tach not working, gearbox in limp mode, what to do?

[huse]

My tach is not working, It moves a tiny bit if i slam the throttle.
My "rear antilock" light stays on.
And on my transmission only 2 gear works (limp mode).
My Truck is a 1990 F250 7.3idi e40d tranny.

This is what i have done so far:

• replaced the Rpm/Tach sensor with a new one- didn't help

• checked the Wires from the rpm/tach sensor to the tcm-ok

• checked the signal from the rpm/tach sensor at the connector for the tcm-ok

• checked for voltage and earth to the ecm-ok

• removed and opened the tcm to check see if i could see any problem- looked ok.

• Removed the instrument cluster and checked for ground and voltage to tach, also checked the tach/rpm signal, Voltage and ground. voltage and ground- ok, tach signal is not ok (around 60-80mV) should be 300 mV or more

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Any body that has an idea? What should i check next.
Should i get a new tcm? Is there any known issues conserning tcm failure?
What else would mess up the tach?

Im thinking it might be the tcm power relay? Where is the relay located? Any way to test it?

Thanx

 

[oldmisterbill]

How about the tach sensor mounted n the timing gear cover near the inj pump?

 

[huse]

pot-ato pota-to..

How about the tach sensor mounted n the timing gear cover near the inj pump?

Thats the same as rpm the sensor just a different name. that was the first thing i checked, it checked out fine but i replaced it with a new one anyway.
And tested that too.

 

[liddledoggie]

check the post by psychohawk he had a bad relay on the fire wall that caused him simular problems. joe

 

[94turboidi]

I know it sounds simple but have you checked out your battery cables every bit of them to make sure they are tight getting a good connection? Also do you have your engine ground cables to the cab on and in good shape? I had trouble with all my dash gauges when I didn't have those cables on. I also had trouble with the e40d once with bad battery cables.

 

[Agnem]

Have you tried running the engine with the TCM disconnected? If a component in there has failed it could put sufficient load on your tach signal to kill it. I think the tach will still work with it disconnected.

 

[huse]

Agnem- The two tach signal wires goes from the sensor to the tcm, the tach signal then goes from the tcm to the tachometer as one wire.
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94turboidi- Brand new heavy duty battery cables from custombatterycables an two new deka 665mf batteries was recently installed.
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I have checked the tcm/eec power relay, and it seems to be working properly, what should i check next?

 

[OLDBULL8]

Im thinking it might be the tcm power relay? Where is the relay located? Any way to test it?

IIRC my 90 there is a couple relays under the hood drivers side close to the firewall just hanging there, might be one of those. My 90 is up in Michigan so can't check.

 

[bubblegum]

Huse, not sure if this will be of any help to you at all, but while looting and pillaging a junkyard several months ago I found an '80s truck with an IDI diesel engine and a tachometer - from my limited knowledge on Fords this truck was too old to have any computer in it, yet it had a tachometer, and its rpm sensor looked identical to that in my own truck (I know because I pocketed it and had someone measure it up) - so obviously the tachometer can operate without a computer, which to me means that maybe your tachometer should be capable of the same? Is it possibly worth it to investigate why the tachometer itself is not working properly, like maybe start following its wires from the gauge back to where they may be going and see if there is a damage anywhere?

Edit: just saw Oldbull8's post - that is correct, my '91 has two relays next to the coffee can there - one is for the computer, the other is for the running lights. Not sure which is which, I am not that advanced...

 

[Shadetreemechanic]

I didn't see "check all fuses" on the list of things you have done. I had a very similar condition that started with abs light and proceeded to limp mode on the tranny. Though I don't remember a tach problem it ended up being a blown fuse associated with the abs circuit. The abs has to use tach signal and speed sensor signal somehow to know when to work.

 

[huse]

IIRC my 90 there is a couple relays under the hood drivers side close to the firewall just hanging there, might be one of those. My 90 is up in Michigan so can't check.

I found it, it checked out ok .. Funny thing is that i was looking everywhere for the relay exept next to the tcm test output in the relay box

 

[huse]

I didn't see "check all fuses" on the list of things you have done. I had a very similar condition that started with abs light and proceeded to limp mode on the tranny. Though I don't remember a tach problem it ended up being a blown fuse associated with the abs circuit. The abs has to use tach signal and speed sensor signal somehow to know when to work.

I've checked the fuses. that was the first thing i did. I didn't put it on the have done list because i thought that was pretty obvious but then again i have done the worst case senario before and it turned out to be a fuse but not on my truck

 

[huse]

Huse, not sure if this will be of any help to you at all, but while looting and pillaging a junkyard several months ago I found an '80s truck with an IDI diesel engine and a tachometer - from my limited knowledge on Fords this truck was too old to have any computer in it, yet it had a tachometer, and its rpm sensor looked identical to that in my own truck (I know because I pocketed it and had someone measure it up) - so obviously the tachometer can operate without a computer, which to me means that maybe your tachometer should be capable of the same? Is it possibly worth it to investigate why the tachometer itself is not working properly, like maybe start following its wires from the gauge back to where they may be going and see if there is a damage anywhere?

Edit: just saw Oldbull8's post - that is correct, my '91 has two relays next to the coffee can there - one is for the computer, the other is for the running lights. Not sure which is which, I am not that advanced...

If the truck didn't have a e4od transmission, the tach wouldn't affect the transmission.

According to wiring diagrams the two wires from the tach sensor goes in to the Transmission control module (TCM). From the Tcm there is a single wire going to the tachometer - tach signal wire. atleast from what i can make of the wiring diagram i have here (hayes manual). I have ordered the wiringdiagrams made by ford, but im still waiting for them to drop in my mailbox.

Any how i can't see how the tach would work without the TCM

I was also thinking that maybe there is a short or something like that to the wire going from the tcm to the tach, and maybe that short would screw up the TCM.
Im just fishing here

 

[huse]

I have checked the wiring from the tcm to the tachometer gauge and it checked out ok. I decided to order a new TCM, hope this will fix my truck. Now i just need to wait for fed-ex to deliver it to me in Norway..

 

[huse]

New ECM didn't work .. So i have investigated further and found burned wires on the solenoid pack connector on the transmission.

Just ordered the connector pigtail.

Hope this works.!!