advantagedent

iam trying to remove the rear blower motor on my 89 c4 its been said that it cant be done and that the hvac will not work. the resistor works in the 90 and up cars but not the 89 so being a electric tec iam trying to figure it out(there is always a way ) so far looking a the wiring diagrams there is a difference in the rear blower relay, the newer cars have a resistor in them the 89 does not if the cpu is looking for a certain load then the resistor value that worked for the newer cars will not work for the 89 becaues it does not take into account for the resistors that are in the dual relay. also the motor bass resistence will change
when it is running ,electric motors make a feed back power so if say its 20 ohms stationary while running it would be 16. this might also be how the cpu sees if the motor has siezed and why it allows a few seconds to function until it shuts down if you have a blown fuse or if the rear blower is disconnected .it does read that as a fault , siezed, short and open .
iam going to dig into this this week end or so i would like the look of the pipe in place of the blower .
any input would be help full and if it works i will give the info and new resistor value

Dave White

The hvac works good enough to do it, then try to make it work properly later, this is where I am at. I also am electrically inclined, so I will share where I am at. Did the complete delete. A/C works fine, all speeds. Heat, shuts off immediately with the temp full on. I modified my original relay by soldering jumpers to basically mimic the "relay jumper mod" that is the sole change the later cars need to do to make this work. Works for a while at lower settings, then intermittently shuts off and starts back up.

My mechanic got the idea and is in the process of installing a dummy motor to the circuit, basically, a small fan motor or something from used Porsche parts, without anything on the shaft, that is currently hidden from view under the car. I don't know if this will or has worked yet, I as have not gotten the car back. I just re-printed out the schematics for our car vs a 92 to see what the differences are last night, and will refer to them now. The climate unit might be looking for an AC signal back from the turning motor as you mentioned on Climate control G19, coming from pin 5 on the relay. Our cars do have the resistor in the motor circuit, as that is what gives you slow vs high speed.

I just looked at the 89 vs 92 schematics and the resistors aren't in the newer relay! Hadn't checked that before. The controller provides a ground to switch one side or the other for low or high speed, as the other sides of the coils are tied together and to 12V on pin 8. I know our controllers are different than the newer ones, maybe they are using current draw across the resistor to determine that it is activated. Or a combination of the feedback line with this to determine that all is working. If the motor seized, blew the fuse, the controller would not get the current draw on either speed line, which is pins 6 for slow, 1 for high. I bet they simplified the circuit in the controller so that they could use a standard relay without the resistors, that may be the only difference! I will let you know if the dummy motor idea works, then, maybe a small timer chip circuit could be made to mimic the motor.

Dave White

Oh yeah, and you really want to do it!!

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Dave White

Got it backwards, the 91 and up relays have the resistors. I jumped the output of the slow speed side to the high speed side, (pins 5 and 7)which is where the feedback line is connected. This mod alone should work for all the later cars, as this is the equivalent of the jumper mod, only this one shouldn't drain your battery as it is still switched by the climate unit.

Dave White

Update from mechanic, says the dummy motor load doesn't work, only with the original blower motor. So it may need proper resistance, and the feedback line signal.

Ducks964

There is a jumper process for the rear relay. I cannot search on my Ipad - try looking for heater bypass.

crg53

I have this picture, don't know if that helps.

Dave White

From the original thread, this causes battery drain. Then someone added a relay to replace the jumpers, problem is the owner didn't know what kind of relay, or how it was connected as a mechanic did it for him. My best guess is that it was a dual make relay that switches 30 to 87 and 87c, and the relay is powered by a switched ignition source. Haven't had the chance to locate one of these and try it yet.

Ducks964

Does not cause battery drain on my car.

advantagedent

going to get into it today

GT TWO

I also have a 1989 car so am watching with interest. My bypass tube is on its way from Germany and then my tech will get onto it.

Will report back if we make progress.

Dave White

There is another, more current link here: Latest blower mod update for early cars
Basically, my mechanic tried all of the known fixes, plus some of mine and his, couldn't get it to work. So my climate control unit was swapped with one from a 92 and up and all works fine. Note that a few wires will need to be changed to update to a newer climate unit. I am using the dummy motor load instead of the relay jump, , with no resistors, temp sensor, etc. so all looks normal. For those not looking to drop the extra money for changing the climate unit, I would still do it. The heat worked good enough for a cooler day. I was able to swap my unit for $100 I think. Maybe there was something wrong with my unit, maybe not, but now everything works perfect, so I am happy.

advantagedent

i was looking into it today going to try to find the amp draw the temp sensor and risistor in pipe are not afected by the air flow (which i was concerened about thoght it might work like a mas air flow and look for temp change sp far no so now going to get a amp meter and see the draw and a pot resistor and star playing around ( on a side note i wonder if the relocation of the wires for the new unit have anything to do with the rear blower .going to look in to it will keep posted.ther will be away some one made it some one can break it.