battery. And just as you mentioned SunCr, I also check the wire between the Relay and ECM and I have good continuity.

I believe I have a problem with the pin that connects the ECM to the Blue/White wire. Atleast I am hoping that is the problem and not the ECM.

I know that there is a tech article on how to have both fans come on at once. It is just a matter of connecting both signal wires from the ECM that ground the relay to turn on. But now I am paranoid about the pins connecting the wires to the ECM. Who's knows if the primary fan signal will also loose contact.

When I get a chance, I am going to check all pin connections going to the ECM.

65Z01

When you force the ECM to call for the secondary fan On what is the voltage reading on the Blu/Wht wire to that fan's relay?

If the ECM cannot sink sufficient current the voltage should be rather high, else it should be close to 0Vdc.

BTW, I've been running with the jumper wire between my booster fan relay and the main fan relay for 5 yrs now with no problems nor damage to the ECM drivers from bring on both fan relays.

If you go this route don't get into soldering wires, just use the clip-on (insulation displacement) connectors from Radio Shack.