Engine/Transmission (1994 - 1998) Serious Problems?

[ScottLewis]

I began throwing a Code P0720 with MIL coming on at about 15 MPH. I was advised that it was a TSS problem and that I should get it scanned. I lacked a good scanner and the symptoms seemed to match what others on this sight reported for TSS problems so I bought a new TSS. Now I have no speedometer and when the truck reaches about 5 MPH it starts slipping badly. There are no MIL codes showing at all! I'm worried it may be the PCM.



Since the truck barely moves, can I take the PCM in to have it checked?



What are my other options?



Is there someone in the Manteca area with a scanner who could assist me?

 

[ScottLewis]

Please????? Any ideas?



Can someone supply me with a pinout of the '98 PCM?



I have a manual ordered from Geno's but until it gets here I'm dead in the water!

 

[REFLOYD]

what is the TSS

 

[ScottLewis]

Transmission Speed Sensor. Used on '98 (and others) A/T w/ 2WD. Mine is a 2 wire unit.

 

[REFLOYD]

the only other thing i could tell you to do would be to check the wiring and see if anything is loose or broken. also check the fuses(all of them)

 

[ScottLewis]

OK, here's where it is now. Symtoms and testing so far:



1-) No Speedometer

2-) No intake heater grid

3-) No shift from 1st and begins slipping above 3 MPH or so

4-) Tach OK

5-) A/C OK

6-) Alternator charge OK

7-) No further trouble codes



All above are controlled by PCM so I still suspect that is the problem. All fuses appear ok as does wiring.

Anyone with PCM knowledge want to venture an opinion?



Dan

 

[Howard Durand]

Is it possible that the Towlock and Presure Lock components are causing trouble?

 

[ScottLewis]

I guess it's possible. I've had the pressure-loc on for several years without problems. I had the Towloc on for about a month with the only problem being setting of trouble code 0720. Speedometer worked OK and it shifted fine. I talked to BD staff who said they didn't believe it was the problem. Because of the progression of faults, I agree. I have made an appt. with the dealer to check and probably install a new PCM. If that proves NOT to be it, I guess removal of the Towloc is next.



This is the first time I've had any real trouble with the truck but it was enough that my wife was able to convince me to buy a new Laramie 4X4. I pick it up tomorrow. I guess when I finally get the '98 fixed, it will go up for sale.



Dan

 

[Howard Durand]

Your list includes failure of the intake heater grid. This would be the first thing to happen when turning on ignition. Would a bad heat sensor cause any of the other problems?

 

[ScottLewis]

On retrospect, the heat grids may not actually be failing. When I checked the ambiant temp. was approx. 70* so the grids may not have initiated. Even if they had failed, I doubt that all the other problems would have resulted. I think I'll just wait to see what the dealer comes up with.

Thanks for the input Howard. I've read your prior posts with interest and appreciate your comments. You've given me some good ideas.



Dan

 

[Kirwin]

Speed sensor

On your 98 the VSS (Vehicle Speed Sensor) Should not affect transmission operation until you are in 3rd gear. I can verify this when I get home, but the early second gen trucks did not have all the bells and whistles that the later trucks have. It does affect the speedometer and the cruise control and the shift to OD. Essentially all PCM controlled functions that use vehicle speed which does include the manifold heaters inhibit, will not function. Note that the shift up to 3rd gear is not PCM controlled so your issues are not ALL PCM related. Hope this helps Ken Irwin

 

[ScottLewis]

Somewhere in the manual, I don't recall what section, I read that the PCM uses the signal from the VSS above 3MPH to control "certain transmission functions". Since the 3MPH coincides with the point at which the transmission loses it's grip, and since I replaced the transmission VSS, the PCM seemed the next step. I've done resistance tests of both the TSS and the ABS speed sensor on the differential. Both appear within spec.



Dan

 

[Kirwin]

98 Vss

Now that I think about it, I thought that the 98 only uses the ABS sensor for speed input. Do you have a VSS and a anti lock speed sensor on the differential plus a VSS on the transfer case tailshaft? Should look like the old speedometer cable plug (about 1. 75" in diameter) with 2 wires coming out its connector. Ken Irwin

 

[ScottLewis]

Mine is a 2WD but yes, there is a sensor on both the transmission tailshaft (TSS) and on the differential (ABS). As I said, I replaced the TSS and when that had no effect, did a resistance check on both the old one and the new one. They were within a few ohms of each other. I then checked the ABS sensor and found low resistance on it, slightly higher than the others but still (I think) in the right ball park. The manual doesn't give much specifics on testing them. I have a lot of other irons in the fire right now so don't have time to dedicate to the '98. I guess the dealer shop will get their shot at it when the new PCM I ordered comes in.

 

[Howard Durand]

Based on my PCM configuration, which may have nothing to do with yours, a group of sensors depend on a "signal reference" pin for ground return. If you can find one on yours, you could check it with an ohm meter for actual ground connection.

 

[ScottLewis]

Cured!!!!!

Well, I spent about $740 on a rebuilt PCM and new Speed Sensor and discovered that wasn't the problem. I then started tracking down wiring gremlins. There was continuity on both wires from each speed sensor to the connectors at the PCM. I untaped and removed the wire covers on each connector and checked at the back side on the connector. That was OK too. I finally found one connector wire that was pulled slightly out from the connector. It had barely made contact, then worked out a little more, breaking connection. The locking barb was apparently not holding it in. I pressed it in as far as possible, then taped everything up tight, replaced the wire loom and plugged everything in.

Viola! The truck runs like it's supposed to with no trouble lights and the TowLoc works great.



Thanks to all with their suggestions and comments. Without your help,I doubt I ever would have tracked this elusive devil down.



I now have a new truck that has spoiled me and this one is for sale.

 

[Howard Durand]

Outstanding job of trouble shooting, thanks for posting your findings. It will encourage us to dig deeper before buying parts.

 

[JO'Leary]

Glad to hear you got it figured out Thanks for sharing you findings.

It's always good when you can end a thread with a fix.



Joe