idosubaru Posted July 4, 2007 Share Posted July 4, 2007 1991 XT6 automatic, AWD i need to swap transmissions since my speedometer driven gear is broken and my speedometer doesn't work. but that's down the line, so i want to bypass the speed sensor so that my engine quits cutting off (not often, and it's predictable, but it is annoying) and my power steering will work. i've traced down the pinout for the "car speed reed switch" (which i assume is the "speed sensor") in the speedometer cluster. fortunately it even has a dedicated pinout on a test connector, so i don't have to bother with cutting or separating or anything. what exactly is a reed switch and how do i 'properly' ground this wire? it probably varies resistance dependent on speed, but i just want it to give a signal of any kind. do i need to ground it with a resistor in line? and do i need a fuse on that ground wire as well? Link to comment Share on other sites More sharing options...
DaveT Posted July 5, 2007 Share Posted July 5, 2007 what exactly is a reed switch and how do i 'properly' ground this wire? I am not famoiliar with XTs. General electric / electronic information: A reed switch is a magnetically activated switch. They can be normally open or normally closed. A magnet nearby changes the state. All of them that I have seen are a small glass tube with a contact in each end. One is the magnetically sensitve piece that moves. Proper ground = good ground. The braket that mounts the hood release has a couple of screws that work for ground. Also, the ECU mounting nuts. Link to comment Share on other sites More sharing options...
idosubaru Posted July 5, 2007 Author Share Posted July 5, 2007 someone else mentioned that what i said won't work. they said the speed sensor works on pulses or something. the FSM shows one end going to ground...then the reed switch...then to the ECU. i was wondering if just grounding the wire that goes to the ECU would be enough? it seems to me i'd need something in there to mimick the resistance of the speed sensor. the speed sensor must vary something right...voltage or resistance, somehow differentiate different speeds? the FSM is very vague on the speed sensor, often ignoring it. Link to comment Share on other sites More sharing options...
GeneralDisorder Posted July 5, 2007 Share Posted July 5, 2007 The speed sensor produces a square wave. On all the EA's (no reason it should be different for the XT6 as far as I can see) it's a voltage signal that varies from 0 to 5 volts - the frequency of the square wave indicates the speed. Thus the more quickly it varies the faster you are going. The speed sensor for the EA's is inside the speedometer but the fact that the XT6 is a "reed switch" would tend to indicate it's probably the same or at least similar. Have someone hookup an occiliscope to a working unit for you..... This wave *can* be replicated, but you will need to build a small circuit to do it. Do a search for square wave generator circuits and you will probably find a ton of easy solutions. GD Link to comment Share on other sites More sharing options...
idosubaru Posted July 5, 2007 Author Share Posted July 5, 2007 gah! thanks a ton for the info i needed GD. this is quickly getting into tasks i'm not good at. electrical work and building circuits is...well, something i've never really done. probably just need to deal with it until i can manage the down time to swap trans. i was hoping for more of an EGR type work around with a resistor and some wire! you are correct, the XT6 set up is identical to the EA82. thanks again, i'm going to post that info at xt6.net. Link to comment Share on other sites More sharing options...
idosubaru Posted July 5, 2007 Author Share Posted July 5, 2007 here's one of those ideas that is probably really, really bad but i'm electrically stupid so i don't know why. what would happen if i fed the rear wheel speed sensor signal to the VSS line in the speedometer? i believe the rear wheel sensor signal will read half the speed of the front VSS, so it's signal is "half" of the fronts, because i can't think of a better way of saying it. anyway, i'll need to verify it's a square wave and in the same voltage range. curious if that sensor/circuit will put any dangerous loads on the ECU as well. Link to comment Share on other sites More sharing options...
Gloyale Posted July 5, 2007 Share Posted July 5, 2007 The rear speed sensor is not a reed switch. It is a hall effect pick-up. IIRC it also creates 30 pulses per revolution not 4 like the reed switch. The curious thing is that the system is designed to use either sensor for ECU control of the engine. Are you sure the lack of VSS signal is the culprit? Where it really sucks is that with only one operational, your AWD is probably not working properly. My guess is that you have a C sol. lock switch, so no worries. Link to comment Share on other sites More sharing options...
idosubaru Posted July 5, 2007 Author Share Posted July 5, 2007 the lack of a working speedometer and a VSS signal is definitely the cause of the problem. it slowly died over the past couple months. when it works...everything works, no problems at all. when it doesn't, the speedo shows zero and the power steering is quirky (speed sensitive), the engine rarely shuts off, and the cruise doesn't work. all of that works fine if it decides to start working again...which as of the past week or two is really never. thanks for the info on the rear wheel speed sensor, makes sense. i forgot it was a hall effect sensor. i knew it sounded too easy! yes i've been wondering what that does to my AWD. and you are correct, i have Duty C control if needed. Link to comment Share on other sites More sharing options...
idosubaru Posted July 6, 2007 Author Share Posted July 6, 2007 an update - i'm trying to splice the rear wheel speed sensor signal into the speedometer vehicle speed sensor. yes, there's a 30:4 ratio they will be off. but...if it works it should quit the rare stalling, the cruise may work, the power steering should work better and the trip meter might work (but it'll be inaccurate by 15:2 of course). i found the wire for the speed sensor in the instrument cluster and i'm trying to figure out which of the three wires in the rear wheel speed sensor may work. i have it installed but not sure of the results yet.... i guessed sort of on one of the 3 rear wheel speed sensor wires. Link to comment Share on other sites More sharing options...
daeron Posted July 7, 2007 Share Posted July 7, 2007 good luck, intrepid wrench warrior! always fun to see people 'sperimentin. Link to comment Share on other sites More sharing options...
idosubaru Posted July 7, 2007 Author Share Posted July 7, 2007 i tapped into the ECU input, but the ironic thing to this is that it looks like the output of the speed switch splits into many parts right at the speedometer cluster to go to the trip meter, cruise control, power steering, ECU..etc. so, i may have just sent input to the ECU. haven't driven it enough to see if the stalling is now gone since it happened so rarely. still looking into getting the input to all of those other items. Link to comment Share on other sites More sharing options...
Gloyale Posted July 7, 2007 Share Posted July 7, 2007 I don't think this is a good idea. It is a completely different type of signal, not just a different rate of pulse. I just looked at the Diagrahms for at least an 89 XT6. It appears the Black wire of the 3 is the signal wire. the other 2 are a ground and a shield. I think it will be a ground continuity pulse and not a Volatge pulse the the in dash VSS. I though that both sensors where connected to the ECU but appartently only the dash one drives the ECU. The TCU has both connected to it. Link to comment Share on other sites More sharing options...
idosubaru Posted July 8, 2007 Author Share Posted July 8, 2007 I don't think this is a good idea. shhh, don't tell me that, it only encourages me! i agree, i wouldn't recommend this kind of stuff to anyone, kind of silly really. but now i'm interested and want to see what happens. i've got it hooked up and am driving 200+ miles tomorrow. see what happens. my electrical tools, extra ECU, extra TCU, extra RWSS....bunch of goodies, are in the trunk just in case. Link to comment Share on other sites More sharing options...
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