VDO Tach Help - Please!

I have an older vdo tach in my car that recently quit working and I cannot figure out why. So here is the short story:
- TR6 ignition
- XFI sportsman that does currently display rpm
- Continuity check is good from plug on side of tr6, cannot remember pin number but found it on here, through to tach lead ( the one behind alternator with 2 light colored wires) and plug at tach.
- Had tach tested today at local old school gage shop that happens to be a vdo dealer. They say tach is good. Showed it to me working with square wave input.
- Confirmed power and ground at tach
- DC volts at tach lead with key on, car not running, is about 5 v. Sticker on back of tach says "5V" so i assime that is some sort of reference.
- Guy at gage shop said to check "AC" voltage with digital volt meter at tach lead with car running and it should change with RPM. When I do that I got like 1.4 volts but no change with rpm.
Any thoughts? I really want to put my dash back together.
Thanks ahead of time!
 

750H.P.V6

Brutal 6 Racing
Charles,

I had a similar problem when I swapped an older VDO tach for a new version in a Kenne Bell dash I had. The new tach would not function so I sent it back for a replacement after troubleshooting the problem as best I could. They sent out a replacement and that one wouldn't work either. I contacted VDO and they didn't have any helpful input.

Ultimately I contacted John Spina at Caspers electronics. My recollection is that the tach signal from the CCCI ignition system wouldn't work with the newer VDO tach. At the time he sold a part to fix the problem. Since you said the tach did work and doesn't now maybe none of this applies. Hopefully I haven't served to further confuse you.

Neal
 

turbobitt

Well-Known Member
Charles,

I had a similar problem when I swapped an older VDO tach for a new version in a Kenne Bell dash I had. The new tach would not function so I sent it back for a replacement after troubleshooting the problem as best I could. They sent out a replacement and that one wouldn't work either. I contacted VDO and they didn't have any helpful input.

Ultimately I contacted John Spina at Caspers electronics. My recollection is that the tach signal from the CCCI ignition system wouldn't work with the newer VDO tach. At the time he sold a part to fix the problem. Since you said the tach did work and doesn't now maybe none of this applies. Hopefully I haven't served to further confuse you.

Neal
IIRC the factory tack signal is weak and not a true 0-12 volt square wave. I think it only goes to something like 0-7 volts. I had measured it on a scope at one point but its been a long time.
AG.
 
Well now that is a good question. I haven't tried it in a long time but will next time I get a chance.
What is the connection?
 

GNRick

Retired member
Caspers sells a module that you will need. You will also need a VSS from Autometer that works with their module .
 
Caspers sells a module that you will need. You will also need a VSS from Autometer that works with their module .
I guess I'm not following you....
The tach used to work but doesn't now. What would the module and VSS do for me? Also, my tach is VDO not Autometer.
 
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