Redi2RocZ

Hey Guys,

I have had a good browse thru the Electronics Board & Tech/General Board but can't quite see the answer to my problem.

Wondered if somebody can diagnose why my '86 Iroc Tpi Tach is off by quite a lot!

Idle @ 800 in Park and Tach shows 2000 approx. Start to drive and it goes off into Red Zone.

I have just taken the original 85mph cluster out and replaced it with a 115mph version (Can't afford a 145 Version!)

Everything works fine apart from this Tach Problem.

When I switch engine off and then on again, but without firing the engine, the needle stays at zero.

Have I disturbed something?

How do I swap the Tachs over if I have too?

Any ideas or suggestions gratefully received.

Thanks In Advance for any replies.

Mark A Shields

Are gauges are known to go bad. Whether that's the case I'm not sure. You swapped from a 85 to 115mph. I remember when we dropped a 302 in my 4 banger Mustange, the instrument panel was set for a 4 cyl. My Dad took the instrument panel off and on the back, there was a setting for 4 cyl or 8 cyl. I'm not sure if this helps at all, but I'm sure someone can give you more help.

Chevy83Z28

Could it be a bad ground wire to the gauges? My friend has a similar problem but he never bothered to check it out... curious to see what the actual problem is. Do the other gauges work? (fuel/oil pressure/water temp... the speedometer is mechanical.)

Redi2RocZ

Thanks for your replies guys.

Like I said, everything works fine, apart from the Tach reading incorrectly.

The 85 to 115 is a straight swap fit (85mph cluster from my 1986 Iroc and the 115 was from a 1987 Iroc)

The fact that the needle zeros when ignition is turned off and then turned on again might indicate that there could be a fault on the electronics side?

Maybe I should disconnect the power?

Chevy83Z28

The tach is supposed to read 0 when you turn the key on w/ the engine off. I think it reads the speed the engine was last running when you shut it off or something. If you changed out the entire gauge cluster, maybe you put in one that had a bad tach, I thought you were saying you kept the old tach just for a new speed-o.

Redi2RocZ

Thanks for your reply Chevy83Z28,

How easy would it be/is it to swap over the Tachometer from my 85 Gauge Cluster to my 115 Gauge Cluster.

How would I do it?

Any tips/know how from anybody would be appreciated.



Thanks in advance

Redi2RocZ

Managed to swap out my Tachometer, just removed the self tap bolts and pulled it out.

Put the whole 115 cluster back in (with my Tach from my 85 mph Gauge Cluster) and everything is back to normal.

Even managed to swap over the speedo heads from the two clusters so that my mileage is correct which also means that I still have a working Trip because the one in the 115 was fubar!

Chevy83Z28

How hard was it to swap out the speedo heads? I've wanted to get a higher mph speedo for a while now but have been holding back b/c I didn't want to change the mileage on the engine ya know... it just adds too much confusion. Once you change it is the gauge still accurate for the higher speedo?

Redi2RocZ

With the Gauge Cluster out of the car, clear plastic cover and metal bezel removed and on a workbench (or kitchen table in my case!) look at the back of the cluster where the speedo cable fits to the head.

Undo the the 1 small bolt that holds the VSS sensor head in position (which is a plastic rectangular block with wires coming from it leading to the (Yellow?) module.

Then remove the two bolts (bigger heads) that are either side of where the first bolt that you have just removed.

Doing the above should release the complete Speedometer from the cluster housing.

The problem that you will now have is getting the Speedo Needle off (it will be easy or hard) without breaking it (unfortunately like I did) so that you can undo the Face Plate (by removing the two slotted screws) and gain access to the inner head unit that holds the mileage tumbler rolls.

You can split the inner part of the speedo head (the plastic unit that houses the Number Tumblers and Tripometer) from the metal inner/outer part by unscrewing the two small bolts once you have the face plate off. Doing this will let you install your original correct mileage tumbler housing into your new 115/145 metal inner/outer head if you need to.

Then just bolt the whole re-assembled speedometer back into the cluster housing (hopefully with an intact speedo needle?!)

Then out came the super glue for sticking the needle back (I had already marked the resting position of the needle in case I broke it).

Thats all there was to it and everthing works perfectly.

This was an IROC 85 to 115 swap. I am presuming that the same/similar applies to a 145 swap?

I don't know how accurate my speedometer is now because I broke the needle and stuck it back again but it is within a few mph either way.

I will check the mph accuracy soon by arranging to follow a buddy in their car and talk on the cell to determine speeds etc.

HTH

Chevy83Z28

Sounds good to me!