Page 1 of 2 12 LastLast
Results 1 to 20 of 28

Thread: front blower mystery-resistor bypass?

  1. #1
    Van Enthusiast
    My Van(s):
    97 Previa DX SC AWD...alive! 95 Previa LE SC AWD (RIP) 93 Previa LE AWD (rustbucket w/ no reverse...parts van) 91 Previa LE AWD (RIP)
    Join Date
    Nov 2014
    Location
    Midwest/Southwest/Northwest
    Posts
    185
    Rep Power
    1

    front blower mystery-resistor bypass?

    normally when the resistor is shot, the front blower will only work on high. My blower only works on high. However, I bought a new resistor and it still only blows on high.

    The previous idiot...I mean previous owner...altered the wiring for reasons I know not. The resistor wiring is untouched, but the wiring to the blower itself was cut and the connector removed, and an entirely new wire installed that runs to the blower. I'm guessing (and only guessing, because I am not in possession of electrical know-how) that the resistor might be completely bypassed, therefore installing a new one did nothing.

    I know there is also a blower switch somewhere that sometimes burns out, but I've never heard of that causing all of the lower speeds to go out while leaving high speed intact. That's usually resistor. And getting at the switch isn't fun, so I'm not itching to try.

    Given my limited electrical know how, is there a simple way to test whether the resistor has simply been bypassed? If it has i could then contemplate whether I have the smarts to remedy that....I suppose tracing the new wire to the blower might be a first step.

  2. #2
    Van Obsessed
    My Van(s):
    1988 4WD LE auto dual range t-case (the mistress) 1988 4WD LE auto(the donut van)
    Join Date
    Aug 2012
    Location
    Tacoma, Wa
    Posts
    677
    Rep Power
    1

    Re: front blower mystery-resistor bypass?

    Quote Originally Posted by Previologist View Post
    I suppose tracing the new wire to the blower might be a first step.
    This would be a good start. You may find the original wiring in the process.You could also test for voltage at the resistor itself. Unfortunately, figuring out and fixing other people's "repairs" is typically not a fun job.

  3. #3
    Van Enthusiast
    My Van(s):
    97 Previa DX SC AWD...alive! 95 Previa LE SC AWD (RIP) 93 Previa LE AWD (rustbucket w/ no reverse...parts van) 91 Previa LE AWD (RIP)
    Join Date
    Nov 2014
    Location
    Midwest/Southwest/Northwest
    Posts
    185
    Rep Power
    1

    Re: front blower mystery-resistor bypass?

    Being 4 terminals at the resistor and me having only half a brain, i would need instructions on how to test the resistor connector for voltage. If there were only 2 that would mean i only have a 50% chance of getting it wrong but with 4...thats the wild west, to me.

    I'm pretty sure I saw where the new wire came from while I was doing something else, but I didn't pay close attention at the time. I will check again tomorrow. They also ran it in a different location than the original harness so it should be easy to trace

  4. #4
    Van Obsessed
    My Van(s):
    1988 4WD LE auto dual range t-case (the mistress) 1988 4WD LE auto(the donut van)
    Join Date
    Aug 2012
    Location
    Tacoma, Wa
    Posts
    677
    Rep Power
    1

    Re: front blower mystery-resistor bypass?

    There are three resistors on the resistor pack each of a different resistance value, one for each of the three lower fan speeds. There is one terminal connected to each of the three low speed outputs of your fan switch. The forth terminal is the ouput to the fan. All three resistors are tied to the output terminal on one side. When the fan switch sends voltage to the resistor pack, it flows through the appropriate resistor and a reduced voltage is sent to the fan. Without knowing which terminal is which, just check all four. Ground your voltmeter negative lead to the chassis and with the fan on any position except high, probe all four terminals of the resistor pack with the voltmeter positive lead. Repeat for each fan speed. If there is no voltage at anytime, it is indeed bypassed.

