1989 Laredo VSS/trans issue

Okay, I will preface this by saying, while I am mechanically 'ept', and have rebuilt everything that can be built on a vehicle except for an auto trans (and wouldn't it figure that's what I'm having problems with), this one would have me pulling out my hair, had I any left!

1989 Cherokee Laredo, 218k miles. Trans was rebuilt about 1k ago to try to resolve a shift dropout issue. Shifted great after that, except the dropout was still there (driving along in OD, suddenly it drops to neutral; let it cool down and it's good to go again for a while). Guy who rebuilt the trans was at a loss and couldn't replicate the issue, but then again, wasn't driving it like we did, just took it out on short jaunts.

We've since pulled the TCM plug and have been shifting manually, but my wife wants an auto trans to WORK like an auto trans. I did some research and made sure the TCM fuse was clean and intact, resistances of all shift solenoids was correct, but I couldn't rig up a meter to measure the VSS without throwing the TCM into a tizzy. That was implicated, so I figured I'd just replace it.

I went online, and couldn't find ANYONE who had a VSS for an '89, even the dealer. Most places listed it as '90-up or '91-up. I ordered the '90, figuring one year wouldn't make that much difference. WRONG! Here's the problem:

I CAN'T FIND THE BLOODY THING ANYWHERE.

The tailhousing of the t-case (NP231) has the CABLE for the speedo, and the vacuum switch for the front axle (center-disco). The cable is uninterrupted from the t-case to the speedometer, with no through-put sensor (no cruise control). No other wires coming from the t-case.

The trans has two modules on the driver's side, both completely occluded by a bracket that seems to support one end of the shifter for the t-case. Are these for shift solenoids, or...? Neither one looked like the item I purchased (http://tinyurl.com/ybqaxl6). On the other side of the trans, there's the NSS (already rebuilt that, using info from this great site). Another one up top is, I presume, from the reverse position of the floor shifter to trigger the back-up lights.

I need DEFINITE answers, pictures, diagrams, SOLID "This is what you need and this is where it goes" sorta stuff. Does this old rig even have a VSS?! I feel like a doofus.

Nick

90 and earlier did not have a VSS.

However... they do have an OSS (Output Speed Sensor)! It is a reed switch mounted on the tail housing of the transmission that picks up a 1 pulse per revolution magnetic signal (this is accurate info from 87 through 97 iirc, after that it is a 4 pulse per revolution coil or hall pickup.) It is connected from ground to an input on the TCU, which has a pull-up resistor to either 5 or 12 volts I believe.

If you can put an oscilloscope on it you should see a 0 volt blip for every rotation the shaft makes.

I normally wouldn't give detailed electronics/tech stuff, but I see you're a Ham, so I figure you can probably handle it just fine!

On 98 and later, there is also an ISS (Input Speed Sensor) which measures the rotations of the input shaft of the transmission right after the torque converter. You don't have one of these so it is not a concern.

I'm not sure what the one "up top" you speak of is, it could be either the throttle valve cable (leads to the throttle, used to adjust fluid pressure and thus shift "stiffness" depending on gas pedal position) or a vent hose. The other module covered by the shift linkage on the side of the transmission is the bushing where the wiring for the shift solenoids enters the transmission housing.

- W1KAS

Kas,

Thanks for the detailed explanation, I appreciate it. Yes, I'm a ham; no, I haven't kept up very well. I am also, however, an EE/T, so I follow your tech just fine. I figure the business I saw up top (and mistook for wiring, as it is in wire loom) must be the vent.

So, to the OSS- is this unit replaceable? If so, is it this unit from 4WDH? http://www.4wd.com/productdetails.aspx?partID=11456# If not, where would one get this thing, and is it reasonable to assume that our shifting (or rather, not shifting) issue is related to it? I'd really rather not have to buy a refurb TCM on a presumption; I can't test the OSS as I don't have a scope anymore. Many years back I had a sweet little HP 20MHz unit that I later gave to my stepson; wish I had that back! Alas, he doesn't have it anymore either, and fests up here in NW MO (I have to update my profile, we're no longer in IL) are rare.

73 de n9viw

Hmmm... you could probably actually test it with an LED and a ~470 ohm resistor now that I think about it. Connect cathode to ground through the resistor, poke the two wires from the sensor with the other... one should result in the LED being on, but flickering off once per revolution (e-brake on, transfer case in neutral for safety.)

I would say to use a standard noid light but I'm not sure if that sensor signal line can produce enough current to drive one.

As for replaceable, not sure if you want to pay new prices or not... it's part number 8350 3722. It's mentioned in the parts catalog clear up till model year 97 though, so you should be able to pick a few of them up at an extremely reasonable price at a junkyard. The one RockAuto has is actually miscategorized as a vehicle speed sensor, costs around a hundred bucks (Airtex 5S6318.) For some reason they do not have it listed under the 89 but they do have it listed under the 96 even though the parts catalogs show it to be the same part.

As for the TCU - I would not buy a refurb, I'd buy two from the junkyard for ten bucks a whack, probability says at least one of them works.

kastein said:

As for the TCU - I would not buy a refurb, I'd buy two from the junkyard for ten bucks a whack, probability says at least one of them works.

Click to expand...

yes, this.
and most of the renix era electronics are not SMT, good old fashioned pins through circuit boards, so don't be sfraid to crack one open and start looking forthe problem. I found cracked solder joints on one, touched it all up with an iron and good as new.

Good point. Even up till 96, almost all of the parts (or all, I forget, will have to look at my spare 96 TCU again tonight) on the TCU board are through hole with your normal 0.1" spacing. In 97 the TCU changed to the small one, about half the size of the 96-down one, and everything except a few power semiconductors, filter caps, and the connector went to SMT.