Tach and speedo problems - help please!

[eichhoma]

So, I just had new motors put in my cig. Went from the stock 525sc's (which were 454 blocks) and went to 502 blocks and running TBS blowers at 5 lbs of boost. I am in my break in period right now and I noticed that once the tachs reach 3800-3900 rpm's, they start fluttering and freaking out; actrually going backwards. Its not the motors - they sound and run great. Does this sound like an electrical issue? Ground maybe? I am kind of stumped. The tachs worked fine before the rebuilds, but now, they work up to 3800 and freak out. What should I be looking at? modules? wiring? grounds? The GPS speedo also does not work. Just seems strange that these problems arise now when they all worked fine prior to bring the boat in to the shop a few months ago...Any feedback or thoughts would be much appreciated. Thanks! Mark

[VetteLT193]

did you add MSD's?

Sounds like you have the tachs hooked up to the coil on MSD ignition and they have to be hooked up to the MSD tach output instead. If I guessed correctly you have to fix this ASAP because the MSD can fry the gauges.

[eichhoma]

No, I have the stock TB IV's with the V6M modules.... they work great to that point (about 3800 rpm) then start going back towards zero.....

[VetteLT193]

No, I have the stock TB IV's with the V6M modules.... they work great to that point (about 3800 rpm) then start going back towards zero.....

darn, thought I had this one nailed.

This is both tachs, correct?

Is anything else strange on the dash, (voltage gauge especially)

[eichhoma]

darn, thought I had this one nailed.

This is both tachs, correct?

Is anything else strange on the dash, (voltage gauge especially)


yes, it is both tachs doing the same thing....

Something else strange, is when you turn on the panel lights, the gps speedo flickers, and when you turn off the panel lights, it resets it self..... still not working though.... I am all kinds of confused.

[VetteLT193]

The grounds are usually daisy chained through the whole dash. check closely around the panel light switch and of course the tachs. pay close attention to the order of the daisy chain. you can usually see a pattern of oddness starting at a certain point in the daisy chain and carrying through the rest of the chain.

Also check the grounds on the engines. If the engines were painted and the paint wasn't sanded off under the ground(s) that could also be a cause.

There is little to no chance that both gauges went bad at the same time.

Another option is to make a jumper wire, put it on one gauge ground then to a known good ground and see if it works. then you will know the ground is bad Vs. something else the signal wire.

[Griff]

I'm guessing you might have weak ground that may be connected to a painted surface on the new engines.