  5. #5
    Van Enthusiast
    My Van(s):
    97 Previa DX SC AWD...alive! 95 Previa LE SC AWD (RIP) 93 Previa LE AWD (rustbucket w/ no reverse...parts van) 91 Previa LE AWD (RIP)
    Join Date
    Nov 2014
    Location
    Midwest/Southwest/Northwest
    Posts
    185
    Rep Power
    1

    Re: front blower mystery-resistor bypass?

    No power to the resistor connector, as I suspected. I haven't found where those wires originate, and may not be able to.

    I traced one of the new blower wires to the front heater relay. The other one runs under the instrument panel and I presume goes to ground. Still trying to trace it


  6. #6
    Van Enthusiast
    My Van(s):
    97 Previa DX SC AWD...alive! 95 Previa LE SC AWD (RIP) 93 Previa LE AWD (rustbucket w/ no reverse...parts van) 91 Previa LE AWD (RIP)
    Join Date
    Nov 2014
    Location
    Midwest/Southwest/Northwest
    Posts
    185
    Rep Power
    1

    Re: front blower mystery-resistor bypass?

    So here's the setup. One of the new red wires runs from a terminal inside the Front Heater Relay to the blower, and the other new red wire is crimped to a presumably original blue/ wire that is apparently (I can't see) soldered into the underside of the Front Heater (HI) relay next to it.

    relays.jpg

    I couldn't find the origin point of the original wires, although I presume (perhaps incorrectly) that the blue one is part of it. I found no disconnected wires.

    Here is the new wiring from the relays to the blower, seen under the hood. They ran them through the firewall with the cable that opens the fresh air flapper.

    blower.jpg

    And here is the original wiring near the resistor, with the resistor connector intact and the blower wires cut.

    old wires.jpg

    So now what? I can't help wonder why they did this in the first place, and if that holds the key to what to do next. But that might be giving the previous owner, who also owned the shop that did this, too much credit. and I'm not sure I have the electrical skills to fix this myself
    Last edited by Previologist; 09-20-2020 at 07:51 PM.

  7. #7
    Van Enthusiast
    My Van(s):
    91 Previa 5 Speed M/T Alltrac, 93 Previa 5 Speed M/T RWD
    Join Date
    Mar 2013
    Location
    SL,UT
    Posts
    137
    Rep Power
    1

    Re: front blower mystery-resistor bypass?

    Did you get this fixed yet?

    I wonder if they did all this to "fix" a failed resistor. That seems the most likely explanation.
    The resistor is inside the blower housing. Taking the blower out is required to see what's going on.

    When looking at the resistor, the 4 terminals are marked H, M2, M1, and L on the plastic plate. Test resistance between H and M2, H and M1, H and L. Resistance is really small, like 1 - 2 ohms or something, and you should see a slight increase in resistance doing them in this order as you move through the terminals.

    Pic of a smashed apart resistor to see the workings. This was all encapsulated in porcelain or something before it ran into a hammer. Basically it's a bunch of coils in series. The terminals/fan switch include or exclude coils to increase or decrease resistance in the circuit. That beefy looking "5th terminal" is simply a support for the big coils and just mounts directly into the plastic.
    DSCN4787.jpg


    Here are the terminals. The supporting thingy goes to the point marked PM (connects to nothing)
    DSCN4788.jpg

    If it was me, I'd take your current blower out and test the resistor to verify whether your problem is indeed there, or not.
    If not, I'm out of ideas.
    If so, find a complete spare blower assembly from the scrap yard (so you have a spare motor, too), test the motor and resistor to verify function, wait until a point in the year when you can live without heat or AC for a couple of days so that it doesn't matter if things don't go according to plan, and reinstall everything back to stock. Maybe just clip out the existing wiring minimally so you can easily return it to the high speed only situation if for some reason the new parts don't fix the blower speeds problem. (I can't imagine why it wouldn't, but with my modest understanding of electrical stuff that doesn't mean much!). Looks like you might have to solder or crimp to that blower harness wire.
    Last edited by Jonny; 12-25-2020 at 11:40 PM.

