Tach Sensor testing???

abnkeis · Sep 30, 2012

My tachometer only works a portion of the time, so I have installed a new tach sensor and still have the same results. I have since tested the new tach sensor with the ohm check and get the 2500 ohm range as I have read it should be, but I cannot get the correct voltage reading from the leads with the engine on. I actually went and bought a second tach sensor thinking I possible had a faulty one and I am getting the same exact readings.
Is there any chance the voltage test isn't a truly accurate test and the tachometer reading issue could be further down the wiring or into the instrument cluster?

OLDBULL8 · Sep 30, 2012

First of all. Welcome to the OB.

Put your truck in your signature, difficult to answer questions if we just guess.

Maybe this will help troubleshoot your problem. These are from an 89 Ford Manual, but should be close. Corroded connectors are usually the problem.

abnkeis · Sep 30, 2012

Sorry, I forgot to add my truck in my signature... updated now.

Those pages look like they will help quite a bit, thanks!

OLDBULL8 said:
First of all. Welcome to the OB.

Put your truck in your signature, difficult to answer questions if we just guess.

Maybe this will help troubleshoot your problem. These are from an 89 Ford Manual, but should be close. Corroded connectors are usually the problem.

abnkeis · Oct 10, 2012

Well I made it through the testing procedures from the pages that you posted on here. I am thinking it is the computer that is the issue. I am wondering how common this is or if there is anymore test's on the computer itself to verify. My other question is when I check for resistance from the tach sensor connection (tach unhooked and testing at the truck wiring only) I get very little resistance to ground on either terminal of the connector. Shouldn't one side be complete resistance? However once I unplug the computer at the firewall then I do get resistance.
Any other thoughts before I start looking for a computer?

OLDBULL8 said:
First of all. Welcome to the OB.

Put your truck in your signature, difficult to answer questions if we just guess.

Maybe this will help troubleshoot your problem. These are from an 89 Ford Manual, but should be close. Corroded connectors are usually the problem.

iverger · Oct 11, 2012

After a lot of testing and replacing of parts, like tach sensor, gauge cluster and wire connections, I ended up taking my 89 to a local trans shop. The computer was the issue for my lazy tach. It had water damage that I attributed to the missing rubber seal along the top of the cowl. While I don't think it's very common it is obviously possible.

Bob

riotwarrior · Oct 11, 2012

I am at a complete loss as to what computer you folks are speaking about? These diesels DO NOT HAVE A COMPUTER for any portion of operation.

Please enlighten me and the others here...

About the closest thing to a computer these diesels have for running the engine is the printed circuit on the back of the instrument cluster.

The E4OD automatic transmission itself has a stand alone computer, it is not required to run the trucks engine however. I cannot say for certain it does not affect the tach though, I do highly doubt it since the C6 doesn't use a computer and tachs work...so do 4sp and 5sp std trans.

There is a stand alone ABS computer too for rear ABS, though again I highly doubt it plays any function with the tach.

JM2CW
Al...

iverger · Oct 11, 2012

The tach sensor, ECU (Computer) and speed sensor all play a critical role in the operation of the E4OD. The ECU on E4OD trucks is buried in the firewall below the brake booster. The tach sensor on an E4OD truck runs through the ECU before going to the dash. Those in the know can go into greater detail, but the ECU uses, at least, the speed sensor and tach to determine shift points and pressure. Depending on the year the speed sensor is either at the transmission next to the speedo cable or in the rear axle as part of the ABS sensor.

That's as far as I can take it.

P.S. Al, I'm sure you know, but I just had to say, you live in one of the most beautiful places in NA. That coming from someone who lives in the Smoky Mtns, which are pretty but just little bumps compared to the Rockies. :hail

riotwarrior · Oct 11, 2012

iverger said:
The tach sensor, ECU (Computer) and speed sensor all play a critical role in the operation of the E4OD. The ECU on E4OD trucks is buried in the firewall below the brake booster. The tach sensor on an E4OD truck runs through the ECU before going to the dash. Those in the know can go into greater detail, but the ECU uses, at least, the speed sensor and tach to determine shift points and pressure. Depending on the year the speed sensor is either at the transmission next to the speedo cable or in the rear axle as part of the ABS sensor.

That's as far as I can take it.

P.S. Al, I'm sure you know, but I just had to say, you live in one of the most beautiful places in NA. That coming from someone who lives in the Smoky Mtns, which are pretty but just little bumps compared to the Rockies.

Thanks for the info on the tach sensor I'll check my factory manuals and look further into this. However I'm not certain if it affects the tach but I guess if it goes to the transmission ECU then up to the instrument cluster it could.

Thanks for the compliment, yes, I do live in a magnificent area :love:

I like to reckon it as BC's California coastline....we got..Mountains, deserts, lakes, vineyards and wineries, beaches, orchards, thongs,

and so much more here! Not necessarily in that order ;Sweet

abnkeis · Oct 11, 2012

The computer was actually new to me too.

Can anyone verify the resistance to ground readings from the tach sensor wiring (not including the sensor). I get little to no resistance on either terminal to ground. Is this accurate? I don't see how both can go to ground and that is why I am led to the computer because once I unplug it at the firewall I no longer get the path to ground reading.

abnkeis · Oct 14, 2012

Any chance someone could verify the resistance of the tach wiring I posted above? Any help would sure help make buying a new part a lot easier.

bbjordan · Oct 14, 2012

Hmm...I guess the automatics won't make good Doomsday trucks then.

Unless you have a lead box with a spare Transmission Control Unit...
Come to think of it, I need to build a lead box to keep a couple of spare voltage regulators in. Doh!

Sorry, not trying to Hi-jack the thread.

lindstromjd · Oct 15, 2012

Here's what I got when I tested mine... (1985). I unplugged the sensor from the plug, started the truck, and took readings from both sides of both terminals:

42v - 37v
8v - 14v

That's from the tach sensor itself (first number), and the sensor plug in the harness (second number). Don't know if that's what you're looking for or if it helps, but I have a really good industrial meter that I can measure just about whatever you want or need me to.

abnkeis · Oct 16, 2012

Thanks so much for your help here, I really appreciate it!!!
I have actually never been able to get a voltage reading from the tach sensor itself, which may be part of the issue which im checking out now. However I was originally trying to get someone to check resistance to ground from NOT the tach sensor but the connection itself so as to test for a short to ground in the wiring going away from the sensor connection back to the firewall. From the schematic it looks like one wire should be positive and one negative, but I get little to no resistance to ground on both wires.
If you happen to be out again and could check the wiring to ground that may eliminate my troubleshooting of the wiring or actually the computer, since the ground short goes away once i unplug the computer harness at the firewall
Thanks again!

lindstromjd said:
Here's what I got when I tested mine... (1985). I unplugged the sensor from the plug, started the truck, and took readings from both sides of both terminals:

42v - 37v
8v - 14v

That's from the tach sensor itself (first number), and the sensor plug in the harness (second number). Don't know if that's what you're looking for or if it helps, but I have a really good industrial meter that I can measure just about whatever you want or need me to.

lindstromjd · Oct 16, 2012

So you want an ohm reading? I can do that as soon as I get home tomorrow from work. Unfortunately, I don't have a computer in my truck. I've got a T-19.

abnkeis · Oct 21, 2012

If you still have a chance to test the ohm reading it would be much appreciated.
Thanks so much!

Tach Sensor testing???

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