Tach/Transmission Problem
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Tach/Transmission Problem
Hi all,
I've read some posts about dead tachs and shifting problems but not sure the tach (RPM) sensor is my problem.
90 F350 diesel automatic transmission
Problems: dead tach, shifting erratic.
I tested the tach sensor two ways that I found in this forum but not sure I'm doing it right.
First: I removed the senor and tested the DC resistance. It read 3.1 when the meter was set to 200K ohms. I've never used one of these meters before so I'm not sure what the reading means.
Second: With the sensor installed I unplugged it, started the engine, checked the AC volts and it read .049 when the meter was set at 2 volt range. Increased the RPM's and the volts went up to .07.
I think both of these tests show the sensor is good but again, i don't know how to use the meter.
I've cleaned the sensor and it looks in really good shape with no cracks in the wires.
Based on the above, is the sensor good or bad and if good, what else should I look for as the cause and how?
Many Thanks,
I've read some posts about dead tachs and shifting problems but not sure the tach (RPM) sensor is my problem.
90 F350 diesel automatic transmission
Problems: dead tach, shifting erratic.
I tested the tach sensor two ways that I found in this forum but not sure I'm doing it right.
First: I removed the senor and tested the DC resistance. It read 3.1 when the meter was set to 200K ohms. I've never used one of these meters before so I'm not sure what the reading means.
Second: With the sensor installed I unplugged it, started the engine, checked the AC volts and it read .049 when the meter was set at 2 volt range. Increased the RPM's and the volts went up to .07.
I think both of these tests show the sensor is good but again, i don't know how to use the meter.
I've cleaned the sensor and it looks in really good shape with no cracks in the wires.
Based on the above, is the sensor good or bad and if good, what else should I look for as the cause and how?
Many Thanks,
Post Fiend
Joined: Jan 2005
Posts: 8,402
Likes: 10
Going on memory the tach sensor should read 2000ohms. You need to pull it from the engine and hold it away from anything metallic and then check the resistance across it.
200k ohms is 200,000 ohms so you should pick a lower scale.
The most likely cause of the tach not working is a bad sensor.
If you need to get one, the Ford parts stores refer to them as the engine speed sensor.
A bad tach sensor will give hard shifts and usually, a blinking OD light as will a bad vehicle speed sensor.
200k ohms is 200,000 ohms so you should pick a lower scale.
The most likely cause of the tach not working is a bad sensor.
If you need to get one, the Ford parts stores refer to them as the engine speed sensor.
A bad tach sensor will give hard shifts and usually, a blinking OD light as will a bad vehicle speed sensor.
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Thank you Lazy K,
I guess I was just over-thinking the 200K thing thinking it might be in thousands up to 200K therefore 3.1 might mean 3,100 ohms.
I just checked it at 2K and the meter read 1.6 which I think means 3,200 ohms. I read on this site the norm is between 2,000 and 3,000 ohms.
The overdrive light on the dash doesn't light at all no matter how many times I push the on/off button. That could be a bad bulb.
Does 3,200 ohms mean the sensor is bad? New ones here in the Dallas/Ft. Worth area run $75 plus tax and they can't be returned.
I guess I was just over-thinking the 200K thing thinking it might be in thousands up to 200K therefore 3.1 might mean 3,100 ohms.
I just checked it at 2K and the meter read 1.6 which I think means 3,200 ohms. I read on this site the norm is between 2,000 and 3,000 ohms.
The overdrive light on the dash doesn't light at all no matter how many times I push the on/off button. That could be a bad bulb.
Does 3,200 ohms mean the sensor is bad? New ones here in the Dallas/Ft. Worth area run $75 plus tax and they can't be returned.
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Freshman User
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Okay... I bought a new meter and learned how to use it. The tach sensor reads 2,475 so that should tell us it's good.
I checked the fused but since you asked I went ahead and replace the two under the dash that said Overdrive Transmission and Tachometer. I also replaced two that had to do with speed control.
Still no luck!
I checked the fused but since you asked I went ahead and replace the two under the dash that said Overdrive Transmission and Tachometer. I also replaced two that had to do with speed control.
Still no luck!
Post Fiend
Joined: Jan 2005
Posts: 8,402
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Ford Fuel Injection » How To Run a Self-Test Here is a link on how to get any trouble codes. I would think you would have to wire a meter or small test light as shown as you are unlikely to have a check engine light. Also during the test you would have to hold the gas pedal full down (engine off) otherwise you will get a false TPS code.
However before you do any of that you need the tach working. From your test results on the sensor it would seem OK. If you had a frequency meter or a hand held tach you could check that, with engine running, you get a frequency change with engine speed.
Most tach faults are due to a bad sensor but there have been guys here who fitted a new one and still had a fault
I think I would try posting this problem on the Pre-Powerstroke page in the diesel section. Plenty of guys there have had the same problem. Tell them the checks you have already done on the sensor.
However before you do any of that you need the tach working. From your test results on the sensor it would seem OK. If you had a frequency meter or a hand held tach you could check that, with engine running, you get a frequency change with engine speed.
Most tach faults are due to a bad sensor but there have been guys here who fitted a new one and still had a fault
I think I would try posting this problem on the Pre-Powerstroke page in the diesel section. Plenty of guys there have had the same problem. Tell them the checks you have already done on the sensor.
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Post Fiend
Joined: Jan 2005
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Good man. I have an IDI diesel and the tach has never worked right so I have limited experience in fault finding. The big difference is that my truck has a C6 auto which doesn`t need input from the tach, otherwise I would already have followed your path and could be of more help.
Anyway, good luck in getting it fixed.
Anyway, good luck in getting it fixed.
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1996, c6, f250, ford, problems, read, rpm, sensor, tach, tachometer, transmision, transmission, troubleshoot, truck, working