  8. #8
    Van Enthusiast
    My Van(s):
    97 Previa DX SC AWD...alive! 95 Previa LE SC AWD (RIP) 93 Previa LE AWD (rustbucket w/ no reverse...parts van) 91 Previa LE AWD (RIP)
    Join Date
    Nov 2014
    Location
    Midwest/Southwest/Northwest
    Posts
    185
    Rep Power
    1

    Re: front blower mystery-resistor bypass?

    Quote Originally Posted by Jonny View Post
    Did you get this fixed yet?
    No its not fixed, I'm currently worrying about other things like where to get the money to replace my engine for this van that I got ripped off on. But I would like to fix it.

    However I think I explained above, or I meant to, that I bought a new resistor hooked it up and it didn't work either. I never mounted it, and had been driving without a resistor and high fan speed only until the head gasket blew. But I don't think there was anything wrong with the old resistor.

    So I have both of them here in front of me and with my analog meter I don't pick up any resistance on any of the terminals on either of them. I doubt the new one is bad, maybe you are testing a bad one? Also I'm pretty sure there were no fan speeds at all before they did this sloppy "repair", because I have the receipt saying they repaired it by running the new wire. haha. So that suggests it wasn't the resistor

  9. #9
    Van Enthusiast
    My Van(s):
    91 Previa 5 Speed M/T Alltrac, 93 Previa 5 Speed M/T RWD
    Join Date
    Mar 2013
    Location
    SL,UT
    Posts
    137
    Rep Power
    1

    Re: front blower mystery-resistor bypass?

    It looks like I need to learn to read, eh? Especially the first paragraph of the first post!

    I looked at the wiring diagram and when the switch is in high position it activates the "heater high relay" providing a separate no-resistance path to ground, effectively bypassing the whole resistor part of the deal. What I'm not so clear on is the "heater relay", but I believe that it is activated whenever the blower switch is at any setting except off, and sends current through the resistor to ground. The high setting and heater high relay just creates the other bypass in addition.

    Since the new wiring seems to be related to the two relays I wonder if this is what they did....
    i) solder the contacts together on the under side of the "heater relay" so that it is always closed.
    ii) cut the original motor wire as the easiest way of disabling the original circuitry.
    iii) run new wires from the relays to the motor so that the stratagem for high-only function comes to fruition.
    The question remains why? Maybe there was an undiagnosed electrical gremlin unrelated to relays and resistors and this was the expedient way of getting at least some blower function. There must have been something unusual going on or you'd think they would have found the culprit component and just replaced it.

    Did you test the relays, just to be sure about them, too (the heater relay especially)?

    Re: the resistor terminal resistance values. They're really small. It surprised me how small they are. 1/2 an ohm between H and M2, 2 ohms between H and L. Maybe it's a meter sensitivity thing? Would have been nice if the manual gave an idea of the values one is meant to see, but it doesn't. It just shows you should be seeing some resistance. At least that's the way it is in the '93 manual I'm looking at.

    Edit: I realize now that if the red wire in your first pic goes from the battery positive side of the heater relay to the positive side of the fan it would bypass the heater relay and create an always live motor just waiting for the heater high relay to be activated by the high switch setting. Basically since the original wires were cut the entire system may have been reduced to a simple circuit with an on/off switch, the switch high setting / heater high relay acting as that switch. A faulty heater relay would cause failure of all fan speeds and is beginning to look suspicious, but if that's the case someone went to a lot of trouble to avoid spending $25 for a new one. It still doesn't make sense. Sometimes though, I suppose, it doesn't have to.
    Last edited by Jonny; 12-26-2020 at 01:34 AM.

  10. #10
    Van Enthusiast
    My Van(s):
    97 Previa DX SC AWD...alive! 95 Previa LE SC AWD (RIP) 93 Previa LE AWD (rustbucket w/ no reverse...parts van) 91 Previa LE AWD (RIP)
    Join Date
    Nov 2014
    Location
    Midwest/Southwest/Northwest
    Posts
    185
    Rep Power
    1

    Re: front blower mystery-resistor bypass?

