tachometer reading high

[mitchlevy7]

Ever since I got my 91, the tach has read high. I thought it could be idle, but it has been a problem through 4 different engines, all 6cyl, and original engine is 6cyl, so the coding plug should be right. With the m20, m50, and s52, set to normal idle, it would idle at 950-1050, and would be able to go way past the rev limiter (it can hit zero on the mpg scale). It idles at about 1100 on my m52, set at 900 rpm idle for the cams. When I go in gear and let the rpms drop it stutters at about 800-900 rpm, when that should be more like 500. Seems to be about 400-500 off everywhere. Anybody had this problem? Im gonna try a whole different working cluster and see if anything changes. Anything that could cause a high idle or vacuum leak is new, maybe a new coding plug? When the car is off it goes back to zero normally.

[Gregs///M]

Make sure your coding plug is indeed from a 6 cylinder. I have spare ones if you need. It sounds like you might have 4 cyl plug in there.

Engine speed signal originates from the DME. Feeds into the cluster, through the SI board, and into the tach. I would make sure your SI board batteries are in good condition or they can give you inaccurate results.

[mitchlevy7]

alright. thanks.
Ill try a known good cluster to see if it fixes it.

[siggy]

Another angle: someone may have removed the tach needle and re-installed. This is tricky to get back exactly right. Easy to have a working tach that had the needled replaced but 500 rpm off on the high side. Fixable...

[ZM Blue Devil]

if you use the internals from a Euro e28/m535 in your coding plug your rpms will be correct. Just using a 6 cyclinder coding plug in a 24V makes it closer then a m42 coding plug but not exact.
L