Another Tach Question

[rodman68]

Ok on ny 94 f350 with an automatic the tach is not working correctly it shows real low at idle but when you accelerate it will be higher than normal. The engine doesnt sound like its running at 3000 rpms but the tach shows it is. All I can find is when its not working at all nothing that shows a fix for this and yes the tranny is acting up also.

 

[rodman68]

Another issue is this is a good one so we have those stupid roundabouts out here so if I go to fast around them the transmission freaks out light starts blinking and it shifts hard.

 

[FoolhardyIDI]

From my knowledge of the tach system there are 3 places they can mess up. One is the sensor on the timing cover. Next is the relay on the driver side fender. Then finally there is always the connection on the back side of the tach.
O and there is another sensor on top of the rear differential (VSS) that can also be tied into the tach I think... not to sure....

 

[79jasper]

Nope, not tied into the tach, but the tcm, yes.
You can ohm the sensor iirc, but I don't know the values. It's been posted here before.
Check the wires going into the sensor, and also remove it and clean the end off.

Sent from my SM-T537R4 using Tapatalk

 

[Kistthesky]

You can ohm the sensor iirc, but I don't know the values. It's been posted here before.
Check the wires going into the sensor, and also remove it and clean the end off.

Sent from my SM-T537R4 using Tapatalk

guess i should do that too, when i first got my truck i thought i was redlining @ 60mph until it would kick down occasionally to the proper reading!

 

[rodman68]

I will be checking all I can today and I will get me a scan tool also. All of the previous post I found it sounded like the tach wasnt working at all or very erratic like it would work then stop. Mine it goes way to high, hopefully this is part of the trans issue.

Anybody have an idea on the going around roundabouts, causing it to go nouts?

 

[OLDBULL8]

Your tach is the problem, not your tranny. Without the tack working properly it causes the tranny to act up, ie hard/erractic shifting. Check the wires at the sensor, if there bare they may be touching slightly, fan will blow them together sometimes. Disconnect the tack and screw it out, may have metal chips on it. Check connection for corroded pins. The tack puts out an millie volt AC square wave pulse, can't check it without a scope. The tack meter has no connection to your problem, it only reads what it see's. I think the 94's still have the tach relay under the hood, hangs by itself by the firewall. Contacts may be dirty. Sensor resistance is 2000 to 3000 ohms you have to remove the sensor and check it in free air with no metal around it.

 

[OLDBULL8]

Here is basicly the tach circuit.

 

[rodman68]

Some years ago i used that liquid electrical tape on the bare wires so that should be ok but of course I will look at it. Thank you for the info.

 

[gandalf]

What they're all trying to say is that very likely the problem is the tach sensor, and possibly the wires or connections for the teach sensor. You know that the tach sensor is mounted on the IP gear tower, with about a 1 inch head shaped for a wrench. It has 2 wires coming out of the top. Those wires may be frayed, but more likely the sensor has gone bad.

If you disconnect the wires for the sensor, and unscrew it, pull it out, you may find that it's dirty. Clean it off and put it back in. If it works now works, consider that a temporary fix. It's probably on it's way out, so start preparing for a new one from the dealer.

Yes, the tach sensor feeds the TCM, transmission control modual, as does the VSS on the rear differential. If either of these sensors becomes erratic the transmission will go into limp mode, shift hard.

 

[rodman68]

Ok this leads to another question if the VSS is having issues would the ABS light come on also? I will know more when I get the code ready but that wont be until tuesday.

 

[rodman68]

Ok tach sensor reads 2.4k and I can get an ac reading when running across metal using max hold I can get about 300-900mvac. The wire connections seem to be ok for the tach sensor and the VSS.

 

[rodman68]

Going to check FIPL next I replaced that years ago when having problems with the trans wonder if its broke already

 

[FORDF250HDXLT]

the FIPL/TPS isn't the reason for the bad tach.the e40d needs to have a clean and smooth tach reading.remain focused here.

 

[rodman68]

I know while I am working on this might as well check as much as possbile thats the only reason why.