    Yeah my analog meter might just not pick up a resistance reading that small, so its possible that it was there.

    I did not test the relays. I'm electrically challenged and would need instructions.

    Let me tell you some of what else they did, and going to a lot of trouble to avoid a $25 (actually closer to 50) relay might not seem so unthinkable.

    They installed a rebuilt engine and never hooked up the oil pressure sensor, and either removed or never replaced the check engine light bulb. They installed SADS couplings and within 2 months all but one bolt had broken and it damn near came off. The head gasket on the rebuilt engine blew after less than 8000 miles.

    Nothing would surprise me at this point. The seller, who owned the shop that did this work, is either an incompetent shop owner or an unethical bamboozler, or both.

  11. #11
    Van Enthusiast
    My Van(s):
    97 Previa DX SC AWD...alive! 95 Previa LE SC AWD (RIP) 93 Previa LE AWD (rustbucket w/ no reverse...parts van) 91 Previa LE AWD (RIP)
    Join Date
    Nov 2014
    Location
    Midwest/Southwest/Northwest
    Posts
    185
    Rep Power
    1

    Re: front blower mystery-resistor bypass?

    Quote Originally Posted by Jonny View Post
    It looks like I need to learn to read, eh? Especially the first paragraph of the first post!

    I looked at the wiring diagram and when the switch is in high position it activates the "heater high relay" providing a separate no-resistance path to ground, effectively bypassing the whole resistor part of the deal. What I'm not so clear on is the "heater relay", but I believe that it is activated whenever the blower switch is at any setting except off, and sends current through the resistor to ground. The high setting and heater high relay just creates the other bypass in addition.

    Since the new wiring seems to be related to the two relays I wonder if this is what they did....
    i) solder the contacts together on the under side of the "heater relay" so that it is always closed.
    ii) cut the original motor wire as the easiest way of disabling the original circuitry.
    iii) run new wires from the relays to the motor so that the stratagem for high-only function comes to fruition.
    The question remains why? Maybe there was an undiagnosed electrical gremlin unrelated to relays and resistors and this was the expedient way of getting at least some blower function. There must have been something unusual going on or you'd think they would have found the culprit component and just replaced it.

    Did you test the relays, just to be sure about them, too (the heater relay especially)?

    Re: the resistor terminal resistance values. They're really small. It surprised me how small they are. 1/2 an ohm between H and M2, 2 ohms between H and L. Maybe it's a meter sensitivity thing? Would have been nice if the manual gave an idea of the values one is meant to see, but it doesn't. It just shows you should be seeing some resistance. At least that's the way it is in the '93 manual I'm looking at.

    Edit: I realize now that if the red wire in your first pic goes from the battery positive side of the heater relay to the positive side of the fan it would bypass the heater relay and create an always live motor just waiting for the heater high relay to be activated by the high switch setting. Basically since the original wires were cut the entire system may have been reduced to a simple circuit with an on/off switch, the switch high setting / heater high relay acting as that switch. A faulty heater relay would cause failure of all fan speeds and is beginning to look suspicious, but if that's the case someone went to a lot of trouble to avoid spending $25 for a new one. It still doesn't make sense. Sometimes though, I suppose, it doesn't have to.
    I wonder if this could simply be a bad speed control switch? The shop that did this work is completely incompetent and its possible they went to all this trouble because they couldn't find a switch. Its not usual for all the low speeds to go out when the switch goes bad and that's why I've been ignoring that possibility but i suppose its possible.

    I will pull the relay to see if your guess about soldering it is correct, but that would suggest a mixture of incompetence and problem solving that I don't think they possessed. How do I test the relay?

  12. #12
    Van Enthusiast
    My Van(s):
    91 Previa 5 Speed M/T Alltrac, 93 Previa 5 Speed M/T RWD
    Join Date
    Mar 2013
    Location
    SL,UT
    Posts
    137
    Rep Power
    1

    Re: front blower mystery-resistor bypass?

