I have a 1995 Jimmy. My service engine light does not come on when you turn the key, The speedometer doesnt work, It is in limp mode, I have had problems with the fuel pump not coming on. It does with the jumper wire but on a couple of occasions I have had to get it running with the jumper and after a min it will stay running and be fine. It is slow to start, as though the pump isnt doing the initial prime.

I took it in to get scanned and the scanner said it could not communicate or find the ecm. They told me it was probably the VSS. I changed that. I have resistance all the way up the wire to the ecm from the VSS.

Anybody have any suggestions? Is there a way to check the ecm and make sure its good?

 

There are three different ECM configurations used in '95. It sounds like yours is the OBD1.5 setup which is not fully OBD1 nor fully OBD2, but a mix of the two which requires a special scan tool to read all of the stored codes. Some of the old OBD1 codes can be read by grounding out a specific pin on the data link connection. I do not recall the specifics, but there is a post out there detailing it.

Concerning the VSS sensor, there should be two wires that go from the sensor to the ECM. Both wires need to show continuity with near zero resistance from end to end. The newer sensors have a resistance range of 1300-2700 ohms across its pins and this reading should be duplicated at the ECM. The sensor should also show >0.5mV-AC output when the driveshaft is rotated. I believe that your truck's VSS should follow these requirements as well.

 

Thanks for the reply. I do have continuity and a 1500 ohm reading at the VSS and at the ecm plug. I have not checked the voltage yet. I will try today and will see if I can find a scan tool for a 1.5.

Any reason why my service engine light doesnt come on when I turn the key? The bulb is good. I should also note that I have replaced the wiring harness under the steering column. I found a spot on the big red wire that had the coating rubbed off and had black on it where it shorted out at sometime

 

Not sure why the light wouldn't be coming on at key-on.

Did you replace the whole wiring harness and if so, which harness? Have you found any blown fuses as a result of the short?

 

I replaced the harness from the ignition switch to under the dash. I dont recall any fuses blown (I've been trying to figure this out for a month). I'm wondering if There is a voltage problem somwhere because the feul pump doesnt seem to come on when you first turn the key. You have to crank it for a bit to start it

 

Here's the thread for pulling OBD1 type codes from the OBD1.5 system:
How to read obd1.5 codes

If you replaced the whole ignition switch, that should have included the contacts that typically will burn out causing all sorts of problems, but if your fuel pump isn't kicking on all of the time, you could have a problem with the relay or that whole circuit...

 

Okay... still no luck. I have tried to get it scanned 2x with the expensive snap on scanners. Says cant communicate with ecm on both. I took an ecm from a blazer same year and number that I new worked. Still no speedo and in limp mode.

I am a loss of what to do. could ot be a ground somewhere? Anything in the speedometer cluster?(I took it out and looked for anything out of ordinary appearance. Nothing)

I'm thinking it is something to do with the wiring from the computer but have no Idea what to look for.

I still have no service engine light at any time. Even when I first turn the key. The speedometer does not work and the trans is in limp mode.

Have i missed something? ANY suggestions?

 

Did you check the voltage output from the VSS? All of the wiring check out ok?

 

Found the problem. The plug under the glove box to the body computer came unhooked. I have no idea how because we didnt even take the panels off. Must have not been plugged in right by previous owner. The only way I found it was I stripped the whole dash to look for bad wires.

Thank you very much for your help swart. I'm sure I will need it again