Factory RPM Tach question

[tb3]

Where does the Factory tach get its rpm signal on 86/7 cars? I know the boost gets it from the map sensor. My factory tach will light up halfway across, and just stay. I want to check the rpm signal to it and make sure thats not problem before I send it off to caspers for repair.

tia

 

[turbofish38]

I do believe it gets its signal from the ignition module. Sorry I dont have a pin out of the connector handy.

 

[tb3]

hmm... so, I assume the factory tach and the ECM get the rpm signal from the same wire? I know about the other lead that isn't used by the alt.

 

[blown okie]

pay no attention to what the factory tach says, they are not that accurate get a scantool of any sort and go off what the ecm say the rpms are.

 

[Anthony P]

fixing factory tach

If you need it repaired, I agree John Spina (Casper's) is the place to go.

Do you, by any chance, have the green and grey single wire plug connectors behind the AC compressor plugged in to each other. These are not to be connected to each other. They are test leads for diagnostic equipment. I believe the green plug is for the tach and the grey is for applying a 12V power source to run the fuel pump. If these two leads are connected, your tach will not work. I don't recall if it will light up half way and stay there or not light at all. Check to see that these connectors are not connected to each other first before you start chasing the problem. -Anthony P

 

[tb3]

no, they wheren't connected.
I worked on this last night. I finally found a print for the factory tach in the service manual. I have direct scan, and that was reading correct rpm, so I figured it to be the guage. I pulled it out, and swapped it with a good one from another car, only to have same problem. The print shows the input signal coming direct from pin E on the ccci module. It only makes one stop along the way at that remote tach connector behind the alternator. It is a white wire that goes into pin B on the boost/tach unit. I ohmed it out, and was showing open. I checked that remote conector behind alt, and one of the wires had came out of it. (This is the conector that has the 2 white wires going to it). I respliced, and now works ok.
The ECM gets a seperate signal for its rpm off the ccci module.
htp someone else. thanks for the replys.

 

[54Rich]

no, they wheren't connected.
I worked on this last night. I finally found a print for the factory tach in the service manual. I have direct scan, and that was reading correct rpm, so I figured it to be the guage. I pulled it out, and swapped it with a good one from another car, only to have same problem. The print shows the input signal coming direct from pin E on the ccci module. It only makes one stop along the way at that remote tach connector behind the alternator. It is a white wire that goes into pin B on the boost/tach unit. I ohmed it out, and was showing open. I checked that remote conector behind alt, and one of the wires had came out of it. (This is the conector that has the 2 white wires going to it). I respliced, and now works ok.
The ECM gets a seperate signal for its rpm off the ccci module.
htp someone else. thanks for the replys.

Could I possibly get a copy of the schmatic? Mine tach is peged to the right and I know that I could send it into Caspers, however, I really don't care how acturate it is, just want the fancy lights to show something other than full on. That and the tach is not worth the cost to me to fix it. I plan on getting something acturate (After Market)in the near future. I'm betting I can come up with resistors to make it work.

Thanks,

Rich

 

[tb3]

I don't think the schematic for the actual rpm/boost guage "componet" is in the service manual, but I'll check when I get home. Reason i think that is because when it gives the pinouts to the connector, and what the grounds and voltages should be when trouble shooting, if everything checks out ok, it just says to "replace unit". I did find this link when I was doing search.

http://www.installationinstructions.com/FYI/tachboost.pdf

Some good info there that might help you

 

[54Rich]

I don't think the schematic for the actual rpm/boost guage "componet" is in the service manual, but I'll check when I get home. Reason i think that is because when it gives the pinouts to the connector, and what the grounds and voltages should be when trouble shooting, if everything checks out ok, it just says to "replace unit". I did find this link when I was doing search.

http://www.installationinstructions.com/FYI/tachboost.pdf

Some good info there that might help you

Thanks, I've seen this, actually that artical is what got me thinking that a simple A/D converter would be very easy to fix. That is if I could find the schmatic.

Rich

 

[donny1973]

If you need it repaired, I agree John Spina (Casper's) is the place to go.

-Anthony P

Does anyone know a ballpark price for this service?

 

[54Rich]

Does anyone know a ballpark price for this service?

$89.00 plus shipping, plus core. That's why I was looking for the schmatic. I'm sure that Caspers does a good job, however, with a schmatic I could get it to work (not calabrated) for a couple of dollars.

Rich

 

[donny1973]

$89.00 plus shipping, plus core. That's why I was looking for the schmatic. I'm sure that Caspers does a good job, however, with a schmatic I could get it to work (not calabrated) for a couple of dollars.

Rich

Thanks Rich. That's a good price. Can you tell me how to get in touch with Casper's?

 

[54Rich]

Thanks Rich. That's a good price. Can you tell me how to get in touch with Casper's?

Here you go:

Caspers Electronics, Inc.

Rich