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914World.com _ 914World Garage _ Tach drops in RPM with brake pedal
Posted by: boxstr Mar 11 2009, 11:25 PM
Okay I cannot for the life of me remember what the problem is???
Seems like I have run across this before.
Step on the brake pedal and the RPM on the tach needle drops? Severe volt drop when brakes are applied and or the lights are turned on.
Posted by: southernmost914 Mar 12 2009, 04:35 AM
"Tach drops in RPM with brake pedal"
Press in the clutch. 
I had the same problem when the main ground behind the fuse block worked loose. 
Steve
Posted by: 6freak Mar 12 2009, 10:48 AM
Yes bad ground
Posted by: surfcity914 Mar 12 2009, 11:28 AM
Can someone please post pictures or describe exactly where the ground points to be checked are located? Thanks.
Posted by: southernmost914 Mar 12 2009, 11:37 AM
Can someone please post pictures or describe exactly where the ground points to be checked are located? Thanks.
Start with these:
Front fender wells behind headlights. Left and right.
Behind fuse block under dash.
Next to relay board in engine compartment.
Battery ground strap.
Transmission ground strap.
Steve
Posted by: rhodyguy Mar 12 2009, 11:45 AM
occurs when brakes are applied. i spoke with craig. something to do with the illumination system in the rear. i imagine a +wire was attached to a ground terminal. some shop told the owner that a new harness was required. wow.
k
Posted by: pktzygt Mar 12 2009, 12:15 PM
Okay I cannot for the life of me remember what the problem is???
Seems like I have run across this before.
Step on the brake pedal and the RPM on the tach needle drops? Severe volt drop when brakes are applied and or the lights are turned on.
This problem isn't on the black '73 with the tan interior and riveras is it?
Posted by: brp986s Mar 12 2009, 10:06 PM
When I had this symptom, fuse 9 would also blow. Unfortunately, about half the car's electrical system goes thru this fuse so there are many possibilities. Also, many of these things meet at the hazard switch. The way I tracked mine down was to put a fuse and ammeter in-line with the fuse 9 terminals. Then operate various controls (turn left...turn right...brake...etc) until hopefully you find something that pulls inordinate current or blows the fuse. If it's an intermittant short, it can be a real 
....lotsa luck
Posted by: SGB Mar 12 2009, 10:17 PM
I think maybe I would check the brake lights first too see if they are shorting when the brakes are applied...
Posted by: Joe Sharp Mar 12 2009, 10:26 PM
My 72 had a ground at the rear trunk latch. It screwed with the turn signals but I would check the grounds under the dash.
Posted by: Borderline Mar 12 2009, 10:44 PM
I have the same problem plus the tach drops with the headlights turned on and also fluctuates with the turn signals. I've checked all the grounds I can find...please help us.
Posted by: oldschool Mar 13 2009, 12:28 AM
Okay I cannot for the life of me remember what the problem is???
Seems like I have run across this before.
Step on the brake pedal and the RPM on the tach needle drops? Severe volt drop when brakes are applied and or the lights are turned on.
I had the same thing about to me also, and the generator light would go on also.
it was a bad fuse. play with the fuse box.
Posted by: computers4kids Mar 13 2009, 08:30 AM
I've also seen the incorrect bulb in the brake lights/tailights do some amazingly weird things.
Posted by: oldschool Mar 13 2009, 10:11 AM
I've also seen the incorrect bulb in the brake lights/tailights do some amazingly weird things.
so true that also happen to me.
Posted by: rick 918-S Mar 13 2009, 10:26 AM
I've also seen the incorrect bulb in the brake lights/tailights do some amazingly weird things.
so true that also happen to me.
On of my techs did this to a customer vehicle once. It was so simple 6 guys in the shop couldn't figure it out. The customer came in with the suggestion... fixed! 
sorry.
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