'89 high speed cooling fan mod...UPDATE!

[drtidmore]

NOTE! THE FOLLOWING ONLY APPLIES TO '89-'91 models. '88 models use a resistor to accomplish low speed cooling fan operation.

Well, I have good news to report. First off, a bit of background info. In studying the '89FSM cooling fan schematic, operational logic and ECM QDM operation, I came up with the following. FYI, the cooling fan relays are mounted in the engine compartment relay box (drivers side forward of the air cleaner)

1st. In low speed, as most already know, the low speed fan relay (relay G) alone is energized which connects the front pusher fan to +12V, BUT the ground side of that fan is routed through high speed fan relays C&D's normally closed contacts such that it is connected in series with the rear puller fan and the two fans run at roughly 1/2 speed.

2nd. The ECM QDM1 (QDM = quad driver module), which controls the high speed relay coils does NOT, I repeat, does NOT, report E026 error as there are a number of normal operating conditions that can actually result in a mismatch between QDM1 commanded state (i.e. high or low) and the actual monitored state ('89 FSM page BD1-36). ECM QDM2 which controls the low speed relay coil DOES monitor AND set E026 if there is a mismatch. So as long as QDM2 is happy, there will be NO E026 codes, at least related to the cooling fans (E026 is set when QDM 2 thru 4 sense a mismatch and further testing is required to determine which QDM is unhappy)

3rd. The QDM modules are capable of grounding significantly MORE current than required by the fan relay coils as it is a common design used regardless of what is being controlled.

4th. In HIGH speed mode, the low speed relay coil (G) AND both the high speed relay coils (C&D) are energized.

5th. For those that might wonder if the ECM actually monitors the cooling fan speed, the answer is NO, ONLY the state of the low speed relay coil ground side voltage

With that knowledge, I set about to do some measuring.

With NO fans commanded on (i.e. engine cool, NO AC), the voltage at low speed relay coil (G) ground connection (pin 5) and both high speed relay coil ground connections (pin 2 on either) measures roughly 14.36v (i.e. battery positive). This high voltage is due to the fact that the ECM is NOT supplying a ground path to any of the relays, so the voltage measures roughly the same as the opposite side of the coil.

In low speed mode, relay G (low speed relay), pin 5 drops to roughly 200mv as the ECM QDM2 grounds relay coil G pin 5. At this point the measured voltage at pin 2 on relays C&D measures roughly the same as NO fan mode. In this mode as describe above, the two fans run in series at roughly 1/2 speed)

As I already knew from the FSM that grounding either pin 2 on relays C or D would mimic the ECM commanding HIGH fans, I next simply inserted a short piece of stranded wire into relay C pin 2, then reinserted the relay. With the engine running, and the AC on, I first re-measure to confirmed the low speed conditions once again. Then I simply grounded the attached wire. Instantly, as expected, the fans switched to high speed mode. With the fans running in high speed, I verified that NO E026 (no error in fact) was being reported even though the monitored state of QDM1 was in mismatch state (remember QDM1 does NOT throw E026 codes).

So far so good. Then I simply connected my piece of wire over to relay G pin 5, low speed relay,(i.e. shorting it to relay C pin 2). I started the engine with NO AC and NO fans came on. Again, so far so good. Then I turned on the climate control and the fans fired up in High Speed mode. I verified NO E026 codes (HURRAY!) and returned to the engine compartment to once again measure the voltages as a sanity check. The voltage at relay G pin 5 had increased slightly to 394mv but that was expected as more current (i.e. two additional relay coils) were being routed through the ECM QDM2 connection. Remember the QDM is ONLY controlling the relay coils, NOT the actual fan, so the current remains small.

I want to RESTATE, that this is EXACTLY the same high speed relay/cooling fan operating characteristics as GM designed. Low speed will NEVER happen as all three relays always energize whenever low or high speed is commanded by the ECM. QDM2 remains happy as, even grounding 3 relays coils (C< D & G) rather than the single one as originally designed (G), the voltage is sufficiently close to battery negative as to not trip E026.

As the currents thru the cooling fan relay contacts is IDENTICAL to original, the relays should last as long as originally designed.

So, what does all this mean....well, to effect high speed operation anytime cooling fan operation is commanded, there is NO need to swap wires OR try and trick the ECM to prevent an E026. Just make a connection from pin 2 on either relay C or D (their coils are connected in parallel) AND pin 5 on relay G. This can be done from the back side of the relay box for a permanent installation OR a short piece of 18g stranded wire from the top side by inserting into the appropriate pin location and then re-installing the relay which locks the wire into place.

Mc_Reatta has several years experience with running the cooling fans at high speed year round with no detrimental effects. Granted this may wear the bearings in the fans a bit faster, but I would rather have the increased airflow through the AC condenser and the radiator even if I have to periodically replace a cooling fan motor.

For those that are concerned about running 3 relays off a single QDM port, the QDM module design also handles the coils in the EGR valve that pull more current than the 3 relay coils such as I have described.

[drtidmore]

Spoke a bit too soon!

Had the Reatta out for a drive and I have discovered that I am apparently right on the edge, voltage wise, of where QDM2 sets E026. When I did my initial testing at idle, I never got the E026 but at 45-60 mph speeds I am intermittently setting E026. My original thoughts on connecting the high and low relays together was to use a diode, but when I tried a jumper wire and it worked without setting the code, I stopped short of trying the diode. A diode will limit the current a bit and may be adequate to prevent the intermittent E026. I will update once I give this a try. Worse case, I will connect a reed relay in parallel with the low speed relay and then use the contacts on the reed relay to ground the high speed relays.

I can say that running the fans at high speed makes a difference in the engine temps. I was running a solid 200-209 in stop and go and now that is down to 190-195 and more importantly the AC vent temp is far more stable between steady speeds and stop and go.

[D-a-n-i-e-l]

I had already done this and ran it for a couple years. I can tell you in my experience that the actual engine temp really did not get much lower, if any, in my driving. In fact I went back to the original set up and had my turn on points changed in the ECM programming. Now my car runs nearer to 170-175 when driving on the freeway and 180-190 when parked in the Hawaii heat and humidity. Take into account I have a huge radiator now and a 165 thermostat.

All in all I think the time and money spent modifying the fans would be better used on a prom adapter and a new program for the ecm.

Daniel

[ssmithmiami]

I have done the above procedure on my 89, and it works great, but of course it is throwing the eo26 code.  Where exactly would you connect the diodes in this configuration?  On the ground wire?  I am a complete idiot with electronics, but this was an easy enough thing to do. 

 

If I want to use a reed relay, what kind would I buy, and again, where would I put it?

 

Thanks for any help