Resolved Does anybody know the location of the tachometer sensor?

[flyneagle]

sorry if posted in wrong area... but id appreciate it if you guys helped me out a little... ok.. so when i first bought my car.. i bought it with the tach not working...it wasn't a big deal.. because i had a safc... and i just looked at the rpm's on that...but heres the problem.. i sold all my parts and the safc is off the car... so now that i sold that... there is no way to tell what rpm your at... the guy that i bought the car from told me its the tach sensor or something.. and all i need is a new one... he told me its a little clip or something that just clips in and the tach will work... does anybody know what im talking about... he said its located under the hood sorta near the firewall?????... if anybody knows anything.. because now that im selling my car.. i cant sell it without the tach working....thank you very much.... if thread needs to be moved... no problem... just let me know where to...

 

[steve]

On a '89-'90 the tach signal is generated by a unit attached to the coil.
On the later cars it generated in the Power Transistor Module.

There is some interaction with the ECU so you might want to make sure you have an 89-90 ECU installed. http://simon.chi.il.us/ECU/ECUID.html

Steve

 

[flyneagle]

thank you, steve. so if i have the 90 ecu, it is something off the coil. alright, that makes sense because the guy i bought the car from pointed in that direction when he said it was a clip in the engine bay.

i'm going to have someone that knows what it looks like point it out to me because my friend is parting out a non-turbo. is the sensor the same? because i'll just take his. thanks, nick

 

[SpeedFreak03]

Are you completely sure it's the "sensor"? I mean, if the SAFC was getting the correct RPM reading (it gets it from a line tapped off the ECU harness) then wouldn't that mean the "sensor" is working? I would look at the tach itself or some wiring.

 

[flyneagle]

he said... that the clip just wasn't in and once i clip it back in that the tach will work... idk.. im going to have someone look at it...that knows what they are doing...i just thought maybe someone went through this before.. and could tell me what to do.. or where the sensor is exactly located...i was hoping to find it.. and go and take it off my friends car and clip it in.. and somehow my tach will work... i thought thats how it worked.. it sounds like its a little complicated...thank you though...

 

[steve]

Are you completely sure it's the "sensor"? I mean, if the SAFC was getting the correct RPM reading (it gets it from a line tapped off the ECU harness) then wouldn't that mean the "sensor" is working? I would look at the tach itself or some wiring.

Most people don't actually have their SAFC connected the what the car uses as the Tach signal. The tend to follow the diagrams in the book or off one of the various web sites and wind up using a single coil lead (and have to set their SAFC to 2 cyl) or the CAS signal.
On a 1G the tach is driven off what comes from either the coil (89-90) or from the PTM and goes to pin 109 in the ECU.

So your SAFC could be reading correctly and your tach not work because their using two different "sensors".

Steve

 

[SpeedFreak03]

Oh my bad, I thought they tapped it off pin 21 (CAS signal on a 1G) for the RPM signal - I just assumed it's done that way because that's how they do it with aftermarket tachs and turbo timers that use an RPM signal. Do you know if you tapped it off the ECU wire or the way Steve described? Well good luck man!

 

[SpeedFreak03]

"Does anybody know the location of the tachometer sensor?"

http://members.shaw.ca/dsm.1000q/Engineprimer/1G/cas.htm

Just saw the thread again and realized I never answered the original question

 

[steve]

I'm slow tonight, please explain to me how that answers the question. The CAS is not directly related the the signal that drives the tach in the guage cluster. A bad CAS can indirectly cause funny tach readings but it should also cause other driveability issues and toss a 22 CEL if it's bad enough.

If you look at the MPI Wiring diagram you'll see that the tach is connected to the same wire that goes to ECU pin 109. That diagram is for 91-94 cars where the signal is generated by gating the coil primary drive terminals in the Power Transistor Module. On 89-90 cars the gate is part of the coil pack and uses a slightly different type of gate. (an OR vs NOR gate on the later cars)

Unfortunatly the Visual Locater you pointed out doesn't have either the coil pack or the PTM shown.

Steve