Tachometer works, tranny works. Tach doesn't work, tranny slips bad

dan-o

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My 1989 ambulance is acting weird. Started up great today, but the tach was reading 0. I put it in reverse. No problem backing up. Put it in drive and it acted like it was VERY low on transmission fluid. Dip stick showed it a little low after letting it warm up, so I adding 1/2 qt. Still the same problem. I drove it around the driveway anyway. Then the tach started working and the torque converter engaged and it worked fine the rest of the day.
I checked the tach sensor resistance. It checked out.

Any of you guys ever see this before?
 

ih8minimumwage

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Your transmission reads from the tach sender. If the sender ohms out okay, you can pull it and clean the end off. It's a standard mag pickup that operates off of the gear teeth, and can collect all the microscopic metal in the oil and stop reading correctly.
 

dan-o

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Well, the gremlin is back. After putting in a new OEM sensor it ran ok for months. Today I got in to drive it and no tach reading, no transmission function. I cleaned the sensor and ohmed it out. Still nothing. I checked the fuse to the tach. It's in the #8 position on my fuse panel. It was ok.
Now I'm stumped. DOes anybody know where the signal goes from the sensor? Is there a way to read that signal?
I was planning a camping trip in a few weeks. Now it looks like freakin' tent time, again. UGH!!
 

TNBrett

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Your tach in the dash gets its signal from the transmission controller.
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Perhaps the issue isn’t the tach at all. Maybe the tach is just the most visible symptom when the transmission controller stops working. Maybe the controller is bad, maybe it’s just a bad ground or loose connector. Here’s the diagnostic procedure for the tach FWIW
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dan-o

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Thanks a bunch. I broke down and bought the service manual on DVD. It was thirty bucks. Learning to read it is where I'm at now. I'm trying to figure out where the "Break Out Box" is and what it looks like. When I find it I will be able to start checking it out the right way.
It acted up again yesterday morning when I was getting ready to take the Mrs. camping. I had all but given up, worked on another project for a few hours and then the tach started working again. I gambled and we took the trip. It started up this morning and we drove home.
I'm not a gambler, by nature, but sometimes you have to roll the dice!!
Thanks, again.
 

TNBrett

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A breakout box is a special service tool. It’s not in the truck. It’s just a tool to make isolating individual circuits in a multi pin connector easier. They look something like this.
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You can make do without one. I usually use alldata for factory service manuals FWIW. Best of luck to you on hunting down this issue.


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Big Bart

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Dan - O,

You said your tach in the truck is also not working when this happens. Guessing your tach signal is needed for the E40D, I am not a E40D expert as I have a C6.

The tach sensor is a big threaded plug with 2 wires on top of the IP timing cover. (Front of engine above the water pump.) You may want to check/clean those connections and trace back to the instrument cluster. That sensor could be bad or just the wiring two from is loose, corroded, or has failed. A-7 in the manual above suggests a test you can use to determine if your sensor is bad. Keep in mind you need to test when the tach is not working. If working likely the sensor will test good.

Let us know what your issue turned out to be!
 

dan-o

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I haven't noticed the OD light on or flashing but, I will check it at night to see if it is dim.

I'm starting to lean on it being a bad ground. Somewhere. The reason I'm thinking that is the sound of the buzzer fluctuates and, also, if the tach is NOT going to work, the instant I turn the key the tach needle takes a negative dip, before the engine even cranks over, then sits on zero while the engines runs.

Here's another thing that makes my head explode. I had looked at the fuse (#18, 15 Amp). My wife swore it looked like the fusible link in the fuse was damaged. I, in my years, have never seen a 'kind of bad' fuse. Nor someone 'kind' of dead, or 'kind' of pregnant! To make her think I valued her opinion, I changed it. Started the truck and it worked fine after that. I don't think that was it but, for now it is running. (For the record, I do actually value her opinion...she DID marry me.)

Thanks for getting back to me, guys! I will double check that OD light and, even though the tach sensor is new, clean and torqued down, I will try and jiggle wires and try to trace down the ground wire.

I'm getting to old to work on this stuff when I get home from work. I should have been born rich instead of so good looking. ( I know you all had the same problem )
 

Big Bart

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Part of the problem with men is as we age, we get cheaper and cheaper. (It gets harder to earn that money and feels worse parting with it!) So even if you were born rich and good looking, you would still likely fix this yourself to save a buck!


Also, I think many of us know how these intermittent things go at the stealership or shop around the corner, so we bang our heads on the engine block trying to fix these odd issues.

They say your tranny valve body is shot, you need a tranny. You get it back and same issue happens. They say you need a new tranny controller, you get it back, still not fixed. Then they say you need a tach sensor, nope that did not do it. Then they say, must be the neutral safety switch, wrong again. Then you get it home and your wife says, honey, is this fuse supposed to look like this? Problem sovled!

But they are not going to refund your $5,000 they charged fixing nothing, all because they are not trained technitions. (Or even actually opened the manual to troubleshoot. Or god forbid they jump on this site and get some awesome advice!) Rather many are just part changers happy to steal your money for things you don't need.

Stick with it, it only can be so many things. Hopefully your wife fixed it! :)
 

dan-o

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Right on, bro, on all counts. I will track it down. I will have some time over the weekend to stick my head under the hood and poke around.
It's amazing that the older I get the more patience I have. It's counter to reality. I have fewer days ahead than behind so. you would think I would be in more of a rush.
When I was younger, I had a lot of days ahead but, I had to have everything right then and there.
I guess I learned that rushing things never works out. Besides, once I'm gone there will still be old trucks that need fixed!
 
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