Wierd tach issue

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Cruisergreg

Cruisergreg

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I have a problem where my tach either doesn't work at all, or does work but the needle regularly "jumps" several hundred RPMs even when the engine speed is steady. Wiring circuits all appear good.

Granted, the problem could be in my computer or the 6 cyl - 8 cyl tach interface, but I wanted to see if others have seen this with a stock setup.
 
I doubt its in your ecu. The tachs get their signal from the - side of the coil, so if the ecu was not getting this signal, you probably wouldn't get very far. What type of tach interface are you running and what model tach?
Gary
 
Waggoner5 said:
I doubt its in your ecu. The tachs get their signal from the - side of the coil, so if the ecu was not getting this signal, you probably wouldn't get very far. What type of tach interface are you running and what model tach?
Gary
Click to expand...
I agree about the ECU, but was trying to include it in a mental exercise where the signal was somehow being "corrupted" by it.

I use the stock FJ60 tach with an interface sold by Marks Adapters. It basically takes the 8 cyl signal and applies a multiplier so that the 6 cyl gauge reads the correct RPM.
 
Maybe it's the tach Greg.

We just had a 60 tach that was dead repaired by Albuquerque Speedometer. It woks fine and the bill was like 85 bucks. Maybe they could change it internally to deal with an 8 cylinder signal.
 
Absolutely. I would bet that its either in the interface piece or wiring. I have done many 60 series conversions and have not seen a bad tach yet. Although there is always a first. I guess the truck doesn't do anything crazy when the tach is acting up right?

I like Dans idea to get rid of the interface although its just a potentiometer and probably the same one used to do a tach modification by a speedo shop.
G
 
I have an after market tach because of my v8 and I have had similar issuses when adjusting the dimmer on the dash lights. I've assumed the guy who put it in wired it wrong. If I make sure the dimmer is turned all the way up it usually solves my problem.

Brad
 
Brussell said:
I have an after market tach because of my v8 and I have had similar issuses when adjusting the dimmer on the dash lights. I've assumed the guy who put it in wired it wrong. If I make sure the dimmer is turned all the way up it usually solves my problem.
Click to expand...
How could it be wired wrong? It just has a signal wire from the coil and ground, right? Or is there a separate power supply wire also? If your PO took power from the dash light circuit it would explain the dimmer thing...
 
I don't think its wired wrong. What Brad is probably talking about is, and I am taking a wild guess here, is when probing for a ground wire in the dash a light wire is sometimes mistaken for a ground wire because it is coming from the dimmer, which is resistive, according to how the dimmer is set. In other words the dash lights have 12+ going to them when the lights are on, but only get a dimmer determined amount of ground. Most tachs require a 12+, a 12-, and the tach signal wire. People get the 12- hooked up to what they thought was a sufficient ground, but was really the dimmer ground. Thats why his tach only works when the dimmer is wide open.
I don't think this has anything to do with your problem.
Gary
 
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