Tachometer troubles after guage swap.

[BJA814]

I swapped the full guage cluster from my 1986 2.8l 5 speed into my 1988 4.0l auto. Everything seems to be working like it should, except for the tach. When the key is off, the tach sits at 2800 rpm. At idle it is up around 3500. The actual rpm's are fine, but the guage seems to be off. Is the guage junk, or is there something I've missed?

 

I can always put the idiot light cluster back in, but I'd prefer not to.

 

Thanks in advance.

[Geonovast]

I believe there are electrical differences between 86 and 88. You're probably going to need a mid 87-90 cluster.

[BJA814]

hmmm......I was afraid of that. Anyone know if the rest of the guages (oil press, fuel, battery, temp) can be swapped into a dummy light cluster?

 

Brad

[HOrnbrod]

hmmm......I was afraid of that. Anyone know if the rest of the guages (oil press, fuel, battery, temp) can be swapped into a dummy light cluster? Brad

 

No they can not. The foil printed circuit on the back of the housing is completely different.

 

Your tach should have an adjustment potentiometer inside. Try adjusting it full cw, then test. If the needle RPMs are not lowered, adjust the pot fully ccw and retest. You should be able to dial it in against a handheld tach if the dash tach is working now but inaccurate.

[BJA814]

Thanks hornbrod. I'll have to try that when I pull the dash for my new speedo cable. The tach does work, but like you mention, the needle is just off.

 

It was kind of cool to see a 4.0 rev to 8k plus :rotfl2:

[streetjeep2.5]

Thanks hornbrod. I'll have to try that when I pull the dash for my new speedo cable. The tach does work, but like you mention, the needle is just off.

 

It was kind of cool to see a 4.0 rev to 8k plus :rotfl2:

Was the needle off before putting it in and plugging it into the harness? If it was not, but is when in plugged in, sounds like incompatible electronics to me. Adjusting the potentiometer has nothing to do with where the needle is when the ignition is off. You may have "hurt" the tach when putting it in. Did you remove the tach from the cluster or maybe bent the needle in some way? I put an 88 or 89 in my 87, and hooked up an external tach to adjust the potentiometer, but had no problem with it setting at 0 when off before nor after adjustments. Something goofy with the matchup. Another thought; did you insert the plug in on the back right side up? I don't know what it would do, but I think it possible to plug it in upsidedown.

[BJA814]

Tach never came out of the cluster. It did sit at 3k rpm when I pulled it, but I have no idea if it worked in the previous truck, as is didn't run or have a battery. Needle is fine, not bent or spun, etc. I don't think it is possible to put the plug in upside down, because the tabs are two different widths I believe.

 

Tach is at 0 rpm with the key ON.

Tach slowly climbs to about 1800-2000 rpm with the key OFF.

Tach is at roughly 3000 rpm at idle.

All other guages work as they should.

 

Hope that clears up what the symptoms are exactly