    We can get this figured out. First, let us know if you have a repair manual.

  13. #13
    Van Enthusiast
    My Van(s):
    97 Previa DX SC AWD...alive! 95 Previa LE SC AWD (RIP) 93 Previa LE AWD (rustbucket w/ no reverse...parts van) 91 Previa LE AWD (RIP)
    Join Date
    Nov 2014
    Location
    Midwest/Southwest/Northwest
    Posts
    185
    Rep Power
    1

    Re: front blower mystery-resistor bypass?

    I don't have a hard copy, but I think I may have access to the pdf manual in 1000 pieces for the 96 that used to be on the yahoo site. Now if only i could remember where it is...

    OK, found it.

  14. #14
    Van Enthusiast
    My Van(s):
    91 Previa 5 Speed M/T Alltrac, 93 Previa 5 Speed M/T RWD
    Join Date
    Mar 2013
    Location
    SL,UT
    Posts
    137
    Rep Power
    1

    Re: front blower mystery-resistor bypass?

    I was going to point you to the page of the manual where the relay testing instructions were, but looking at my '91 manual there is an unfortunate typo and the testing method is subsequently wrong!

    Below is a photo of the main heater relay (the left one in the photo of the relay bank as you noted in post #5) sitting atop the page of my manual showing the internal wiring, which is correct!

    DSCN4909.jpg

    Most relays work the same way - a low current circuit with a coil induces a magnetic field in the coil when electrified, and the magnetic field causes a separate internal switch to close which completes the high current circuit used to operate the device.

    In your relay, the low current circuit is between posts 1 and 3, and the high current circuit is between posts 4 and 5.

    With the relay just sitting in front of you not connected to anything, you should see continuity with some resistance between 1 and 3, and continuity with basically no resistance between 2 and 4.
    Now carefully connect 1 and 3 to a 12 v battery (polarity when connecting doesn't matter for this test) and you should hear a distinct click as the internal switch operates. While 1 and 3 are still connected to the battery you should also have negligible resistance between 4 and 5. This shows that the internal switch and its contacts are functioning and current is being allowed to flow between 4 and 5.
    Last edited by Jonny; 05-25-2021 at 04:50 PM.

  15. #15
    Van Enthusiast
    My Van(s):
    97 Previa DX SC AWD...alive! 95 Previa LE SC AWD (RIP) 93 Previa LE AWD (rustbucket w/ no reverse...parts van) 91 Previa LE AWD (RIP)
    Join Date
    Nov 2014
    Location
    Midwest/Southwest/Northwest
    Posts
    185
    Rep Power
    1

    Re: front blower mystery-resistor bypass?

    Thanks. Everything checks out. And to make sure of what I was doing, I checked the resistance between 4 and 5 when the power was off, and there was infinite resistance, but little to none with the power on. So the relay seems to be working.

  16. #16
    Van Enthusiast
    My Van(s):
    91 Previa 5 Speed M/T Alltrac, 93 Previa 5 Speed M/T RWD
    Join Date
    Mar 2013
    Location
    SL,UT
    Posts
    137
    Rep Power
    1

    Re: front blower mystery-resistor bypass?

    Good. One item to rule out.

    If you have page 19 of the wiring diagrams at the back of the manual, take a look at that.
    If not, here it is anyway from my manual...

    DSCN4910.jpg

    The following comments are based on the original wiring before it was all cut up..

    Power comes from the battery (not in photo), through the ignition "on" switch (partly obscured by the paint can holding the book open), through the 40A circuit breaker (on the right side of the pic) to the main relay you have just tested. The relay has power on the battery + side but is waiting for the coil circuit of the relay to be activated in order for the switch to close.

    The Y-B wire coming out of the lower right side of the relay coil goes via the 15A A/C fuse (off photo lower right) to the ignition and is live when the ignition is on. If you follow the Y-L wire out of the other side of the coil it goes to the blower switch, so the blower switch + side is actually live all the time too. Note that when you put the blower switch into any of the lower speeds, the coil circuit is completed via the slide contacts (shown by rectangular bars in the diagram) and the W-B wire to ground. With the main relay switch now on, the motor circuit is completed via the motor and the resistor. The current can't take the other fork of the L-B wire because the Heater HI relay is open.

    When the blower switch is in the HI position the heater high relay also turns on and the motor current is allowed to bypass the resistor portion of the circuit completely.

    So back to your problem...
    Since the main relay works, and the heater high relay works, you're looking at resistor or switch as most likely reasons the wiring bypass was done. I suspect you are right that the resistor is fine - I think you would have picked up an open circuit with your meter had there been a problem. That leaves the switch itself as the last likely culprit. Dashboard surgery might be in your future if you want to pursue this further.
    Last edited by Jonny; 05-26-2021 at 12:24 AM.

  17. #17
    Van Enthusiast
    My Van(s):
    97 Previa DX SC AWD...alive! 95 Previa LE SC AWD (RIP) 93 Previa LE AWD (rustbucket w/ no reverse...parts van) 91 Previa LE AWD (RIP)
    Join Date
    Nov 2014
    Location
    Midwest/Southwest/Northwest
    Posts
    185
    Rep Power
    1

    Re: front blower mystery-resistor bypass?

    Before going in any further I should ask what would I do if I found out the switch was bad, which seems likely, because they have whacked off the original harness that goes to the switch. I wonder if I might finally get in over my head if I continue...

  18. #18
    Van Enthusiast
    My Van(s):
    91 Previa 5 Speed M/T Alltrac, 93 Previa 5 Speed M/T RWD
    Join Date
    Mar 2013
    Location
    SL,UT
    Posts
    137
    Rep Power
    1

    Re: front blower mystery-resistor bypass?

    Well, if your intent is to return the blower to full functionality and stock circuitry, you'd buy a new switch (8473295D00 $83, or get one from the scrap yard) and determine all the parts of the wiring harness which have been cut so that you can replace those too. Depending on what exactly was cut it seems like you might at least need a scrap yard blower motor harness sub-assembly (see attachment).
    https://parts.toyota.com/a/Toyota_19...2440-8712.html
    There is nothing to lose except time and patience by at least taking the dash apart to inspect the switch. It's a bit of a pain, but the van is still drivable with the center dash partially disassembled, so you don't have to worry about getting it done all at once.
    https://www.toyotavantech.com/forum/...sassemble+dash

    If you would like, and depending on how frequently Previas show up in your area, I can go to the yard next time a Previa shows up in SLC and look for some stuff for you. Send me an email.

  19. #19
    Van Enthusiast
    My Van(s):
    97 Previa DX SC AWD...alive! 95 Previa LE SC AWD (RIP) 93 Previa LE AWD (rustbucket w/ no reverse...parts van) 91 Previa LE AWD (RIP)
    Join Date
    Nov 2014
    Location
    Midwest/Southwest/Northwest
    Posts
    185
    Rep Power
    1

    Re: front blower mystery-resistor bypass?

    its not the switch itself that scares me, its the prospect of replacing the harness. If I can use the jury-rigged wire to run off a good switch (which I probably have, in my 93) that would be somewhat less intimidating. Being nearly electrically illiterate doesn't boost my confidence.

  20. #20
    Van Enthusiast
    My Van(s):
    91 Previa 5 Speed M/T Alltrac, 93 Previa 5 Speed M/T RWD
    Join Date
    Mar 2013
    Location
    SL,UT
    Posts
    137
    Rep Power
    1

    Re: front blower mystery-resistor bypass?

    Finding the faulty component is still necessary. Check out whether the switch is bad - it's usually the pigtail connector contacts which burn up - and decide from there. This thread also has switch and dash disassembly info...
    https://www.toyotavantech.com/forum/...=switch+blower

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •